Weed Management in Horticulture Crops (1+1)

CLASSIFICATION OF WEEDS cont...

VIII. Based on morphology
Based on the morphology of the plant, the weeds are also classified in to three categories. This is the most widely used classification by the weed scientists.

(a) Grasses: All the weeds which come under the family Poaceae are called as grasses which are characteristically having long narrow spiny leaves. The examples are Echinocloa colonum, Cynodon dactylon.
(b) Sedges: The weeds belonging to the family Cyperaceae come under this group. The leaves are mostly from the base having modified stem with or without tubers. The examples are Cyperus rotundus, Fimbrystylis miliaceae.
(c) Broad leaved weeds: This is the major group of weeds . All dicotyledon weeds are broad leaved weeds. The examples are Flavaria australacica, Digera arvensis, Tridax procumbens

IX. Based on nature of stem
Based on development of bark tissues on their stems and branches, weeds are classified as woody, semi-woody and herbaceous species.

(a) Woody weeds: Weeds include shrubs and under shrubs and are collectively called brush weeds e.g. Lantana camera, Prosopis juliflora
(b) Semi-woody weeds: e.g.Croton sparsiflorus
(c) Herbaceous weeds: Weeds have green, succulent stems and are of most common occurrence around us e.g. Amaranthus viridis

X. Based on specificity
Besides the various classes of weeds, a few others deserve special attention due to their specificity. They are: a). Poisonous weeds, b). Parasitic weeds and c). Aquatic weeds.

a. Poisonous weeds

The poisonous weeds cause ailment to livestock resulting in death and cause great loss. These weeds are harvested along with fodder or grass and fed to cattle or while grazing the cattle consume these poisonous plants e.g. Datura fastuosa, D. stramonium and D. metel are poisonous to animals and human beings. The berries of Withania somnifera and seeds of Abrus precatorius are poisonous.

b. Parasitic weeds

The parasite weeds are either total or partial which means, the weeds that depend completely on the host plant are termed as total parasites while the weeds that partially depend on host plant for minerals and capable of preparing its food from the green leaves are called as partial parasites. Those parasites which attack roots are termed as root parasites and those which attack shoot of other plants are called as stem parasites. The typical examples are;

• 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 and Loranthus longiflorus on mango and other trees.

c. Aquatic weeds:

Unwanted plants, which grow in water and complete at least a part of their life cycle in water are called as aquatic weeds. They are further grouped into four categories as submersed, emersed, marginal and floating weeds.

• Submersed weeds: These weeds are mostly vascular plants that produce all or most of their vegetative growth beneath the water surface, having true roots, stems and leave e.g. Utricularia stellaris, Ceratophyllum demersum.
• Immersed weeds: These plants are rooted in the bottom mud, with aerial stems and leaves at or above the water surface. The leaves are broad in many plants and sometimes like grasses. These leaves do not rise and fall with water level as in the case of floating weeds e.g. Nelumbium speciosum, Jussieua repens.
• Marginal weeds: Most of these plants are immersed weeds that can grow in moist shoreline areas with a depth of 60 to 90 cm water. These weeds vary in size, shape and habitat. The important genera that comes under this group are; Typha, Polygonum, Cephalanthus, Scirpus, etc.
• Floating weeds: These weeds have leaves that float on the water surface either singly or in cluster. Some weeds are free floating and some rooted at the mud bottom and the leaves rise and fall as the water level increases or decreases. e.g. Eichhornia crassipes, Pistia stratiotes, Salvinia sp., Nymphaea pubescens.