Weeds are also like other plants but have special characteristics that tend to put them in the category of unwanted plants.

• Most of the weeds especially annuals produce enormous quantity of seeds, e.g. wild oats (Avena fatua), produces 250 seeds per plant, whereas wild amaranth (Amaranthus viridis) produces nearly 11 million seeds. It has been observed that among 61 perennial weeds, the average seed-production capacity was 26,500 per plant.

• Weeds have the capacity to withstand adverse conditions in the field, because they can modify their seed production and growth according to the availability of moisture and temperature. They can germinate under adverse soil-moisture conditions, have short period of plant growth, generally grow faster rate and produce seed earlier than most of the crops growing in association

Weed seeds remain viable for longer period without losing their viability, e.g. annual meadow grass (Poa annua) and scarlet pimpernel (Anagallis arvensis) remain viable for about 8 years; creeping thistle (Cirsium arvense) for 20 years and field bind weed (Convolvulus arvensis) for about 50 years.

• Weed seeds have a tremendous capacity to disperse from one place to another through wind, water and animals including man. Many of times, weed seeds mimic with the crop seeds due to their size and get transported from one place to another along with them.