Any plant not sown in the field by the farmer and grow where they are not wanted is called a weed. Weeds are like any other crop plants in size, form, morphological and physiological characters but possess the following characteristics on account of which they are considered as enemy of crops.
A weed may also be defined as any plant growing where it is not wanted. For example, a plant may be valuable or useful in a garden, or on a farm or plantation – but if the same plant is growing where it reduces the value of agricultural produce or spoils aesthetic or environmental values, then it is considered a weed. However, some plants are weeds regardless of where they grow.
Characteristics of Weed:
1) The weeds seed germinate early and the seedlings grow faster. They being hardy compete with the crop plants and deprive them of light, moisture and nutrients.
2) They flower earlier, run to seed in profusion and mature ahead of the crop. They are, therefore, difficult to control and it may be even impossible to eradicate some weeds completely.
3) They are unuseful, unwanted and undesirable. 4) They are harmful to crops, cattle and human-beings.
5) They can survive even under adverse conditions.
6) They are prolific and have a very high reproductive capacity.
7) Viability of seeds remains intact, even if they are buried deep in the soil.
8) The seeds may have special structures like wings, spines, hooks, sticky hairs etc. on account of which they can be easily disseminated over long distance.
9) Many weeds like doob are vegatatively propagated and spread rapidly all over the field even under adverse conditions.
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Classification of Weeds:
Classification according to anatomy of weeds: Depending upon their cycle, weeds can be classified as
i. Annual weeds:
Annual weeds more commonly complete their life cycle in one season. An annual weeds can be either
a. Summer annuals
b. Kharif annuals e.g. foxtail
c. Winter annual e.g. lambs quarter
ii. Biennial weeds: Biennial weeds complete their life cycle in two years in the first year they remain vegetative, and in the second year they produce flowers and set seeds. E.g. Daucus, Carota, Nulicaulis, etc.
iii. Perennial weeds: Perennial weeds grow for three or more years. E.g. Shallow rooted perennials – Bermuda grass, Quack grass. Deep rooted perennials :Johnson grass
Classification of Weed according to cotyledenous character:
Classification according to nature of stem: Depending upon the development of bark tissues on their stems and branches, weed can be classified as
(a). Woody e.g Lantana
(b). Semi-woody and c. Herbaceous species.
Classification according to association : It is classified into three classes:
(a). Season bound,
(b). Crop bound,
(c). Crop associated.
Classification according to situation: Depending upon their occurrences there can be nine important groups of weeds.
(a) Crop-land weeds.
(b) Falloland weeds.
(c) Grassland or pasture weeds.
(d) Non-cropland weeds
(e) Aquatic weeds
(f) Forest and woodland woods
(g) Lawn and garden weeds.
(h) Weeds of plantations.
(i) Orchard and vineyard weeds
Classification according to origin of weeds:
Many of weeds in India originated in some other part of the world. For e.g. Convolvulus sp –Eurasia, cyperus –Euphorbia,-Europe, Lantana –Africa.
Classification according to soil:
Of the several variable of soil, soil pH is implicated most frequently with the distribution of weed species. Weeds, such as red sorrel, corn sporry and bracken tend to comminute with weed flora on saline and alkali soils.
Classification according to climate: Temperature, rainfall, air quality, humidity and solar energy have pronounced effect on the distribution of weeds. One could classify weeds as hillside weeds, weeds of plains, temperate weeds, tropical weeds,etc.
Facultative and obligate weed: Facultative weeds are those weeds that grow primarily in wild communities but often escape to cultivated fields. E.g. Opuntia Spp. Obligate weeds on the contrary, occur only in cultivated or otherwise disturbed land. For e.g.: Field bind weed.
Noxious weeds: Noxious weeds is a plant arbitrarily defined as being especially undesirable troublesome and difficult to control for e.g. Chicory, thistle, bermuda grass (hariyali)
CHEMICAL: Herbicides are a key part of IWM in conventional and some organic systems. In conventional crops, using multiple effective herbicide modes of action (MOA) is essential for effective control of resistant weeds. This involves combining multiple MOA in tank mixes, and varying MOA between applications and seasons. For MOA with high occurrences of resistant weeds, avoid repeat use in consecutive seasons.
CULTURAL: Cultural tactics are crop management decisions that help the crop be more competitive against weeds and help optimize the effectiveness of herbicide applications. Common examples include timely scouting, row spacing, crop rotation, crop variety selection, timing of planting, and cover cropping. Information about using these tactics for weed management is found throughout this site (Hover on the Weed Management Tools tab, and select Cultural.
MECHANICAL: Common mechanical tools to disrupt weed growth and survival include cultivation, tillage, burning, and hand-weeding. Mechanical IWM tools also include emerging technologies like harvest-time seed destructors, cover crop rollers, and robotic weeders. Mechanical tools should be integrated when appropriate as part of a larger IWM program. Many of these mechanical techniques are available to no-till growers.
BIOLOGICAL: A less common IWM strategy is the use of living organisms, including livestock, insects, nematodes, fungi, and bacteria, to target weeds. Many biological agents target specific weed species, while livestock are relatively more generalist in weed consumption and may avoid eating certain weeds.