In this article we will discuss about the classification of weeds: 1. Annual, Biennial, and Perennial Weeds 2. Grasses, Sedges, and Broadleaf Weeds 3. Woody and Herbaceous Weeds 4. Parasitic Weeds 5. Crop-Associated and Crop-Bound Weeds 6. Alien and Invasive Alien Weeds 7. Facultative and Obligate Weeds 8. Noxious Weeds 9. Objectionable Weeds 10. Aquatic Weeds 11. Industrial Weeds 12. Grassland Weeds.
1. Annual, Biennial, and Perennial Weeds:
Annual weeds attain their full growth in one season, living for a few weeks, few months, or at the most for one year. Within this period they produce flowers and set seeds, and then die at the end of season. According to their major growth season these are called Kharif weeds and rabi weeds. Chenopodium album, Phalaris minor, Ageratum conyzoides, Echinochloa spp., and Digera arvensis are a few of the numerous annual weeds with us.
While their regrowth is dependent each time upon seeds, a few annual weeds like Parthenium hysterophorus and Amaranthus spp. can give rise to new plants also from their crown buds. Also, P. hysterophorus and some other wasteland weeds may produce flowers and seeds round the year instead of at the end of a particular season.
The main target in the control of annual weeds is to destroy these before their flowering period to prevent further seed production in them. Both, tillage and herbicides prove more effective against annual weeds when these are treated at their seedling stages than at their grown-up stages.
Several soil active herbicides are now available to control weeds from seeds, before or just after their emergence from the soil.
Biennial weeds are those weedy plants that live for two years. In the first year they attain their full vegetative growth. They produce flowers and set seeds in the second year, after which they die off. In comparison to annual and perennial weeds, the number of biennial weeds is much limited.
Common examples are- Cichorium intybus, Cirsium vulgare, Daucus carota, Gnaphalium obtusifolium, and Senecio jacobaea. A weed like Malva neglecta exhibits features of both annual and biennial weeds. Also, sometimes Cichorium arvense can bolt every year to behave as an annual species. In crop fields the control measures of biennial weeds are planned along with those of the annual weeds.
Perennial weeds persist for more than two years; more usually for a number of years. Their aerial parts may wither every year at the end of a season after producing flowers and seeds, but new shoots develop again from the underground vegetative organs like roots, rhizomes, tubers, stolons, and bulbs at appropriate time.
When major portion of the underground vegetative organs of perennial weeds is limited to the plow layer of soil, these are sub-classed as shallow weeds, e.g. Cynodon dactylon, Agropyron repens, Scirpus sp., Cyperus esculentus, Allium vineale, and Rumex crispus. Many perennial weeds have their underground system located up to 1 m depth or deeper.
These are sub-classed as deep weeds, e.g. Cirsium arvense, Convolvulus arvensis, Sonchus aroensis, Solarium elaegnifolium, Euphorbia esula, Mikania micrantha, Pluchea lanceolata, Cyperus rotundus, and Sorghum halepense. The perennial weeds are very difficult to control. Even the best translocated herbicide available today cannot reach all their underground parts.
Tillage may temporarily destroy the aerial shoots of perennial weeds to give a clean look to the land, but actually it spreads their underground parts to the fresh spots by fragmentation. Each piece of rhizome, tuber, or bulb of perennial weed then grows into a new plant which finally takes the shape of a full-fledged infestation.
It may be noted here that when a new plant of a perennial weed begins from its seeds, at that stage it can be treated for its control like an annual weed. The same is true for some perennial weeds like Oxalis striata, Rumex crispus, and Plantago lanceolata which reproduce only by seeds.
2. Grasses, Sedges, and Broadleaf Weeds:
This is perhaps the oldest and yet the most common classification of weeds used by the weed scientists. It took its roots from the time when the first successful herbicide 2,4-D (and MCPA) was found to easily kill the broadleaf weeds in cereals like wheat, barley, and oat, without damage to the crop plants. Despite discovery of numerous new herbicides since then, this classification of weeds is still in vogue because most herbicides are selective to either of the two categories of weeds, viz., the grasses and the broadleaf’s.
It may be brought home here that in the weed science literature sometimes grasses are referred to as monocot weeds, and the broadleaf weeds as dicot weeds. Strictly speaking, this nomenclature is not very correct since the broadleaf weeds like Eichhornia crasipes, Commelina benghalensis, Cyanotis axillaris, and Monochoria hactata, are all monocot plants.
Likewise, sometimes one tries to talk of grasses as narrowleaf weeds. This is also not always correct since sedges (Cyperus spp.) and cattlails (Typha spp.), are also narrowleaf plants although they don’t belong to the family of grasses (Poaceae). Yet, for all practical purposes, the designation of weeds as broadleafs, grasses, and sedges continues with the weed control scientists, worldwide.
3. Woody and Herbaceous Weeds:
The woody weeds, as the name suggests, have woody or semi- woody, rough stems. They are also called brush weeds. These are broadleaf, largely perennial shrubs and under-shrubs. They are problematic in grasslands, forests, uncropped areas, and fallow fields, although some of these like Zizyphus spp. can settle in crop fields too.
Woody weeds had to be identified as a separate class of weeds because the two popular phenoxy herbicides, 2,4-D and MCPA, destroyed only the herbaceous broadleaf weeds but not the woody ones. Some common brush weeds of importance in agriculture are: Lantana (Lantana camara), mesquite (Prosopis juliflora), wild Indian plum (Zizyphus rotundifolia), poisonoak (Rhus spp.), blackberry (Rubus allegheniensis), and multiflora rose (Rosa multiflora). It may be noted that many brush weeds reproduce only by seeds, despite being perennials.
In variance with the woody weeds, herbaceous weeds have green succulent stems, and are of common occurrence on farmlands. These weeds are easier to control than the woody weeds. Sometimes, semi-woody weeds are considered as different from the woody weeds. Camelthorn (Alhagi pseudalhagi) and arrowood (Pluchea lanceolata) are two such semi-woody, troublesome weeds with us today.
4. Parasitic Weeds:
There are certain plants which parasitise, fully or partially, on specific crop plants, for example, dodder (Cuscuta spp.) on lucerne, broomrape (Orobanche spp.) on tobacco, and witchweed (Striga spp.) on sorghum and pearlmillet. Such weeds are called parasitic weeds. They attach themselves either to the roots or to the shoots of the host plants and survive on food material available in them.
The parasitic weeds are host-specific; they cannot survive in the absence of their host plants. Many weeds are also found to act as host plants to particular parasitic weeds, thus making it possible for them to survive even outside the crop fields.
5. Crop-Associated and Crop-Bound Weeds:
Crop-associated weeds show their preference for association with certain crops for several reasons other than parasitism, as follows:
(i) Adaptation to Specific Habitat:
Weeds like chicory (Cichorium intybus) and swinecress (Coronopus didymus) usually infest crops like lucerne and potato which offer moist, cool, and shady habitat suited to such weeds.
Mimicry- Some weeds survive with particular crops because of similarity in their foliage with the crop plants during their vegetative growth period. Canarygrass (Phalaris minor) and wildoat (Avena fatua) are good examples of mimics in wheat and barley where they easily escape the hoe of the farmer. Likewise, wild rice (Oryza sativa var. fatua), Echinochloa colonum and E. crusgalli survive well in cultivated rice by mimicry.
(ii) Easy Seed Contamination:
Many weeds mature at the some time and height as the crop they infest and are thus harvested and processed with it. This is perhaps the most common way our small grains are contaminated with many weed seeds.
Crop-bound weeds, as different from the crop-associated weeds, are parasitic in nature. These complete their life cycles, fully or partially, on specific crop species only. Their control measures are developed keeping in view the nature of parasitism these depict.
Adoption of proper crop rotations offers the best way to avoid the crop-associated and crop-bound weeds in agriculture.
6. Alien and Invasive Alien Weeds:
When a weed is allowed to move from the place of its origin to a new region, and it establishes itself there, it becomes an alien weed in its new environment. In India, there is a large number of introduced weeds with us which were brought chiefly from tropical America and Australia, during the eighteenth and nineteenth centuries. By now all these introduced weeds have become so widespread in the country that they look like indigenous species.
Several factors have led to the introduction of alien weeds in India. The major amongst these are- greatly increased travels across the countries, introduction of new crops, development of livestock industry, import of food grains and agricultural seeds, and the fast transport systems. In certain cases long distance travel by birds has also introduced new weeds. The seeds of carrotgrass (Parthenium hysterophorus), corncockle (Agrostemma githago), and Solatium elaegnifolium were believed introduced in India with food grains imported from the USA.
Viable seeds of alligatorweed (Alternanthera philexeroides) were brought accidentally with packing material from South America during the Second World War. Chrotnolaena odorata is native to America from where it was navigated to India. Waterhyacinth (Eichhornia crassipes) and salvinia (Salvinia molesta) were brought into India as ornamental plants. Fruits and seeds of Clidemia hirta, a tropical American species first noticed in India in Kerala rice fields, were introduced perhaps by the bird Indian myna.
Lantana (Lantana camara) seeds were also spread from Sri Lanka by this bird. Cocklebur (Xanthium strumarium) is a native of America. Its inflorescence sticks to the body of animals and, perhaps, it was introduced that way in India with the animal imports. Puncturevine (Tribulus terrestris) was brought to India from the Mediterranean region with aircraft tyres.
When any alien weed becomes so aggressive as to fast displace the indigenous flora of the new ecosystem, it comes to be known as Invasive Alied Weed (IAW). In other words, an IAW threatens the native floral biodiversity of the area it infests. In India the major invasive alien weeds include: Ageratum houstorium, Alternantliera philoxeroides, Chromolaena odorata, Eichhornia crassipes, Lantana camara, Mikania micrantha, Mimosa spp., Cuscuta spp., Ipomoea carnea, and Phalaris minor.
The distinction between the native and alien weeds is of main interest to the biological weed control workers. They find the biological control of alien weeds comparatively easy by introducing their bioagents from the places of their origin. The quarantine people also need to become familiar with the alien weeds to be able to check their apparent entry in the new environment.
The origin of many an alien weed is not yet known.
7. Facultative and Obligate Weeds:
Facultative weeds are those weeds which grow primarily in undisturbed or closed communities. But these may sometimes escape to the cultivated fields, for example, Opuntia spp. and Parthenium hysterophorus. Obligate weeds, on the contrary, occur primarily in the cultivated fields where the land is disturbed frequently.
The obligate weeds cannot withstand competition from volunteer vegetation in a closed community of facultative weeds. If a cultivated land is abondoned permanently, sooner or later the obligate weeds give way to facultative weeds. For example- even a heavy infestation of a hardy, obligate weed like bindweed (Convolvulus arvensis) can be outgrown to the point of elimination if the field was abondoned sufficiently long (which, of course, is not possible on farm lands).
8. Noxious Weeds:
A noxious weed is a plant arbitrarily defined as being especially undesirable, troublesome, and difficult to control. The status of a plant as noxious weed will vary with the legal interpretation of a country or a state, as well as with the development of new weed control technologies.
The noxious weeds have immense capacity to reproduce and disperse, and they adopt tricky ways to defy man’s efforts to get rid of them. The noxious weeds are sometimes also referred to as special weeds and obnoxious weeds. Some common noxious weeds in India today are- Cyperus rotundus, Cynodon dactylon, Parthenium hysterophorus, Eichhornia crassipes, Solanum elaegnifolium and Orobanche spp.
The status of a noxious weed changes with time. For instance, Striga is no more a noxious weed in the USA where it has been controlled effectively with ethylene. Similarly, Salvinia molesta, Phalaris minor, and Cuscuta arvenis are now not noxious in India since their control measures have been found.
More recently, in Tamil Nadu wild brinjal (Solanum elaegnifolium), a noxious weed, has forced the farmers to replace their annual crops by the perennial plantations of coconut, eucalyptus, and casuarina where more rigorous weed control measures can keep this weed in control. S. elaegnifolium is a deep rooted, thorny, perennial broadleaf weed.
In advanced countries there are strict laws to prevent the entry of new noxious weed seeds from outside the country, as well as the inter-state movement of the noxious weeds already present in some part of the country.
9. Objectionable Weeds:
Weeds which produce seeds that are difficult to separate once mixed with crop seeds are called objectionable weeds. Wildoat in wheat, dodder in lucerne, and Polygonum plebejum in cumin are few examples of such weeds. Special efforts are needed to prevent seeds of objectionable weeds from entering the crop harvests.
10. Aquatic Weeds:
Aquatic weeds grow within and around the fresh waterbodies. They possess special features to withstand their partial or complete submergence in water. Aquatic weeds infest canals, ponds, lakes, irrigation channels, drainage systems, and the low level paddy fields. Some aquatic weeds of tropics are waterhyacinth (Eichhornia crassipes), waterfern (Salvinia molesta), hydrilla (Hydrilla verticillata), and cattails (Typha spp.).
11. Industrial Weeds:
Weeds invading areas around buildings, highways, air strips, railway tracks, utility rights-of-way, fence-rows, industrial pipelines, electric and telegraph pole bases, observatory structures, gravel walks, petroleum farms, and several other like situations are called Industrial weeds. These form part of non-crop land weeds.
These are adapted to undisturbed soil environment and various biotic stresses. The industrial weeds prevent proper use of highway and airstrips, involve fire hazards, hide pipelines, and hinder approaches to poles and buildings. Usually, the long term control and eradication programmes initiated against industrial weeds, involve non-selective herbicides and soil sterilants.
12. Grassland Weeds:
As the name indicates, weeds belonging to this class invade grasslands, rangelands, and permanent pastures, which offer a completely different ecological environment than the crop lands. The major difference between the two situations, from the stand point of view of weeds, is that while crop lands are frequently tilled and disturbed, the grasslands remain undisturbed for long period.
The grassland weed species, however, must withstand frequent grazing and cutting, as well as trampling by the animals. Some grassland weeds are equipped with mechanisms to keep the animals away, like bitter leaves, poisonous foliage, prickly shoots, and hard stems.
Although the scientists have classified weeds based on their ecological preferences and some other features, yet there are no hard and fast physical boundaries limiting their spread. Frequently, a weed considered to be typical of one agro-ecological situation can invade also the other, widely differing situation, depending upon its plasticity.
Still, the classification of weeds is essential and useful in planning their management strategies. For instance, the industrial weeds will require for their destruction the use of non-selective herbicides, as against the crop land weeds which are treated with highly selective herbicides. In grasslands on the other hand, partially selective herbicides can also be used.