WEED DISSEMINATION – Agriculture Notes – For W.B.C.S. Examination.

আগাছাছাড়ই – কৃষিবিদ্যা নোট – WBCS পরীক্ষা।

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.Continue Reading WEED DISSEMINATION – Agriculture Notes – For W.B.C.S. Examination. 

Weeds compete with crops for water, soil nutrients, light and space (ie CO₂) and thus reduce crop yields.

Definition: Weeds are unwanted and undesirable plant that interfere with utilization of land and water resources and thus adversely affect crop production and human welfare.

Sometimes Agriculture also defined as a battle with weeds as they strongly compete with crop plants for growth factors.

Origin of weeds

Weeds are no strangers to man. They have been there ever since he started to cultivate crops about 10,000 BC and undoubtedly recognized as a problem from the beginning. To him, any plant in the field other than his crop became weed. Again the characters of certain weed species are very similar to that of wild plants in the region. Some of the crops for example including the wheat of today are the derivatives of wild grass. Man has further improved them to suit his own taste and fancy. Even today they are crossed with wild varieties to transfer the desirable characters such as drought and disease resistance. So the weeds are to begin with essential components of native and naturalized flora but in course of time these plants are well placed in new environment by the conscious and unconscious efforts of man. Hence, it is considered that many weeds principally originated from two important and major arbitrarily defined groups.

1. By man’s conscious effort

2. By invasion of plants into man created habits

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 is grouped as weeds.

I. Based on life span:

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

(a) Annual Weeds: Those that live only for a season or year and complete their life cycle in that season or year is called annuals.

These are small herbs with shallow roots and weak stem. 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 annual

Commelina benghalensis, Boerhaavia erecta

b. Winter annual

Chenopodium album

(b) Biennials: It completes the vegetative growth in the first season, flower and set seeds in the succeeding season and then die. These are found mainly in non-cropped areas.

Eg. Alternanthera echinata, Daucus carota

(c) Perennials: Perennials live for more than two years and may live almost indefinitely. They adapted to withstand adverse conditions. They propagate not only through seeds but also by underground stem, root, rhizomes, tubers etc. And hence they are further classified into

i. Simple perennials: Plants propagated only by seeds. Eg. Sonchus arvensis

ii. Bulbous perennials: Plants which possess a modified stem with scales and reproduce mainly from bulbs and seeds. Eg. Allium sp.

iii. Corm perennials: Plants that possess a modified shoot and fleshy stem and reproduce through corm and seeds. Eg. Timothy sp.

iv. Creeping perennials: Reproduced through seeds as well as with one of the following.

a. Rhizome: Plants having underground stem – Sorghum halapense

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 as well under waterlogged and in partially dry condition. Propagation is chiefly by seed.

Eg. 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

Eg. 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.

Eg. Tribulus terrestris, Convolvulus arvensis

III. Based on soil type (Edaphic):

(a) Weeds of black cotton soil: These are often closely allied to those that grow in dry condition. Eg., Aristolochia bracteata

(b) Weeds of red soils: They are like the weeds of garden lands consisting of various classes of plants. Eg. Commelina benghalensis

(c) Weeds of light, sandy or loamy soils: Weeds that occur in soils having good drainage. Eg. Leucas aspera

(d) Weeds of laterite soils: Eg. Lantana camara, Spergula arvensis

IV. Based on place of occurrence

(a) Weeds of crop lands: The majority of weeds infest the cultivated lands and cause hindrance to the farmers for successful crop production. Eg. Philaris minor in wheat

(b) Weeds of pasture lands: Weeds found in pasture / grazing grounds. Eg. Indigofera enneaphylla

(c) Weeds of waste places: Corners of fields, margins of channels etc., where weeds grow in profusion. Eg. Gynandropsis pentaphylla, Calotropis gigantea

(d) Weeds of playgrounds, road-sides: They are usually hardy, prostrate perennials, capable of withstanding any amount of trampling. Eg. Alternanthera echinata, Tribulus terestris

V. Based on Origin

(a) Indigenous weeds: All the native weeds of the country are coming under this group and most of the weeds are indigenous. Eg. Acalypha indica, Abutilon indicum

(b) Introduced or Exotic weeds: These are the weeds introduced from other countries. These weeds are normally troublesome and control becomes difficult. Eg., Parthenium hysterophorus, Phalaris minor, Acanthospermum hispidum

VI. Based on cotyledon number

Based on number of cotyledons it possess it can be classified as dicots and monocots.

(a) Monocots Eg. Panicum flavidum, Echinochloa colona

(b) Dicots Eg. Crotalaria verucosa, Indigofera viscosa

VII. Based on soil pH

Based on pH of the soil the weeds can be classified into three categories.

(a) Acidophile – Acid soil weeds eg. Rumex acetosella

(b) Basophile – Saline & alkaline soil weeds eg. Taraxacum stricta

(c) Neutrophile – Weeds of neutral soils eg Acalypha indica

VIII. Based on morphology

Based on the morphology of the plant, the weeds are also classified in to three categories. This is the most widely used classification by the weed scientists.

(a) Grasses: All the weeds come under the family Poaceae are called as grasses which are characteristically having long narrow spiny leaves. The examples are Echinocloa colonum, Cynodon dactylon

(b) Sedges: The weeds belonging to the family Cyperaceae come under this group. The leaves are mostly from the base having modified stem with or without tubers. The examples are Cyperus rotundus, Fimbrystylis miliaceae

(c) Broad leaved weeds: This is the major group of weeds as all other family weeds come under this except that is discussed earlier. All dicotyledon weeds are broad leaved weeds. The examples are Flavaria australacica, Digera arvensis

IX. Based on nature of stem

Based on development of bark tissues on their stems and branches, weeds are classified as woody, semi-woody and herbaceous species.

(a) Woody weeds: Weeds include shrubs and undershrubs and are collectively called brush weeds. Eg. Lantana camera, Prosopis juliflora

(b) Semi-woody weeds: eg.Croton sparsiflorus

(c) Herbaceous weeds: Weeds have green, succulent stems are of most common occurrence around us. Eg. Amaranthus viridis

X. Based on specificity

Besides the various classes of weeds, a few others deserve special attention due to their specificity. They are;

a. Poisonous weeds:

b. Parasitic weeds

c. Aquatic weeds

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