Classification, Biology, Ecology, Allelopathy, Important Weeds and their management, Herbicides and their classification.
Weed
Jethro Tull first coined the term weed in 1931 in the book Horse Hoeing Husbandry
Weeds are the plants, which grow where they are not wanted (Jethro Tull, 1731).
Weeds can also be referred to as plants out of place.
Weeds are unwanted or undesirable plants compete with crops for water, soil nutrients, light and space (ie CO2) and thus reduce crop yields.
National Research Centre for weed science at Jabalpur (1988).
Weed is such undesirable plant grown in association with crop which snatches major parts of nutrients, water, light, place, and CO2 available to the crop. Due to tough competition crop suffers adversely and the production is reduced.
Followings are the main points
Unwanted plant
Plant growing where it is not required i.e., a plant out of place
Extremely noxious, useless, unwanted, or poisonous plant
Any plant or vegetation excluding fungi interfering with the objectives or requirement of people.
Thus, weed may be defined as unwanted and undesirable plant which interferes with the utilization of land and water resource and adversely affects human welfare.
Criteria to be a Weed
ππ» Whether a plant is weed or not depends upon
Characteristics and habit of plant
Relative position
Time of occurrence of plant
Therefore, all plants may become weeds in a particular situation.
Classification of Weed
A. On the basis of site of predominance
A1) Obligate weeds: Such weeds are grown only in association with man and his agriculture. lt is never found in wild form e.g. Chenopodium, Anagallis etc.
A2) Facultative weeds: Such weeds are grown both as wild and in cultivated habitats e.g. Argemone mexicana, Euphorbia hirta.
B. On the basis of Dependence on Host
Parasitic Weed: Weed that depends for its growth on its host plant.
C. On the basis of Botanical or Morphological Characters
Grasses
Cylindrical and hollow stem having nodes and internodes. Leaf emerges from node, fibrous root, Graminae family e.g., Echinochloa, Phalaris etc. Most of the weeds belong to this group.
Sedges
Triangular stem, no node, very large internode and leaf at top, Cyperaceae family, no branch, usually three leaves at top, pith present e.g., Cyperus (Motha).
Broad Leaf
Broad leaf alternately arranged on stem, cylindrical stem, usually tap root e.g., all dicot weeds like Camellina, Chenopodium album (Bathua).
D. On the basis of Season
Annual
Kharif Weeds: e.g.
Amaranthus viridis (junglee chaulai)
Boerhavia diffusa (Vishkhopra)
Cassia occidentalis (Kasaundhi)
Euphorbia thymiloia (Chhoti duddhi)
Euphorbia dracunculoides (Titali)
Digera arvensis (Lahsua)
Datura festoosa
Rabi Weeds: e.g.
Convolvulus arvenis (Hiran Khuri)
Chenopodium album (Bathua) UPPSC 2021
Argemone Mexicana (Satyanashi)
Anagallis arvenis (Krishna nil)
Desmodium trifolium (Tinpatia)
Cuscuta reflexa
Asphodelus tenuifolius (Vanpyaji)
Parthenium hysterophorus: Congress Grass/Carrot Grass/Gajar Grass (King of Weeds)
Biennial
Dacus carota
Cirsium vulgare
Alternanthera pungens
Eichornia intybus
Perrenial
Cyperus rotundus (Motha)
Cynodon sp. (Dub grass)
Saccharum spontanium
Ageratum conizoids
Achyranthus
Zyziphus rotundifolia
E. Aquatic Weeds and Submersed Weeds
Submerged Weeds: Weeds grow under water e.g., Hyrilla
Emerged Weeds: Anchored weeds growing in water with major foliage on the above surface e.g., Nilumbium speciosum
Marginal Weeds: Weeds grown mostly in shoreline areas with a depth of 60-90 cm water e.g., Typha sp., Polygonum sp.
Floating Weeds: Weed leaves ο¬oat on water surface either single or in cluster e.g., Echhornia crassipes (Water Hyacinth), Salvinia, Pistia (water cabbage), Nymphaea sp., etc.
ππ» Eichornia cressipes is known as weeds of fisherman.
ππ» For control of aquatic weeds 2, 4 - D and Copper Sulphate is used.
F. Other Basis
Relative Weed e.g. Rice in Wheat field.
Absolute Weed e.g. Such weeds are due to its characteristics e.g. Cyperus rotundus.
Rogue: The off type crop variety in the field of certain crop variety is called rogue.
Mimicry Weeds: Such weeds are similar in external morphology with the crop e.g. Phalaris in wheat field and wild rice in the rice field.
Volunteer Weeds: Such weeds are grown from the fallen seeds of previous or preceeding crop in the field.
Noxious Weeds: Such weeds are declared undesirable, troublesome weed difficult to control e.g. Parthenium sp.
Prohibited Noxious: Perennial weeds reproduce by the seeds, underground roots, stems or other parts and difficult to control.
Restricted Noxious: Perennial weeds objectionable in cropping areas but can be controlled. Ex. Parthenium hysterophorus
Objectionable Weed: Problematic weed, whose seed once mixed with crop seed is extremely difficult to control/separate. E.g., Convolvulus arvensis & Phalaris in wheat and wild rice in rice field.
Satellite Weed: Weed that become an integral part of the crop ecosystem.
Most problematic weeds are Salvinia molesta, Cyperus rotundus, Cynodon dactylon, Echinochloa colona, Sorghum helepense etc.
