Classification of weeds

There are at least 450 families of flowering plants and well over 350,000 different species. Only about 3,000 of them have been used by humans for food. Fewer than 300 species have been domesticated, and of these, there are about 20 that stand between humans and starvation. There are at least 100 species of great regional or local importance, but only a few major species dominate the human food supply. Only about 15 plants provide most of the food that humans have consumed for many generations.

Twelve plant families include 68% of the 200 species that are the most important world weeds (Holm, 1978). These weeds share certain characteristics, including

• Long seed life in soil,
• Quick emergence,
• Ability to survive and prosper under the disturbed conditions of a cropped field,
• Rapid early growth and
• No special environmental requirements for seed germination.

They are also competitive and react similarly to crop cultural practices. Weeds are usually defined primarily by where they are and how that makes someone feel about them. The fact that they may have shared characteristics means we may be able to define and classify them based on what their genotype enables them to do.

Weeds can be classified into following types:

A. Phylogenetic relationships

Phylogenetic keys to plant species, based on ancestry and ancestral similarity, include division, subdivision, class, family, genus, and species. A brief description of a plant key for weed species follows:

Division I: Pteridophyta:

• Fernlike, mosslike, rushlike, or aquatic plants without true flowers. Reproduce by spores.
• Representative families: Salviniaceae, Equisetaceae, Polypodiaceae

Division II: Spermatophyta:

• Plants with true flowers with stamens, pistils, or both. Reproduce by seed containing an embryo

Subdivision I: Gymnospermae:

• Ovules not in a closed ovary. Trees and shrubs with needle-shaped, linear, or scalelike, usually evergreen leaves.
• Representative families: Pinaceae, Taxaceae

Subdivision II: Angiospermae

• Ovules borne in a closed ovary that matures into a fruit.

Class I: Monocotyledoneae

• Stems without a central pith or annular layers but with woody fibers. Embryo with a single cotyledon. Early leaves always alternate. Flower parts in threes, or sixes, never fives. Leaves mostly parallel veined.
• Representative families: Poaceae, Cyperaceae, Juncaceae, Liliaceae, Commelinaceae

Class II—Dicotyledoneae

• Stems formed of bark, wood, and pith with the wood between the other two and increasing with annual growth. Leaves net-veined. Embryo with a pair of opposite cotyledons. Flower parts mostly in fours and fives.
• Representative families: Polygonaceae, Chenopodiaceae, Convolvulaceae, Asteraceae, Solanaceae

B. Based on life span (Ontogeny):

• Weeds are classified as Annual weeds, Biennial weeds and Perennial weeds.

a. Annual Weeds:

• Weeds that live only for a season or a year and complete their life cycle in that season or year are called as annual weeds.
• 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

I. Monsoon annual e.q. Commelina benghalensis, Boerhavia erectab.

II. Winter annual e.q. Chenopodium album

b. Biennials:

• It completes the vegetative growth in the first season, flower and set seeds in the succeeding season and then dies.
• These are found mainly in non-cropped areas e.g. Alternanthera echinata, Daucus carota

c. Perennials:

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

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

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

iii. Corm perennials Plants that possess a modified shoot and fleshy stem and reproduce through corm and seeds. Eg. Timothy (Phleum pratense)

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

C. Based on ecological affinities

a. Wetland weeds:

• They are tender annuals with semi-aquatic habit. They can thrive under waterlogged and partially dry condition as well.
• 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.
• g. 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.
• Tribulus terrestris, Argemone mexicana.

D. Based on soil type (Edaphic)

a. Weeds of black cotton soil:

• These are often closely allied to those that grow in dry condition.
• , Aristolochia bracteata

b. Weeds of red soils:

• They are like the weeds of garden lands consisting of various classes of plants.
• Commelina benghalensis

c. Weeds of light, sandy or loamy soils:

• Weeds that occur in soils having good drainage.
• Leucas aspera

d. Weeds of laterite soils:

• Lantana camara, Spergula arvensis

E. Based on place of occurrence

a. Weeds of crop lands:

• The majority of weeds infests the cultivated lands and cause hindrance to the farmers for successful crop production. Eg. Phalaris 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 gigantean

d. Weeds of playgrounds, road-sides:

• They are usually hardy, prostrate perennials, capable of withstanding any amount of trampling.
• Alternanthera echinata, Tribulus terestris

F. 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.
• 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.
• Parthenium hysterophorus, Phalaris minor, Acanthospermum hispidum.

G. Based on cotyledon number

a. Monocots

• Panicum flavidum, Echinochloa colona

b. Dicots

• Crotalaria verucosa, Indigofera viscose

H. Based on soil pH:

a. Acidophile: Acid soil weeds eg. Rumex acetosella

b. Basophile: Saline & alkaline soil weeds eg. Taraxacum sp.

c. Neutrophile: Weeds of neutral soils eg Acalypha indica

I. Based on morphology:

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 dicotyledonous weeds are broad-leaved weeds.
• The examples are Flavaria australacica, Digera arvensis, Tridax procumbens

d. Filamentous, such as Chara Zeylanica, Nitella hyaline

J. Based on nature of stem

a. Woody weeds:

• Weeds include shrubs and under shrubs and are collectively called brush weeds.
• 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.
• Amaranthus viridis

K. Based on specificity

a. Poisonous weeds:

• Datura fastuosa, D. stramonium and D. metal
• The berries of Withania somnifera and seeds of Abrus precatorius are poisonous.

b. Parasitic weeds:

• Total root parasite – Orabanche cernua on Tobacco
• Partial root parasite – Striga lutea on sugarcane and sorghum
• Total stem parasite – Cuscuta chinensis on leucerne and onion
• Partial stem parasite – Loranthus longiflorus on mango and other trees.

c. Aquatic weeds:

i. Submersed weeds:

• These weeds are mostly vascular plants that produce all or most of their vegetative growth beneath the water surface, having true roots, stems and leaves.
• Utricularia stellaris, Ceratophyllum demersum.

ii. Immersed weeds:

• These plants are rooted in the bottom mud, with aerial stems and leaves at or above the water surface. The leaves are broad in many plants and sometimes like grasses. These leaves do not rise and fall with water level as in the case of floating weeds.
• Nelumbium speciosum, Jussieua repens.

iii. Marginal weeds:

• Most of these plants are emersed weeds that can grow in moist shoreline areas with a depth of 60 to 90 cm water.
• These weeds vary in size, shape and habitat. The important genera that comes under this group are; Typha, Polygonum, Cephalanthus, Scirpus, etc.

iv. Floating weeds:

• These weeds have leaves that float on the water surface either singly or in cluster. Some weeds are free floating and some rooted at the mud bottom and the leaves rise and fall as the water level increases or decreases.
• Eichhornia crassipes, Pistia stratiotes, Salvinia, Nymphaea pubescens.

L. Based on economic importance

a. Absolute weeds:

• Weeds which have no economic value and growing out of their proper place are called absolute weeds i.e. Euphorbia hirta, Amaranthus spinosus, Anagallis arvensis

b. Relative weeds:

• Weeds which have some economic importance but are called weeds because these are growing out of their proper place i.e. Saccharum munja and Typha latifolia are used in cottage industry and Phalaris, Avena ludoviciana, Cynodon dactylon can be used as fodders.

M. According to association

a. Season bound weeds:

i. Monsoon annuals (Kharif annuals): Ammania baccifera, Sagittaria Sagittifolia, ludwigia, parviflora, Cyperus difforamis, Echinochloa crusgalli etc

ii. Winter annuals (Rabi annuals): Chenopodiumalbum, phalaris minor, Avena fatua Spergula arvensis, Vicia hirsuta, Molilotus alba etc.

iii. Summer annuals: Solanum nigrum, Portulaca oleracea, Argemone mexicana, Tephrosia purpurea etc.

iv. Multi seasons annuals: Echinochoa colonum, Eclipta alba, Phyllanthus niruri etc.

b. Crop bound weeds or parasitic weeds:

i. Total parasitic weeds: Orabache sp

ii. Semi parasitic weeds: Cascuta sp like Cuscuta reflexa and Cuscuta chinensis and Striga sp.

iii. Non parasite weeds: e.g. Cyperus rotundus, Echinochloa sp, Phalaris minor, Chenopodium album etc.

c. Crop associated weeds:

i. Need for specific microclimate:

• Weeds like Cichorium intybus “chicory” and Coronopus didymus– “swine cress” require for their best growth shady, cool, and moist habitat which is amply available in crop like lucerne and berseem.

ii. Mimicry:

• Wild rice (Oryza longistaminata) in cultivated rice; wild sorghum (Sorghum halepense) in cultivated sorghum; wild sugarcane (Saccharum spontaneous) in sugarcane.

iii. Contamination of crop seeds:

• Weeds like Oryza sativa var. fatua (wild rice) in rice, Phalaris minor (canary grass), Avena ludiciana (wild oat) and Convolvulus arvensis (hiran khuri) in wheat , Cichorium intybus in berseem.

N. According to the site of predominance:

i) Obligate Weeds:

• Those species of weeds which grow primarily in the cultivated land and never or rarely in the wild form, e.g. Chenopodium album, Anagallis arvensis.

ii) Facultative weeds:

• Those weed species that grow primarily in uncultivated land, e.g. Argemone mexicana, Euphorbia hirta, Opuntia spp.
• The facultatine weeds are also called “apophytes”

O. According to habitat:

a. Terrestrial weeds:

i) Weeds of cultivated crops:

• These weeds grow mainly in cultivated crops like Chenopodium album, Echinochloa spp, Phalaris minor etc

ii) Orchard weeds:

• The micro climate of orchards being different than the cultivated crop field due to shade, humidity and excessive soil moisture and some of the weed species like Cannabis sativa, Euphorbia geniculata, Ageragtum conyzoides, Oxalis corniculata find this type of habitat congenial for their growth and become troublesome.iii) Weeds of Lawns and Parks:
• A large number of annual and perennial weeds are found in lawns and parks and deprive the natural beauty. Some common examples are Imperata cylindrica, Eleusine indica, Desmodium triforum, Setaria intermedia, Medicago denticulata, Poa annua

b. Aquatic weeds habitat:

i) Floating:

• Free floating, e.g. Eichornia crassipes, Pistia stratiotes
• Rooted floating, e.g. Trapa bispinosa, Ludwigia adscendens

ii) Submerged, e.g. Hydrilla verticillata

iii) Emerged, e.g. Typha elephantina, Sagittaria sagittifolia

iv) Amphibious, e.g. Ranunculus aquatilis, Scirpus supinus

P. According to nature of stem

a. Erect: Stem of such weeds stands upright and does not require any support, e.g. Chenopodium album,Panicum repens, Melilotus sp. etc.

b. Prostrate: Those weeds instead of being erect have short internodes that bear a crown of leaves borne directly on a root, e.g. Eleusine indica, Digitaria sangunalis, Portulaca oleracea etc.

c. Twining: Those weeds, stem will round the support e.g. Cuscuta sp., Ipomoea quamoclit etc..

d. Trailing: Such weeds stems spread on ground, e.g. Convolvulus arvensis, Ipomea pandurata, Citrallus vulgaris etc.

e. Runner: Such weeds stem grow horizontally along the ground. Usually there is development of roots at the nodes of stem, e.g. Cynodon dactylon, Ipomoea bilobba, Launia asplenifolia etc.