1 Introduction

Weed is defined as ‘a herbaceous plant not valued for use or beauty, growing wild and rank, and regarded as cumbering the ground or hindering the growth of superior vegetation’ (Zimdahl 1999). Weeds are those plants which are harmful, interfere with the agricultural operations, increase labor, add input to the cultivation, and reduce the crop yield (Sen 2000). Weeds grow in a variety of ecosystems including pastures, rangelands, and forests. There are approximately 250,000 species of plants worldwide, of those about 3% or 8000 species act as weeds (Kumari 2016). Weeds have been recognized as a problem since the beginning of agriculture and the battle against weeds is a never ending one (Tiwari et al. 2016). Weeds are believed to have been existing on the earth ever since the man started cultivating plants around 10,000 BC (Macneish 1964). The factors that create weed problems can be classified as: hydrological, habitat modification, changes in succession, disturbances, grazing, competition, diseases, hybridization, reproductive constraints, introduction, etc. (Reid 1998). Weeds differ from other plants in being more adaptive and having peculiar characteristics that make them more competitive (Dangwal et al. 2010). Most of the weeds have characteristics of enormous seed production, variety of seed dormancies, ability to grow and multiply under variable environmental conditions (Sharma et al. 2010). Some plants are naturally weedy and become a nuisance when agriculture invades the areas in which they already grow, whilst others have developed into weeds since people have started to cultivate crops (Swarbrick and Mercado 1987). Weeds reduce crop yield by competing for water, soil moisture, soil nutrients, sunlight, and growing space needed by crop plants. Weeds have the ability to spread rapidly and reproduce in high numbers which enables them to effectively crowd out native and endemic plant populations and establish a plant kingdom of their own within a short period of time. Weeds also act as alternate hosts for insects, bacteria, viruses, and nematodes that affect the crops badly by causing diseases (Younkin 1942). Weeds also inhibit the growth parameters of crop plants by secreting allelopathic chemicals (Oudhia and Tripathi 1998). Most of the weeds are exotic and have been introduced for various purposes like food, fodder, medicinal, ornamental, plantation, horticulture, etc., and they support farming and forestry in a big way. Introduced species became invasive when they are introduced deliberately or unintentionally outside their natural habitats into new areas where they express the capability to establish, invade, and out-compete native species (Sekar et al. 2012). These invasive species are threatening biodiversity by exerting significant impact on the native and endemic plant species or directly by altering ecosystem properties and resulting in the displacement of native communities, hence creating an imbalance in natural and agricultural ecosystems (Vitousek 1986; Kohli et al. 2004). Several exotic disturbances dramatically affect succession and lead to exotic annual communities with low native species richness (Stylinski and Allen 1999).

Most often, the term ‘weed’ is used to denote the invasive species only, but native plants have also the weed potential and can also compete with our crops. Invasive species are defined as the exotic/non-native plants that express the capability to invade or compete with native species. But a weed may be a native or an exotic species that mostly grow and compete with our crops, most of which are non-native. Thus, weeds are both native and non-native, whereas ‘invasive’ is a term related to non-native competing species only. Artemisia roxburghiana is regarded as a weed in Indian Himalayan Region (IHR) (Bisht 2017), but it is native to the Himalayan Region (Samant et al. 1998), so cannot be regarded as invasive as it is a native plant species. The two terms ‘weeds’ and ‘invasive species’ are often used together and are considered one and the same thing. Invasive species can be regarded as weeds, but weeds cannot be regarded as invasive. The present study presents a comprehensive database of native as well as non-native weeds of Northwest Himalayan Region (Himachal Pradesh, Uttarakhand, and Jammu and Kashmir) along with common names, life forms, range of altitude, nativity, and flowering periods and will provide the baseline information about the weeds which will serve as a manual for future weed identification.

Present findings are based on the intensive review of available information on weed species for the Northwest and West Himalaya and also the survey conducted in different parts of Uttarakhand, Himachal Pradesh, and Jammu and Kashmir. Information on various aspects like altitude, life form, flowering period, and nativity was gathered. The samples of the specimens were collected and identified with the help of local/regional floras and research papers (Osmaston 1927; Choudhery and Wadhwa 1984; Naithani 1984; Samant 1987; Singh et al. 2002). Date were compiled and analyzed for diversity, distribution pattern, and nativity. Nativity of the species was identified following Anonymous (1883–1970), Samant et al. (1998), Samant and Pal (2003), and Dhar et al. (2002).

2 Species Diversity and Distribution Pattern

A total of three hundred twenty-three weeds belong to 221 genera and 72 families (Table 1.1); they are of 279 species herbs, 27 shrubs, 15 climbers, and 2 species ferns (Fig. 1.1). Among the families, Asteraceae was the dominant family (57 species) followed by Poaceae (21 species), Lamiaceae (20 species), Ranunculaceae (19 species), Fabaceae (15 species), Brassicaceae (14 species) representing maximum weed species (Fig. 1.2). Out of documented weed species, 278 species represented dicot species within 186 genera and 58 families, while 43 species represented monocots within 33 genera and 12 families and 2 species as pteridophytes within 2 genera and 2 families.

Table 1.1 Diversity, distribution pattern of weed plants of Northwest and West Himalayan Region
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Fig. 1.1
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Diversity of life forms of weed species

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Fig. 1.2
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Dominant families of weed species

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Of the total species, maximum number of species (276) occur in the altitudinal zone, 1601–2400 m, followed by (206) in zone 801–1600 m, (146) in 2101–3200, (96) in up to 800 m, (54 species) in altitudinal zone 3201–4000 m, (18 species) in 4001–4800 (18), and (04 species) above 4800 m, respectively (Fig. 1.3).

Fig. 1.3
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Altitude-wise distribution pattern of weed species

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Ranunculus was the dominant genera (09 species) followed by Veronica, Ipomoea, and Artemisia (05 species each), and Polygonum, Chenopodium, and Viola (04 species, each).

2.1 Richness of Native and Non-native Species

Out of 323 plant species, 56 were native to Himalayan Region and 267 plant species were non-native or exotic (Fig. 1.4). The native species grows within an altitude range of 1000–5600 m, whereas the non-native species grows within an altitudinal range of 200–4500. Only few non-native species grows above an altitude of 4000 m. Further, the peak flowering period of the recorded weeds is between May and September.

Fig. 1.4
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Distribution of native and non-native weed species

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Weeds are the undesirable plants that reduce the crop yield by competing with the crops for moisture, light, space, nutrients, etc. Weeds affect everyone in the world by reducing crop yield and quality, delaying or interfering with harvesting, interfering with animal feeding, etc. (Kraehmer and Baur 2013). There is no reliable study of worldwide damage due to weeds. However, it is estimated that loss caused by weeds has exceeded the loss from any other category of agricultural pests such as insects, nematodes, diseases, and rodents (Abouziena and Haggag 2016). Generally, weeds are invasive in nature but some weeds are native too. Invasive species cause loss of biodiversity including species extinction, changes in hydrology, and ecosystem function (Sekar 2012). Besides non-native weed species, native species also act as weeds and may cause damage to the crops or may reduce the yield. Further, the flowering period of weeds coincides with the flowering period of crops. Weeds mature ahead of crops so that their seeds get mixed with crop seeds, thus resulting in adulteration, and these weeds compete again with the crops in the next season, so the battle against weeds is a never ending one (Tiwari et al. 2016). Weeds cause many billion of crop losses annually, and identification and proper management of weeds will reduce the loss by increasing crop yield and quality.