Weeds are colonizing plants that thrive in the disturbance created by man’s activities. They are successful because they exploit man’s activities both for their habitats and their dispersal, and in many cases because they have evolved with man in the old centres of civilization.
Weeds as colonists
Weeds are usually associated with an aggressive ability to spread quickly and become a pest and may be difficult to control or eradicate.
The term weed is rather imprecise because it means different things to different people. To the farmer weeds are the unwanted wild plants growing in his crops, while to the gardener all the plants he is not cultivating are weeds. However plants cultivated as crops or as gardenmay persist spread, and themselves become weeds in certain situations. To the uninitiated perhaps all wild plants are weeds because they are apparently unproductive and of no particular use to man. The best definition of a weed is a plant which is growing where it is not wanted. In general do actual harm to cultivated plants by shading them out, depriving them of sunlight and by competing with them for nutrients and water. Each climatic zone of the world and each particular soil type has its own particular prevalent weeds which are able to exploit these conditions most effectively. The most aggressive weeds in a country are very often not natives but are introduced aliens. This is perhaps because the normal controls such as herbivorous insects have been left behind in the country of origin. Many weeds are virtually cosmopolitan, having been distributed by man’s movements round the world.
Weed species have a number of common characteristics. They are usually colonists of open or disturbed ground so they are particularly noticeable on cultivated arable land or in gardens, or in other situations where the natural vegetation cover is broken up and soil exposed. These conditions occur temporarily on building and construction sites, on waste ground and roadsides and on rubbish tips. A relatively small number of weeds can compete in closed turf such as lawns. Ponds, lakes and rivers also have their own weeds which may become very troublesome in navigational waterways.
Dispersal of weeds
The capacity to produce prolific quantities ofis a quality which enables weeds to rapidly colonize open situations. This may be helped by the production of several generations of fruiting plants during a year. Perhaps the most important factor that contributes to the success of a weed is the ability to flourish and reproduce in a wide variety of different conditions. Wind dispersal of enables rapid spread and colonization of new habitats. A number of successful species have evolved seeds that can be carried by the wind, especially those which have a parachute of hairs such as the seeds produced by thistles, willowherbs and fleabanes. Many seeds that are eaten by livestock or birds may remain viable after passing through the digestive system of these animals and so dispersed. In some cases the viability of the seeds is actually enhanced by this process. Animals are responsible for distributing other weeds by carrying the seeds externally. or fruits that bear hooks or barbs are designed to become entangled in the coat of animals. Small seeds can be picked up in the mud that adheres to animals’ feet or to the wheels of man’s vehicles.
Vegetative reproduction may be a significant means of multiplication of weeds especially where conditions are relatively uniform. Some of the most ineradicable weeds can regenerate from fragments of rootstock as in the creeping thistle Cirsium arvense. Ploughing and harrowing breaks up theand spreads the plants by scattering the fragments. Similarly the weeds of the genus Oxalis produce tiny bulbs or ‘bulbils’ on their which readily break off to produce new plants.
Man is undoubtedly the most significant factor in the dispersal of weeds both within a country and round the world. It is man’s own commerce that has introduced alien plants into new countries. Foreign plants may be introduced into a country as cultivated plants which are deliberately imported or as foreign weeds which are accidentally introduced. Cultivated plants include those imported as garden plants, as crops or as fruits or seeds for processing. Weeds may be introduced with almost any product that is capable of harbouring the seeds. This is especially true of seeds or grain for cultivation or animal foodstuffs, which may include impurities. Weed seeds may also be attached to wool, hides or furs; included in packing materials; or adhering to people, their vehicles or livestock. In this way a number of weeds have become so widespread that it is now difficult to determine where they originated. Examples of species which are more or less cosmopolitan are bracken, Pteridium aquilinum, knotgrass or ironweed, Polygonum aviculare, andmeadow grass, Poa annua. The widespread weeds of the temperate regions of the world, such as field convolvulus, C. arvensis, may also grow in mountain areas in the tropics.
When Europeans first emigrated to North America they took with them their weeds of cultivation so that many European cornfield weeds, such as couch grass, Agropyron repens, and groundsel,vulgaris, are now troublesome weeds in USA and Canada. Similarly the weeds of New Zealand are largely European in origin with species such as ragwort, jacobaea, and spear thistle, Cirsium vulgarc, being a particular nuisance. In Africa, where the climate is rather hostile, European plants such as the ribwort plantain, Plantago lauceolata, may occur as weeds in wet places.
Weeds of husbandry
A good illustration of the way in which weeds have been distributed around the world by man is given by the plants which adhere to sheep’s wool. Commercially-useful wool is produced by merino sheep which were first raised in Europe in Spain. During the eighteenth century the king of Spain made gifts of merino sheep to the Dutch governor of South Africa and to George III of England. Sheep were then taken from England to Australia in the eighteenth century and were later crossed with rams from the Cape. The Spanish took sheep to South America, from whence they were taken into the USA.