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Introduction

This contribution surveys general aspects of the relationship between people and weeds in Mexico as part of agricultural productivity and diversity, from both an ecological and economic point of view [1]. It does not focus on weed competition or control. Some subjects that have attracted much attention in the last decades, such as in situ selection and incipient domestication, are treated in more depth in other parts of this volume.

First, the meaning of the word “weed” in a Mexican/Mesoamerican context is discussed. Numbers are treated briefly. Then, we ask why Mesoamerican weeds are different, and why they are important. The role of weeds as a product in various parts of the agricultural production system is examined and an overview of different uses given. Finally, the economic role of these plants is analyzed in some detail. In several sections, I make suggestions on how the idiosyncrasy of the Mesoamerican agricultural system may have influenced cultural traits of present-day Mexicans and which topics are still under-researched.

What Are Weeds?

Weeds are creatures o f humans. Most are plants that have adapted to human ways and habitats just as much as cultivated and domesticated plants have. Many species or biotypes could not survive without the habitats created by Homo. This adaptation involves changes in physiology, germination, growth parameters, and morphology, in a way similar to how domestication syndromes develop in domesticated plants [2–4]. But, while the forces acting on weed evolution are largely man-made, selection pressures on weeds are (mostly) undirected by human purpose, in contrast with domesticated plants; some exceptions are mentioned below. However, they certainly fall under the purview of ethnobotany as the study of the relationships between plants and humans.

There are many definitions of weeds, such as unwanted plants or pioneers. For our purpose the most appropriate definition of weeds is one used by Baker [2] and Rzedowski [5] Chap. 4: they are vascular plants that grow, reproduce, and disperse in habitats strongly modified by human beings, without direct help from humans. However, even this very general definition is not always completely accurate, as we will see further on.

It should be noted that Mesoam erican farmers generally do not have a word for weed in an agronomic sense. Mestizo farmers generally refer to the wild plants growing in fields as hierbas, herbs, while jehuite is a nahuatl-derived word for herbs that are useless. Wild vegetation outside of fields is often called monte, without differentiation. Neither of the words implies a value judgment (but also see the nomenclature reported by Chacón and Gliessman [6]). Indigenous languages have their own nomenclature, but the terms related to “weed” generally do not have negative connotations.

Weed species and communities are not homogeneous entities [4]. There are many kinds of disturbance by humans, and plants are selected by type of disturbance. Simple observations show that a sand heap left from construction will be colonized by short-lived species with wind-dispersed propagules. The cracks in a sidewalk will have plants that have strong, deep primary roots and can resist trampling. A rubbish heap will often be covered by resistant cultivated plants that germinated or grew from garden refuse. A roadside is generally dominated by grasses that resist mowing. A rubber or coffee plantation has plants that can grow in the shade. An apple or grape plantation will often have some geophytes that flower early, before the canopy closes. A rye field harbors species that can germinate at low temperatures, just like the main crop. Maize and other row crops have plants that can produce several waves (cohorts) of growth, as the early emergers will be weeded out. And, of course, they are all influenced by soil, climate, intra- and interspecific competition and other biotic and abiotic factors, just like all other plants.

However, we can distinguish two main types of terrestrial weeds (aquatic and silvicultural weeds are not treated here). One type is agrestal weeds that grow in tilled soil (Fig. 12.1). The plants are frequently subject to attempts at eradication, but the soils, nutrient regime, and water status are generally conductive to plant growth and relatively uniform. The disturbance regime is severe, but cyclical an d regular [3, 4].

Fig. 12.1
figure 1

Weeds in a Mexican highland maize field. The dominant species are Cosmos bipinnatus Cav., Bidens odorata Cav., Tithonia tubiformis (Jacq.) Cass., and Simsia amplexicaulis (Cav.) Pers.

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The other type is ruderal plants . These also grow in disturbed places. However, the disturbance is mostly irregular, and only occasionally involves soil movement. These are often sites associated with human settlements: surroundings of houses and farms, garbage heaps, sports fields, parks and gardens, sidewalks, walls, or communication connections (road and railroad margins, rights of way), or the edges of fields and irrigation channels. Plantations, pastures, and some other crops cultivated without tilled soil can also be included in this group. The main disturbance types include mowing, trampling, selective weeding, some general eradication, or difficult growing conditions (drought, poor and meager soils, lack of nutrients).

These differences in environmental conditions strongly influence the ecology. Agrestal weeds are generally annuals or perennials with good capacity for resprouting from segments and for adapting to the crop. Ruderal plants are much more likely to be perennials [4]. Both are selected for and adapt their pollination and dispersal strategies to those media available in their habitat, which may vary considerably.

Many species of weeds have their origin in naturally disturbed vegetation (riversides, burned or storm damaged sites, clearings, sites disturbed by animals); some derive from natural vegetation, for example, desert annuals [7]. In addition to developing biotypes from the variation present in wild populations, some have broadened their genetic base and evolved new or strongly modified taxa through hybridization with relatives, introgression, polyploidy, and selection, just as domesticated species have [2, 7]. People have selected biotypes both negatively (weeding) and positively (tolerance and encouragement of useful forms). Thus, the limit between weed and cultivated/domesticated plant is not always clear.

How Many Weeds Are There in Mexico?

As in other regions of the world, in Mexico about 10–12 % of the flora can be called weedy. As the country has a very rich and diverse flora, its weed flora is also one of the richest in the world—about 3000 species [5, 8, 9]. If species of the general secondary vegetation flora, particularly of secondary forests, are counted, the number is probably much higher. Gómez-Pompa [10] stated that the diversity of many taxa of the tropical flora cannot be properly understood and analyzed without considering their role in secondary vegetation. Preadaptation may have occurred because of the numerous natural forces in the area that cause disturbances in the natural vegetation, such as flooding and hurricanes.

Direct human influence should not be discounted in preadaptations . Humans have been a strong influence on vegetation since the end of the last ice age [11]—through management, fire, influences on the populations of herbivores, and soon afterwards, swidden and permanent agriculture. The vegetation of Mesoamerica, like that of the Mediterranean and some Far Eastern landscapes, has been profoundly shaped by humans. Areas that are really wild, that is with no human influence, probably exist only in some very inaccessible sites. Bye y Linares [12] provide an impressive list of the ways people have influenced their surrounding vegetation and taxa in Mexico.

A remarkable fact about the Mexican weed flora is the relatively low number of introduced plants—somewhat over 600 species, and less than 3 % of the flora [9, 13, 14]. Mexico shares this low proportion with some other regions that are believed to be regions of agricultural origin, such as Israel, China, and Java.

In Mexico, as in other parts of the neotropics [15], the worst terrestrial invasive species are African tropical grasses, which were introduced for and are promoted by cattle, sheep, and goat ranching, a completely new disturbance type for the New World. In contrast, introduced agricultural weeds such as Brassica rapa L., Malva parviflora L., Reseda luteola L., or Raphanus raphanistrum L. while common, appear to have integrated into existing systems, and are not particularly invasive in the sense of displacing native vegetation. A very large proportion of these introduced weeds are useful.

So, the relatively low proportion of invasive species in Mexico may be related to the long history of agriculture, in which local weeds were able to acquire a comparative advantage. If disturbance types change, species that evolved with these disturbance types (such as the large mammals of Africa) will be more successful.

Why Are Mexican Weeds Different?

As in the Near Eastern and Asiatic center of origin of agriculture, weeds in Mexico have a long history of coevolution with human disturbance, particularly agriculture [13, 14], and have developed specialized traits. However, they developed from a different biological and cultural matrix . Mexican weeds are different from most Old World agrestals and ruderals because of (a) the stature of the main crop (Fig. 12.2), (b) the almost complete absence of domesticated animals historically, and (c) the different ecological origin of the crops.

Fig. 12.2
figure 2

The robust stature of maize has led to the development of large weeds, here Tithonia tubiformis

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In the Near and Far Eastern centers of agricultural origin, small-statured grains dominate. It is not possible to walk around in the main crop for intercropping or other purposes. The exception are some large millets in Africa, and their effect on the weed flora is still to be explored. The intercropping of maize was adopted, presumably independently, in other regions of the world where maize was introduced. African farmers combine maize with their own cucurbits (or the Mesoamerican ones), with Vigna beans and with the equivalent of quelites (wild edible herbs) [16]. The same occurs in China (Fig. 12.3).

Fig. 12.3
figure 3

In regions where maize was adopted as a main crop, people developed mixed cropping patterns similar to the Mesoamerican milpa. This maize field with intercropped Vigna unguiculata (L.) Walp.was found in southern China

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Another consequence of the large stature of maize is the ecology of its weeds. In Mexico, the dominant weed species are also large statured, with attractive flowers pollinated by insects and sometimes birds. Of course, there are also smaller species, but they have to be shade tolerant. Their diaspores are dispersed outside of the field by large (humans) and small mammals [17], depending on the size of the plants, and birds. Wind pollination or dispersal is not very successful in maize fields.

In Near and Far Eastern small grain agriculture, crop rotations and the integration of animal husbandry were the most convenient way to restore soil fertility, at least once population pressure limited the more (human-) energy-efficient swidden or slash-and-burn agriculture. In Mesoamerican traditional agriculture, nitrogen has been a limiting factor, although the region has the advantage of naturally fertile soils. Intercropped beans, of course, fix nitrogen. But the requirements of fertility maintenance has led to a system that is very common even today, of permanent plots on fertile soils (that are enriched with whatever biological material is at hand) and satellite plots that are cultivated only periodically. In areas with poor soils, highly regimented swidden systems were maintained even when populations became relatively dense, until the advent of chemical fertilizer [18]. Domestic animals were adopted rapidly, partly because they are manure producers and can transfer nutrients from the natural vegetation the fields.

Most Mexican crops derive from wild plants that grow in the western part of south-central Mexico, that is, the Balsas river basin and adjacent regions to the north. Casual observations indicate that many maize weeds may have the same ecological origin, but there are no studies on the subject. The following table compares some ecological characteristics of domesticated plants in Mesoamerica and the Near East/Europe, identified by Iltis [19] that also appear to apply to Mesoamerican weeds, based on preliminary observations and a few case studies (Table 12.1).

Table 12.1 Comparison of characteristics of Near Eastern and Mesoamerican crops based on Iltis [19], that also appear to apply to their weeds
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Management of Useful Weeds

Most weedy plants in Mexico, particularly agrestals, are useful in one way or the other, and a large proportion are multipurpose [20, 21]. Milpas (maize fields with or without intercrops) tend to have more useful weeds than other crops [21]. While farmers appreciate the contributions of weeds, they recognize that weeds compete with their crops. They also consider some plants noxious because they hinder agricultural practices, such as viny, spiny or stinging species [17, 20, 21], or because they are toxic. They are also aware that some species are more noxious than others [6, 21].

Normal agricultural practices lessen the competitive capacity of weeds and truly unwanted plants are eliminated individually if possible. However, once the critical period of the crop is over and weed growth has less influence on the yield, the spontaneous vegetation is left to grow freely. If it is controlled, it is to make harvesting easier.

Useful agrestal weeds are the focus of a whole scale of management practices that go from none through tolerance, encouragement, selection of best types, to management that is practically cultivation on the road to domestication. Management includes activities such as sparing plants during weeding, leaving a few individuals to seed, or introducing a few seeds if the plant was not yet present. Blanckaert et al. [21] found that 1/3 of the non-forage species were managed in some way.

Incipient domestication or in situ selection have received much scientific interest, since a seminal paper by Robert Bye described incipient domestication of mustards in the Tarahumara Mountains [22]. These processes include both annual plants (mostly weedy agrestals) and various useful perennials. Casas et al. [23] provide useful, general overviews; Blanckaert et al. [24] review the literature focusing on weeds.

Examples of weed taxa studied for the effects of the management and in situ selection include Lycianthes moziniana (Dunal) Bitter [25], Amaranthus [26, 27], Anoda cristata (L.) Schltdl. [28, 29], Melothria pendula L. [30], Jaltomata procumbens (Cav.) J. L. Gentry [31], and Chenopodium (Dysphania) ambrosioides L. [24].

Management intensity is often dependent both on individual idiosyncracy—not all farmers manage weeds differentially [21]—and on land tenure. For example, short-lived plants are apparently managed more intensively on private land [32]. Markets and market access also play a large role, as do considerations of risk, and the relationship of availability and demand [20, 33].

The fact that many of these useful plants depend on some amount of human management is not only shown by differentiated biotypes, but also by what happens if management ceases. A recent, unpublished study by Cristóbal Sánchez on the weed vegetation of San Juan Ixtenco, a community in the state of Tlaxcala, found that many of the edible plants (e.g., Jaltomata procumbens and Chenopodium berlandieri Moq.) have practically vanished from the fields, though people still remember them well. This is mainly due to the changes in agricultural practices and in the social situation over the last 10–20 years: increased mechanization does not allow concurrent collection of weedy quelites, herbicides affect dicots more, and people’s time is worth more, as almost everybody has other sources of income, so that collecting and preparing food comes with a higher opportunity cost.

An interesting detail of the use of wild weeds is the fact that in several cases we registered a gender conflict on weed management. Understandably, male farmers tend to want to make their work easier, particularly the hard physical work of weeding; once herbicides are cheap enough, relative to their time value, they are often happy to apply them. Women, on the other hand, wish to be able to collect quelites and medicinal plants for the family’s meals and health care. They try to get their husbands to leave at least some areas untreated, but they are not always successful [16, 34, 35].

In this context it is significant that, of the small number of useful plants that were exchanged between the New and the Old World before the Columbian voyage, two are weedy, agrestal quelites. Sinapis arvensis L. (=Brassica kaber) was found in archeological contexts in the eastern USA [36], and Portulaca oleracea L. was relatively abundant in several excavations in central Mexico [37].

The Spatial Components of the Traditional Mexican Agricultural System and Their Weeds

The Mesoamerican agricultural system consists of several, relatively well-defined and ecologically distinct components. There are, of course, the cultivated fields and home gardens. However, the field margins and ruderal habitats are also part of the productive system, as are oldfields, secondary and primary forests, and aquatic environments. The following sections deal with fields, home gardens, and field margins.

The Cultivated Fields

A crucial difference between maize and small grains as main crops is the fact that one can walk around in a maize field without damaging the main crop, which is not possible in small grain fields, particularly those sown (originally) broadcast, as was mentioned above. This has an important consequence. In Europe and the Far East, the impossibility of walking in the fields meant that other annual crops needed in smaller quantities had to be cultivated separately, that is, in vegetable gardens. In Mexico, the milpa is also the vegetable garden; this observation has been made repeatedly [38, 39], but has apparently never entered popular consciousness, particularly that of Western-trained agronomists, in detriment of appropriate agricultural policy.

However, the plants in the milpa are not under supervision as they are in a fenced garden. People and animals can help themselves to whatever grows there, and therefore, farmers generally do not want to invest much additional effort in growing their vegetables in the milpa.

With this background, it is not surprising that the Mesoamerican vegetable complement of the food system consists mainly of spontaneously growing plants that can maintain their populations autonomously, or with just a little help from the farmer, as discussed above.

Mesoamerican farmers even took some more or less wild or domesticated European plants and integrated them into this system. A relatively large number of the plants called quelites in Mexico are introduced, and are managed in the same way as the native species. Figure 12.4 shows some individuals of Brassica oleracea L. that resemble the wild type in a milpa in Chiapas; in the humid mountains of Veracruz and Oaxaca, another type of Brassica oleracea occurs in milpas, this one with white flowers. We do not know yet whether these types were introduced and adopted, or if they represent regressions from the domesticated forms.

Fig. 12.4
figure 4

Old World crops were adapted to the conditions of the milpa agriculture. A form similar to wild type Brassica oleracea is encouraged in Oaxaca and Chiapas. The left picture shows a semi-wild Brassica oleracea in a maize field in Oaxaca, and the right the same species as commercialized in the market of San Cristobal de las Casas, Chiapas

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Home Gardens

In home gardens the situation is similar to that in maize fields, but inverted. In Mexico, people grow few annual plants in their gardens. In addition to the planted perennial fruit, ornamental and medicinal plants, we find a complement of weedy perennial and hardy annual herbs, mostly medicinals and ornamentals that have to fend off the turkeys, chickens, pigs, and ducks with which they share the garden. However, they are appreciated and often managed if they appear [40–42]. Examples of annuals are epazote (Chenopodium ambrosioides), cilantro (Coriandrum), or manzanilla (Matricaria), and examples of perennials are diente de león (Taraxacum officinale F. H. Wigg.), hierba maestra (Artemisia ludoviciana Nutt.), and asomiate (Barkleyanthus salicifolius (Kunth) H. Rob. & Brettell). Here, too, it is often difficult to decide if a species is cultivated or weedy.

This tradition has policy implications. There have been numerous attempts to promote European-style home gardens with annual plants such as cabbage, tomatoes, carrots, and beets in Mexico, in order to improve nutrition of rural and urban households. Most of them have been unsuccessful or were adopted by only a few persons. I suggest that this is due to the lack of tradition, which leads to a lack of intimate knowledge of techniques such as sowing small-seeded crops in rows, thinning, timing of cultivation and appropriate spatial combinations. This contrasts with the fact that every rural female in Mexico is very adept at propagating perennial species, and piecitos (shoots) of both cultivated and wild-growing plants are commonly exchanged and given as gifts during visits.

There is an interesting exception to the general rule that milpas intercrop plants consumed as vegetables, and home gardens perennial plants. In Yucatán, in the Maya region, home gardens often have a fenced section planted with annuals and geophytes, such as lettuce, onions, and cabbage; they often have raised seedbeds called ka’anche. Also, milpas may include a maize-free zone, the pach pakal, where other vegetables such as cabbage are cultivated. After seeing similar structures in China, I suggest that these were adopted from the Chinese population that arrived in Yucatán during the henequen fiber boom in the nineteenth century. However, this has to be explored further, probably through historical documents.

In general, Mexicans have a strong tendency not to let any useful plant part go to waste. If a geranium branch breaks, it is stuck in soil somewhere else, a ver si pega (to see if it “sticks”) or is given as a gift; if a tomato plant germinates, even if it is between the flowers, it is tolerated. I consider this a result of the way gardens and vegetation are managed traditionally.

Field Margins and Ruderal Habitats

Field margins and their vegetation are part of the Mesoamerican agricultural system. They serve ecological purposes, for example, to detain erosion, social purposes, to separate holdings as fences and hedges, and also have a productive function. Here, many less valuable perennial plants are cultivated, encouraged, or tolerated. These may include fruit trees, Agave, various cacti, trees for firewood, and numerous medicinal, ornamental, edible, or forage herbs. Almost all of them can propagate by themselves (though they are occasionally planted or transplanted); that is, they are weedy. The production of these components may be highly relevant economically, but they have been studied little [43, 44]; an example of their economic relevance is given in the section on economics.

Once fields are mechanized, these components become an annoyance and are often eliminated. Invasive plants, for example, the kikuyo grass (Pennisetum (Cenchrus) clandestinum Hochst. ex Chiov.) have also been displacing the native, useful vegetation.

Wild Relatives of Cultivat ed Plants

Most wild relatives of the main annual or short-lived Mesoamerican domesticates, such as maize, various species of beans and cucurbits, tomato (Fig. 12.5), chili, tomatillo, and cotton, are weedy. They are often used in various ways, sometimes similarly and often differently from the domesticated forms [30, 45].

Fig. 12.5
figure 5

Wild tomatoes in a maize field

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These wild populations frequently grow near the domesticated plants and introgression has been shown for various crops [46], for example, for maize [47], Phaseolus beans [48], and Cucurbita fraterna-Cucurbita pepo [49]. Understanding the relationships between domesticated plants, their nearest weedy relatives and human activities has also become essential for many modern agricultural problems, such as herbicide resistance and the use of transgenic crops [49, 50]. Also, they are fast becoming the most serious and difficult-to-combat weeds, because of their similarity with the crop [51].

Weedy plants still originate cultivated species. An example is the use of teosinte for forage that has led to improved varieties. One of the numerous weedy Physalis species with edible fruit, Physalis angulata L., has been taken into cultivation in Jalisco [52]. Figure 12.6 shows small plots where normally wild-growing species, Crotalaria longirostrata Hook. & Arn. and Solanum americanum Mill., are cultivated.

Fig. 12.6
figure 6

Weeds being cultivated. The left image shows the cultivation of Crotalaria longirostrata, and the right one Solanum americanum

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Uses of Weeds

Weeds as Food

The focus of most studies on useful weeds has been on their role as food. This contribution only contains some general observations on the subject of wild edible plants, as other parts of this volume treat the subject, and an excellent overview was published recently [53].

Around the world, many weedy plants are used as food [54]. This is partly due to the growth strategy of many of the species, particularly of agrestals, that invest more in vegetative growth and less in defensive morphological structures and chemical compounds, in order to grow biomass rapidly and thus survive the intensive competition with the cultivated crop (r-type selection). Ruderal plants vary more, particularly with respect to chemical defense, as will be seen below.

In Mexico, plants used as green, spinach-type vegetables are known under the name of quelites. This is derived from a Nahua word, quilitl, which means edible herb and is often used as a suffix in plant names [55]. Examples are papaloquelite (Porophyllum), the “butterfly edible herb,” which has leaves that flutter in the wind, ayotquilit (Cucurbita pepo L.), derived from the nahua word for squash, ayotli, totomoxquilit (Sonchus oleraceus L.), allusive to the fact that it grows when the maize is dry (totomoxtle are dry maize bracts). It is also used in combination with Spanish-derived words such as berrosquilit (Nasturtium officinale W. T. Aiton, watercress), derived from the Spanish berro (examples from Molina-Martínez [56]). These plants do not necessarily have to be weeds or even herbs. For example, the young leaves of Leucaena esculenta (Moc. & Sessé ex DC.) Benth. (guaje), a leguminous tree, are also considered a quelite.

Several of these species are well known from the archeological record, particularly Amaranthus and Chenopodium. Also, seed of Portulaca oleracea, Oxalis, Chenopodium ambrosioides, and Suaeda has been found repeatedly, and chipil (Crotalaria sp.), jaltomate (Jaltomata), and chivitos (Calandrinia) occasionally [37].

Wild plants considered quelites are numerous. But those that are most used are almost always weeds of cultivated fields, that is, agrestals. For example, of the 120 species of quelites known to the Tarahumara, only ten are used regularly, and they all grow in fields (Amaranthus retroflexus L., Chenopodium ambrosioides, C. berlandieri, Bidens odorata , Cosmos parviflorus (Jacq.) Pers., Brassica rapa , Lepidium virginicum L., Anoda cristata, Portulaca oleracea, and Urtica dioica L.) [57]. This may be related to both the biological characteristics mentioned above and economics, particularly opportunity costs—it is more efficient to collect/harvest in a place where one goes anyway (fields) during activities that one does anyway (weeding).

Preparation of quelites is often simply frying with oil, onions, and salt. However, there are many other ways. Some are part of soups (Amaranthus, Malva), steamed or parboiled, or eaten fresh. Others are the main stuffing of tamales (chipil, two species of Crotalaria) (Fig. 12.7). Portulaca is part of a standard sauce used for cooked pork. Some species are cooked and the cooking water discarded before consumption, in order to reduce bitterness. Mota-Cruz et al. [58] reported fermentation of Cleome before consumption.

Fig. 12.7
figure 7

Quelites are part of many dishes; this is a tamal with chipil (Crotalaria leaves)

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In the humid mountain regions of Veracruz, Oaxaca and Chiapas people consume several species of Solanaceae (Fig. 12.8), some of them thought to be toxic [58]. They often have to be treated before consumption, or their production managed, for example by coppicing, in order to lessen bitterness, and presumably, lower alkaloid content—this subject has not yet been studied.

Fig. 12.8
figure 8

In the humid mountain areas, several species of the Solanaceae family are consumed as quelites. At the left, Solanum nigrescens Mart. & Gal.on sale in the south of Chiapas; to the right, a meal combining Solanum americanum, tomato, cheese and beans, in the Chimalapa, Oaxaca

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Some other weeds are condiments rather than vegetables. Examples are the widely known epazote (Chenopodium ambrosioides), also an important vermifuge, papaloquelite (Porophyllum macrocephalum DC.), hierba santa (Piper auritum Kunth), and lengua de vaca (Rumex obtusifolius L., R. crispus L.).

A few species are also used for the popular drinks, aguas frescas, for consumption with meals. For example, Portulaca can be combined with cucumber and lemon for a refreshing summer drink. The seed of chia (Salvia hispanica L.), a weed and cultivated plant, now widely promoted as a superfood and source of omega-3 fatty acids, is a popular modifier of the consistency of homemade lemonade.

Quelites—and food in general—are subject to the Mesoamerican hot-cold system, that believes that health is a balance between the two extremes and that several factors, again including food, can disturb this balance and cause illness [56, 59]. Quelites can have cold or hot properties and should not be consumed in an unbalanced way.

A few studies have tried to quantify quelite consumption [16, 20, 60, 61]. Not surprisingly, it varies strongly and is decreasing as people adopt a more urban lifestyle. Today, people even in remote areas eat quelites perhaps only once, or at most 2–3 times a week, whereas earlier workers reported practically daily consumption during the rainy season [57, 62]. One study showed that quelites significantly contribute iron, calcium, Vitamin B2, and riboflavin to the diet [56].

Quelites confront problems that are partly the same as for vegetables in general: people do not have time for cleaning and cooking. However, quelites also have a serious image problem: they are strongly associated with poverty, and people substitute them with other vegetables (or the cheeses, meats, and breads associated with higher status) for this reason [63]. This is not a new phenomenon, but part of the colonial heritage—there are sources from the sixteenth century disparaging this the part of the indigenous diet [56, 64]; even early ethnobotanists were not free of the idea (“Most of these verduras or “greens,” however, supply scant nutriment, serving principally as bulk in the diet” [38]). The idea that wild-growing herbs are inferior still turns up to this day in newspaper articles and other popular literature.

A group at the Botanical Garden of the UNAM has been working to reverse this trend by promoting the use of quelites among the general public, and among restaurant chefs (Fig. 12.9a–c). They have also published recipe books [65–67]. Their efforts have had some amount of success, and in the last few years, the ingredients have been appearing in gourmet restaurants with contemporary cooking (Fig. 12.9d), and even in some chain restaurants.

Fig. 12.9
figure 9

Elevating the image of quelites: (a) gourmet cooking event with top chefs of Mexico City; (b) beef filet with Malva; (c) a salad with verdolaga (Portulaca oleracea), jicama and tortilla strips, both from the cooking event; (d) an omelette with Agave flowers at El Cardenal, an upscale Mexico City restaurant

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Weeds as Medicinal Plants

John Stepp and Daniel Moerman [68, 69] showed that a very large proportion of known medicinal plants in a Tzeltal population in Chiapas, and in North America north of Mexico, and some other regions, are weedy, and not so much derived from tropical forests, contradicting some earlier publications. The authors consider various factors that may cause this relationship: one is convenience for the users—plants growing at a distance are not as useful to the sick. Another factor is biology: long-lived, K-selected species tend to defend against herbivory with large quantities of high molecular weight substances that are metabolically inactive, but less digestible for animals, such as tannins and lignins. Fast-growing, r-selected species prefer to use small, less costly, toxic molecules for defense, such as alkaloids or terpenoids.

In Mexico, it has been shown repeatedly that ruderal weeds are the most common source of medicinal plants (apart from those cultivated in home gardens), by quantity, frequency, and degree of knowledge [34, 59, 70–73] (Fig. 12.10). They are often perennials, such as Artemisia ludoviciana, Waltheria americana L., Buddleia spp., and Heterotheca inuloides Cass., and some introduced species that are cultivated or naturalized (Mentha spp., Origanum, Matricaria).

Fig. 12.10
figure 10

Ruderal habitats are an important source of useful plants, particularly medicinals. (a) The plants in the foreground are Anoda cristata, a quelite, and Grindelia inuloides Willd., a medicinal plant. (b) Agave, Pittocaulon praecox (Cav.) H. Rob. & Brettell, and Erythrina, all useful, on a roadside of northern Oaxaca. (c) Psidium guineense Sw., a relative of the guayaba and important medicinal plant, in a pasture of the Sierra Norte de Oaxaca. (d) Roadside vegetation in the Balsas river basin, with wild Zinnia and other attractive species

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However, plants commercialized on a large scale are often trees and shrubs from dry tropical forests that also have many defense mechanisms [74]. So, some other, still unexplored effects and differences may exist in function of the general climate and vegetation types.

There are relatively few species of agrestals that are widely used medicinal plants; Chenopodium ambrosioides and Datura stramonium L. are examples. They are strongly selected against by farmers, at least in milpas that are also expected to produce animal feed (see the next section).

Weeds as Forage Plants

Whereas weeds as food and medicinal plants have been studied to some extent, their role as forage or fodder plants is not well understood, despite the fact that this use is—or used to be—the most relevant one in terms of number of species, quantity, and economic value; this applies not only to Mexico [20, 21, 44, 75], but also to other regions, such as Brazil [76].

It has been shown repeatedly that a number of maize field weeds are good-quality animal feed [75]. Among these plants are some of the most dominant. They considerably improve maize straw based animal diets [77–79] and, with a few exceptions, lack high quantities of anti-nutritional factors such as tannins or phenolic compounds [80, 81].

Weeds not only add value to farm products (milk and meat) but also to its energy resources (food for working animals [44, 78]). Recycling weed nutrients through domestic animals/manure is an important way to maintain soil fertility and to reduce nutrient lixiviation; as weeds often explore other soil strata than their associated crops, they may even increase nutrient availability (Fig. 12.11).

Fig. 12.11
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In southern Puebla, farmers cut and dry Simsia lagascaeformis DC.for winter forage of their domestic animals (Photo: Cristóbal Sánchez-Sánchez, with permission)

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Other Uses of Weeds

Weeds are an important nectar and pollen source for Mexico’s large—and traditional—honey production. For example, an analysis of the honey produced in Tabasco, Mexico, showed that the pollen and nectar sources were mainly weedy herbs and trees [82]. Particularly Aldama dentata LaLlave is a very important source species for nectar in the Yucatán peninsula.

Some weeds are used for dying, for example, species of the genera Cuscuta, Bidens, Tagetes, Justicia, Dahlia, Commelina, or Baccharis [83]. The Mesoamerican indigo or Mayan blue, Indigofera suffruticosa Mill., is also a weed.

I would like to point out that a large number of ornamental plants used throughout the tropics, and as late-summer annuals in temperate regions, originated as Mesoamerican weeds. The Asteraceae family is particularly prominent, with genera such as Cosmos, Bidens, Sanvitalia, Tithonia, Zinnia, Tagetes, Ageratum houstonianum Mill., and Helianthus. But other families, such as Amaranthaceae (Gomphrena), Convolvulaceae (various Ipomoea), Iridaceae (Tigridia), Lamiaceae (several Salvia), Solanaceae (Browallia), and Onagraceae (Mirabilis), are also represented. Many of these are collected from weedy populations and commercialized regionally (e.g., wild species of Tagetes [44]). Some woody ornamentals, such as the poinsettia (Euphorbia pulcherrima Willd. ex Klotzsch), Brugmansia, or Hamelia patens Jacq., are species of secondary vegetation.

There are also a number of miscellaneous uses, for fiber, pest control, oil and even chewing gum from the latex of Asclepias notha W. D. Stevens, a plant common to field margins [84].

Weeds as an Economic Component of Agricultural Production

Agricultural productivity may refer to various concepts. There is the production of useful biomass per unit area, the gross monetary value of the production per unit area, the net value to the farmer, that is gross value minus cost (which should include home consumption at replacement value but is often not taken into account), the net value to society in general, which includes the net value to the farmer, but also positive and negative externalities. Also relevant is productivity in relation to inputs (energy, water, labor, land value). Here, we look briefly at the role of useful weeds in the net production value to the farmer, and at the value for society.

Traditional agriculture is generally mixed agriculture, in space or time or both. The Mesoamerican milpa is one of the most highly developed mixed systems. However, as explained above, the mixture is composed not only of cultivated plants, but also of other species with various degrees of desirability, usefulness, and management. The cultivated plants may have subproducts as well.

Unfortunately, these secondary products are often, if not mostly, ignored when evaluating the productivity of traditional agricultural systems for two main reasons. The first reason is that they are difficult to measure or document, even by the farmers themselves. Farmers generally have a good notion of the productivity of their main crop. After all, it is their main supplier of calories or income . They are also often harvested at only one point in time during the year and are therefore more memorable. Surveys of farmers can capture this part of the productivity with relative ease.

On the other hand, documenting the secondary products is much more difficult, time-consuming, and expensive. For one, there is much more variability , as they depend to a much larger degree on the individual idiosyncrasy of the farmer and the field. Also, many of these products are harvested and used in small portions over a longer time, and frequencies and quantities are difficult to remember. There is a heavy cognitive bias (saliency bias [85]) to discount their importance, both by farmers and investigators.

The second reason is the fact that most agronomists are trained in agricultural systems with emphasis on monoculture . This is partly related to the fact that the dominant cultures today—European/Western and East Asian—depend on grains with a small stature, traditionally grown in monoculture and rotations (which is an asynchronous mixed system, but is perceived as different from synchronous mixed systems). Also, the opportunity costs of labor in prosperous societies increase motivation for simplified systems, which in turn influences the values of agronomists, policy designers, and extensionists. Thus, training and other perception biases are significant problems, particularly as they lead to perverse incentives in the design and application of agricultural policy and subsidies. Some policies with their insistence on monoculture actually lower the net value of agricultural production (see, for example, Moreno-Calles et al. [44]).

A series of local studies from central Mexico, where small farms integrating maize and animal production are common, have attempted to quantify these secondary products and multiple uses.

Almost 20 years ago, a first study in a relatively well-communicated and productive maize growing area in the highlands north of the city of Toluca showed that farmers purposefully managed maize fields to produce both quelites and forage . While quelites were relevant economically and nutritionally, forage was the main secondary product: it had an average value, after costs, that was 50 % of the value of maize grain production [20]. This showed that these secondary products were not at all trivial, economically.

I should add that a recent, still unpublished, thesis on weeds in the same village showed a drastic change: people no longer have many domestic animals, so they do not use the forage, and the use of quelites has dropped drastically. This is due to the fact that almost everybody now has other sources of income, and farming is becoming a part-time occupation.

A second study explored the absolute production of useful biomass (not the biomass actually used) in a similar but less productive area in Tlaxcala [86, 87]. Here, the value of only the maize grain would have meant a production at loss. However, as the region had a good market for maize straw (that sold at the same price per kg as maize grain), these two products together showed a small profit, after cultivation and harvest costs. However, if forage and quelites were included, the potential production was worth over 1000 US$ per ha. Table 12.2 shows the quantities of the different products obtained, the average from 30 milpas in three different villages:

Table 12.2 Production of useful biomass in maize fields of the region of Nanacamilpa, Tlaxcala [86]
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These are substantial quantities for a temperate region. Of course, not all of this biomass can be used or sold. However, it constitutes a reserve and alternative if the maize crop falters. These results are comparable with those of Díaz-Betancourt et al. [88] who found 1000–3000 kg of edible plants per ha in ruderal vegetation, in both tropical and temperate regions.

A third, unpublished study by Edith Moreno, also in the highlands, documented the production of useful plants, not only in the milpa, but also on its borders (Table 12.3). As explained above, these borders are part of the production system, and harbor fruit trees, agaves, and other useful species.

The following three tables give examples of an economic analysis of individual fields with their borders to illustrate their components and relative importance of their production.

Table 12.3 Case studies of individual milpas in Ixtapan del Oro, Estado de México, Mexico. Data from an unfinished thesis by Edith Moreno
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The examples illustrate clearly that maize is only a small part of a highly productive system composed of several spatial units and numerous species; various wild and weedy plants play a large role. The net values are close to the returns of intensive vegetable agriculture, and quite different from the low returns frequently cited for traditional agriculture. They depend, however, on the price of labor. Calles-Moreno et al. [44] found comparable results in the Tehuacán valley, considering the drier climate.

Apart from productivity, there are other economic considerations in which useful weeds play a role. One is the role of these plants in ameliorating risks to the food supply [44, 89, 90], both for the individual farmer and for society.

The presence of useful weeds diversifies the “portfolio ” of the farmer in a way that is not possible with monocultures. If, for any reason, the main crop fails, the associated spontaneous plants assure at least some harvest [44]. In one example observed near Ixtlahuaca, Mexico, a field of onions failed due to a pest, but the owner later remarked that she earned more by selling the spontaneous Amaranthus and Chenopodium plants that appeared after the field was abandoned than she would have with the onions. It is quite common to see abandoned maize fields whose crop did not prosper, perhaps because of excessive rain or drought. Left alone, useful plants grow and are harvested, together with the abundant weeds for animal forage. The mountain Pima in Chihuahua increased their wild herb consumption drastically in a drought year with widespread crop failure [91]. Also, being able to collect spontaneous plants and sell them is an important fall-back for a large part of the population in emergencies, and for the poorest people as a regular source of income [20, 86, 92].

In another example, Martínez et al. [93] report how coffee farms, when confronted with low prices of the principal product, are not converted to other crops, but rather refocused on other intercropped species and useful spontaneous plants that are often already present. The mixed coffee garden structure is versatile and can adapt rapidly to changing markets, by simply changing efforts from attending to the coffee crop (pruning, fertilizing, etc.) to attending other species, such as fruit trees, vanilla, allspice (Pimenta dioica (L.) Merr.), or medicinal plants. They found over 300 useful species in the coffee gardens of the Sierra Norte de Puebla. Only one-third were cultivated, and the rest were either wild-growing or managed to some extent. Ninety species had commercial value, among them a number of weeds used for medicine, food, or ornament.

Another aspect of risk amelioration , now for society and not so much for the individual farmer, is the conservation and continuing evolution of germplasm of useful plants and relatives of crop plants. Whole societies relying on a narrow genetic base of a few inbred crop plant species is a recipe for long-term disaster. This generalized, increased risk is an externality of modern agricultural practices that has to be attended at the level of society. Ex situ conservation is often not possible or adequate [94].

In a way, small-scale, poor farmers subsidize modern agriculture in rich countries by conserving and improving crops, (weedy) crop relatives, and (weedy) potential crops, and thereby conserving genetic diversity, resistance genes of many sorts, etc., that is, function as a kind of live gene bank [95]. However, this is changing rapidly, as modernization reaches even remote areas.

This author sees only two ways to conserve the function of these systems: one is to pay knowledgeable and interested farmers sufficiently to conserve the systems, and the other is to organize commercial pathways that will sell their mixed products at a price that will recompense their now higher income expectations. Of course, appropriate comparative research on the actual costs and benefits, both for farmers and for society, of modern (simplified) and traditional (diverse) systems may also improve appreciation of traditional systems and the direction of public policy.

A third aspect of risk amelioration is the importance of agricultural diversity, including weeds and the knowledge and experience associated with traditional, low-external-input agroecosystems, in maintaining alternatives to current systems. Modern agriculture is extremely dependent on external energy inputs; agricultural products are often energy negative when they reach the dinner plate of a modern urban dweller [96, 97]. Traditional agroecosystems, by necessity, are always highly energy efficient [96]. The distribution of energy (and its subproducts, such as agrochemicals and improved seeds), in turn, depends on a network of communications of many types, all of which are vulnerable to sudden disruptions, from both natural and man-made causes . And, of course, there is no guarantee that external energy prices will remain low enough to continue making energy-negative food viable. The maintenance of alternatives in cases of catastrophes or drastic changes of circumstances is a quintessential public good, which must be assumed by society as a whole. It requires dedicated strategies. Both germplasm and local knowledge are impossible or extremely difficult and expensive to resurrect, once they have disappeared. As they are part of a larger system, they can be maintained ex situ in only a very limited way.

Perspectives

Weeds are an important and relevant part of Mesoamerican agricultural productivity. They also have a number of general ecological and economic effects that should be explored more thoroughly.

The ethnobotany of Mexican weeds has been studied unevenly. The focus has been on useful plants and a few regions, while other people-plants interactions have been overlooked to some extent.

Agrestal edible weeds used as quelites are relatively well known, at least those of the highlands and the humid mountain areas where their use is most obvious (Fig. 12.12). There is less information from the arid north and the humid and dry tropics. Some data on quantities and qualities of these foods exist, but not nearly enough to be able to make well-supported generalizations.

Fig. 12.12
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The section of edible herbs in the large wholesale market (Central de Abasto) of Mexico City

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Some attention has been paid to agrestal weeds as forage, medicinal, nectar source, dye plants or for handicrafts, but generally only by a single investigator or group and always very locally. It is not yet possible to give a national overview.

In general, this contribution argues for viewing agrestal and ruderal weeds separately. Even though there is a good amount of species overlap, the ecology and selection pressures are distinct. This appears to apply especially to medicinal plants; several well-documented examples show that ruderal vegetation is the principal source of medicinal plants, either in first or second place, after species cultivated in home gardens; agrestal weeds are a minor source. However, the examples from the dry and humid lowland tropics are still few. Ruderal vegetation has been neglected scientifically but is relevant from many points of view—among them, landscape management, ethnopharmacognosy, conservation and invasive plant science. Invasive plants in Mexico, particularly the introduced African forage grasses, tend to occupy ruderal habitats first and are often very dominant and intolerant. They frequently displace the native vegetation of these sites, which exotic agrestal weeds rarely do.

Another area that has received at least some quantitative attention is in situ selection for favorable traits, particularly in edible, agrestal weeds. Medicinal plants or nectar sources should be included in these investigations.

Two areas that have important practical applications are much under-researched. While ethnobotanists commonly speak of the highly adapted milpa system and enumerate its parts, there is very little quantitative analysis of the ecological and economic components, productivity in relationship with other factors, the role of “secondary” products such as useful weeds, sustainability, risk, or the decision processes of the farmers, all of which are good ethnobotanical subjects. Positive examples that integrate various aspects of the milpa agriculture, though also with emphasis on cultivated plants, are summaries of studies on milpas in Yucatán and Chiapas published as books [18, 98–100]. Preliminary and very local data show that the milpa is highly prod uctive, if all in- and outputs are considered. It is astounding that a subject such as this—highly important for understanding and guiding agricultural development—has not received more attention.

Another area that has been explored only very little is the ecology and the adaptations of Mesoamerican weeds. This contribution shows with a few examples that generalizations on Old World weeds do not always apply to New World synanthropic plants, as the selection pressures are distinct. However, even basic data, such as the pollination type, is not known for most native weeds. I suggest this to be a highly interesting subject area for collaborative investigations between ethnobotanists and evolutionary ecologists.