Weeds – undesirable plant in a particular situation

Weeds are the plants, grown where they are not needed or any plant or vegetation that effects the production of agricultural products.

Other definitions of weed:

1. Weeds are the biological limitations towards production of food.
2. Plants that are not intentionally sown.

Example: If the plant in any vegetation or plantation may be important or useful but if the same plant is growing where it decreases the agricultural yield or spoils the environmental values.

It is considered as weed. Both, nature and humans are involved in breeding plants. Nature breeds plant for survival while humans breed for yield.There are approximately 250,000 species of plants worldwide; of those, about 3% or 8000 species behave as weeds.

Characteristics of weeds:

1. Presence of reproductive parts.
2. Seed production is abundant.
3. Buried seeds survive for long.
4. Grow faster than native/local specie.

Disadvantages of weeds

1. Some weeds are parasite to agricultural crops, partially or completely.
2. Increases labour cost and equipment.
3. Reduces crop yield.
4. They compete with the agricultural plants in terms of light, minerals, nutrients, water and space and lowers the quality of harvest product.
5. Some weeds releases toxic chemicals called allelopathy.
6. Some weeds may serve as alternative host to many diseases, insects etc. May provide shelter to insects. Example midge fly of Jowar, leaf minor of soybean & Groundnut, rust of Wheat, tikka of Groundnut, Black rust of wheat.
7. Presence of weeds reduces fishing activities.
8. Blocks drainage and slows the flow of water in canals for irrigation

Advantages of weeds

1. Enhances organic matter in soil.
2. Provides food and shelter to animals. Wild life depends on weeds for food and shelter.
3.  Some weeds are sources of pesticides.
4. Chrysanthemum cinerariifolium that provides insecticide pyrethrum.
5. Serves as major source of genetic material for resistant of insects, diseases etc.
6. Provide nectar to bees.
7. Have medicinal value.
8. Leucas asperais used against snake bite.
9. Used as green manure.
10. Some fix atmospheric nitrogen.
11. Reclamation of soil.

Causes of weeds

1. Reproduce via seeds as well as through vegetative parts.
2. Roots form runners.
3. Seeds remain dormant in soil for long period.
4. If tried to pull them, the break and regrow.
5. No special environmental condition required for germination.
6. Special features for dispersal. Float in water or air or may stick to animals.

Types of weeds:

Their are three types of weeds on the bases of growing characteristics:

1. Annual: Annual weeds germinate and spread by seed, having an average lifespan of one year. Consisting both winter and summer types.
2. Biennial: Complete their life cycle in two years. Germinating in the first year and producing flowers and seeds in the second year.
3. Perennial: Regrow every year.

Common weeds of Pakistan

Common weeds of cotton

1. Petty spurge (Euphorbia prostrata)
2. Field bindweed (Convolvulus arvensis) –
3. Bermuda grass (Cynodon dactylon)
4. Purple nutsedge (Cyperus rotundus)
5. Common purslane (Portulaca oleracea)
6. Common purslane (Portulaca oleracea)
7. Horse purslane (Trianthema portulacastrum)
8. Green amaranth (Amaranthus viridis)
9. Jungle rice (Echinochloa colonum)
10. Wild jute (Corchorus tridens)

Common weeds of wheat

1. Dumbi Sitti (Phalaris minor)
2. Jangli Jai (Avena fatua)
3. Bathu (Chenopodium album)
4. Krund (Chenopodium murale)
5. Jangli Palak (Rumex dentatus)
6. Matri (Lathyrus aphaca)
7. Billi booti (Anagallis arvensis)

 Controlling weeds:

Agricultural weeds can be controlled by the following methods:

1. Cultural control.
2. Mechanical control.
3. Chemical control.
4. Biological control.

Cultural control: It involves maintaining field condition. By crop rotation, minimizing overgrazing, using acclimatize plant species or by stabilizing soil fertility.

Mechanical control: It involves the use of farm equipment.It was initiated when humans start to pull weeds from their cereal crops and tried to grow single plant specie, free from all other competitive species.

From hand pulling humans invented other simple tools such as knife and hoe. Method of weeds plant control that decreases tillage use, conserves soil fertility. Other mechanical control methods include burning, grazing and use of ducks in certain crops specially cotton.

Chemical Control: It involves the use of herbicide (chemicals) to weeds and soil. The chemical may be applied before sowing the seed known as pre-planting, after sowing but before emergence called pre-emergence or after seedling emergence called post-emergence.

Some crops are herbicide resistant, have been made resistant through genetic engineering, known as herbicide resistant crops (HRCs).

Biological control: It involves the use of natural living enemies of weeds/plants to to control the germination and spread of weeds. It uses one living organism to control the other.

Some fungi and bacteria are also applied as biological agents to control weeds. They do not survive year to year and applied annually. This technique is called bio-herbicide technique.