Weed Classification

Out of 2,50,000 plant species, weeds constitute about 250 species, which are prominent in both agricultural and non-agricultural systems. Globally, about 30,000 species are grouped as weeds, and these are classified as follows:

A) Based on Life Span (Ontogeny) Weeds are classified into three categories based on their life span: Annual weeds, Biennial weeds, and Perennial weeds.

i) Annual Weeds; These weeds live for only one season or year, completing their life cycle within that period. They are typically small herbs with shallow roots and weak stems. They produce seeds in large quantities, and their primary mode of propagation is through seeds. After producing seeds, annual weeds die, and their seeds germinate in the next season or year to begin the new generation.

• Monsoon annuals: Commelina benghalensis, Boerhavia erecta
• Winter annuals: Chenopodium album

ii) Biennial Weeds; Biennial weeds complete their vegetative growth in the first season, flower and set seeds in the second season, and then die. These are mostly found in non-cropped areas. Examples: Alternanthera echinata, Daucus carota

iii) Perennial Weeds; Perennials live for more than two years, often indefinitely. They are adapted to survive under adverse environmental conditions and propagate both through seeds and underground structures such as stems, roots, rhizomes, and tubers. Perennials are further classified into:

• Simple perennials: Propagated only by seeds. Example: Sonchus arvensis
• Bulbous perennials: Possess a modified stem with scales and reproduce mainly from bulbs and seeds. Example: Allium sp.
• Corm perennials: Have a modified shoot and fleshy stem, and reproduce through corms and seeds. Example: Timothy (Phleum pratense)
• Creeping perennials: Reproduce through seeds and one of the following structures:
  • Rhizome: Underground stem, e.g., Sorghum halepense
  • Stolon: Horizontal creeping stem above the ground, e.g., Cynodon dactylon
  • Roots: Enlarged root system with numerous buds, e.g., Convolvulus arvensis
  • Tubers: Modified rhizomes adapted for food storage, e.g., Cyperus rotundus

B) Based on Ecological Affinities; Weeds are also classified based on their ecological habitat into Wetland weeds, Garden land weeds (Irrigated lands), and Dry land weeds.

• i) Wetland Weeds; These are tender annuals with a semi-aquatic habit. They can thrive under waterlogged conditions or in partially dry environments. Propagation is mainly through seeds. Examples: Ammania baccifera, Eclipta alba
• ii) Garden Land Weeds (Irrigated Lands); These weeds are not as dependent on water as wetland weeds but also cannot withstand extreme drought like dryland weeds. Examples: Trianthema portulacastrum, Digera arvensis
• ii) Dry Land Weeds; These are hardy plants with deep root systems, adapted to withstand drought due to the mucilaginous nature of their stems and the presence of hairiness. Examples: Tribulus terrestris, Argemone mexicana

C. Based on Soil Type (Edaphic); Weeds are classified based on the type of soil they grow in:

• Weeds of Black Cotton Soil; These weeds are often closely allied to those that grow in dry conditions. Example: Aristolochia bracteata
• Weeds of Red Soils; Similar to the weeds of garden lands, they consist of various classes of plants. Example: Commelina benghalensis
• Weeds of Light, Sandy, or Loamy Soils; These weeds occur in soils with good drainage. Example: Leucas aspera
• Weeds of Laterite Soils; Examples: Lantana camara, Spergula arvensis

D) Based on Place of Occurrence; Weeds are categorized based on where they are found:

• Weeds of Crop Lands; The majority of weeds infest cultivated lands and hinder successful crop production. Example: Phalaris minor (in wheat)
• Weeds of Pasture Lands; Weeds found in pasture or grazing grounds. Example: Indigofera enneaphylla
• Weeds of Waste Lands; These weeds grow in the corners of fields, margins of channels, etc., and grow in profusion. Examples: Gynandropsis pentaphylla, Calotropis gigantea
• Weeds of Playgrounds, Road-Sides; These are usually hardy, prostrate perennials that can withstand heavy trampling. Examples: Alternanthera echinata, Tribulus terrestris

E) Based on Origin; Weeds are classified based on their origin as either indigenous or introduced (exotic).

• Indigenous Weeds; These are native weeds of the country. Most weeds are indigenous. Examples: Acalypha indica, Abutilon indicum
• Introduced or Exotic Weeds; These are weeds introduced from other countries. These weeds are often troublesome, making control difficult. Examples: Parthenium hysterophorus, Phalaris minor, Acanthospermum hispidum

F) Based on Cotyledon Number; Weeds can be classified based on the number of cotyledons as either monocots or dicots.

• Monocots; Examples: Panicum flavidum, Echinochloa colona
• Dicots; Examples: Crotalaria verucosa, Indigofera viscosa

G) Based on Soil pH; Weeds are classified into three categories based on the pH of the soil they inhabit:

• Acidophile – Acid Soil Weeds; Example: Rumex acetosella
• Basophile – Saline & Alkaline Soil Weeds; Example: Taraxacum
• Neutrophile – Weeds of Neutral Soils; Example: Acalypha indica

H) Based on Morphology; Weeds are classified based on the morphology of the plant into three categories. This is the most widely used classification by weed scientists:

• Grasses; All the weeds in the family Poaceae are called grasses. They characteristically have long, narrow, spiny leaves. Examples: Echinocloa colonum, Cynodon dactylon
• Sedges; Weeds belonging to the family Cyperaceae fall under this group. These weeds mostly have leaves originating from the base with a modified stem, sometimes with or without tubers. Examples: Cyperus rotundus, Fimbrystylis miliaceae
• Broad-Leaved Weeds; This is the major group of weeds. All dicotyledonous weeds are considered broad-leaved. Examples: Flavaria australacica, Digera arvensis, Tridax procumbens

I) Based on Nature of Stem; Weeds are classified based on the development of bark tissues on their stems and branches, distinguishing them as woody, semi-woody, or herbaceous species:

• Woody Weeds; These weeds include shrubs and under-shrubs, collectively called brush weeds. Examples: Lantana camara, Prosopis juliflora
• Semi-Woody Weeds; Example: Croton sparsiflorus
• Herbaceous Weeds; These weeds have green, succulent stems and are common in occurrence. Example: Amaranthus viridis

J) Based on Specificity; Certain weeds deserve special attention due to their specific characteristics. These are classified into poisonous weeds, parasitic weeds, and aquatic weeds:

i) Poisonous Weeds; Poisonous weeds cause illness to livestock, often leading to death, and can cause significant economic losses. These weeds are often harvested along with fodder or grass, or cattle may consume them while grazing. Examples: Datura fastuosa, Datura stramonium, Datura metel (poisonous to animals and humans); berries of Withania somnifera and seeds of Abrus precatorius are also poisonous.

ii) Parasitic Weeds; Parasitic weeds are either total or partial parasites.

• Total parasites depend completely on the host plant.
• Partial parasites rely on the host for minerals but can photosynthesize food from their own green leaves. Additionally, parasites that attack the roots are termed root parasites, while those that attack the shoot are called stem parasites.
• Examples:
  • Total root parasite: Orabanche cernua (on tobacco)
  • Partial root parasite: Striga lutea (on sugarcane and sorghum)
  • Total stem parasite: Cuscuta chinensis (on lucerne and onion)
  • Partial stem parasite: Cassytha filiformis (on orange trees); Loranthus longiflorus (on mango and other trees)

iii) Aquatic Weeds; Aquatic weeds are unwanted plants that grow in water and complete at least part of their life cycle in water. They are further categorized as submersed, emersed, marginal, and floating weeds:

• Submersed Weeds: These weeds are mostly vascular plants that grow beneath the water surface and have true roots, stems, and leaves. Examples: Utricularia stellaris, Ceratophyllum demersum
• Emersed Weeds: Rooted in bottom mud, with aerial stems and leaves at or above the water surface. The leaves are often broad, and they do not rise or fall with changes in water levels. Examples: Nelumbium speciosum, Jussieua repens
• Marginal Weeds: These weeds grow in moist shoreline areas with water depths of 60 to 90 cm. They vary in size, shape, and habitat. Examples: Typha, Polygonum, Cephalanthus, Scirpus
• Floating Weeds: These weeds have leaves that float on the water surface, either singly or in clusters. Some are free-floating, while others are rooted in the mud, with leaves rising and falling with water levels. Examples: Eichhornia crassipes, Pistia stratiotes, Salvinia, Nymphaea pubescens