Weed Management in Horticultural Crops

CLASSIFICATION OF WEEDS

Out of 2,50,000 plant species, weeds constitute about 250 species, which are prominent in agricultural and non-agricultural system. Under world conditions about 30000 species are grouped as weeds which are classified as follows:

I. Based on life span
Based on life span (Ontogeny), weeds are classified as Annual weeds, Biennial weeds and Perennial weeds.

a. Annual Weeds
Weeds that live only for a season or a year and complete their life cycle in that season or year are called as annual weeds.
These are small herbs with shallow roots and weak stems. Produces seeds in profusion and the mode of propagation is commonly through seeds. After seeding, the annuals die away and the seeds germinate and start the next generation in the next season or year following.
Most common field weeds are annuals. The examples are:

a. Monsoon annuals: Commelina benghalensis, Boerhavia erecta
b. Winter annuals: Chenopodium album

 
b. Biennials
They complete the vegetative growth in the first season, flower and set seeds in the succeeding season and then dies. These are found mainly in non-cropped areas.
e.g. Alternanthera echinata, Daucus carota

c. Perennials
Perennials live for more than two years and may live almost indefinitely. They adapt to withstand adverse environmental conditions. They propagate not only through seeds but also by underground stems, roots, rhizomes, tubers etc. and hence they are further classified into

i. Simple perennials: Plants propagated only by seeds e.g. Sonchus arvensis
ii. Bulbous perennials: Plants which possess a modified stem with scales and reproduce mainly from bulbs and seeds e.g. Allium sp.
iii. Corm perennials: Plants that possess a modified shoot and fleshy stem and reproduce through corm and seeds e.g. Timothy (Phleum pratense)

iv. Creeping perennials: They reproduce through seeds as well as with one of the followings:

a. Rhizome: Plants having underground stem – Sorghum halepense
b. Stolon: Plants having horizontal creeping stem above the ground – Cynodon dactylon
c. Roots: Plants having enlarged root system with numerous buds – Convolvulus arvensis
d. Tubers: Plants having modified rhizomes adapted for storage of food – Cyperus rotundus

II. Based on ecological affinities

a. Wetland weeds
They are tender annuals with semi-aquatic habit. They can thrive well under waterlogged and in partially dry conditions. Propagation is chiefly by seed e.g. Ammania baccifera, Eclipta alba

b. Garden land weeds (Irrigated lands)
These weeds neither require large quantities of water like wetland weeds nor can they successfully withstand extreme drought as dryland weeds e.g. Trianthema portulacastrum, Digera arvensis

c. Dry lands weeds
These are usually hardy plants with deep root system. They are adapted to withstand drought on account of mucilaginous nature of the stem and hairiness e.g. Tribulus terrestris, Argemone mexicana.