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what makes a plant a weed

What makes a plant a weed

Some have a very high output of seeds. The books say that a Shepherd’s purse plant can produce 4000 seeds.

You could collect a big plant and count the seeds in one “purse”, count the number of purses on a branch and count the branches on a plant to get an estimate of this number, for yourself.

Some produce seed even in poor conditions. Weeds are able to produce seeds on very small plants. eg In one field a scientist found a poppy with 400 capsules, each with 2000 seeds, yet the smallest plant had one capsule with 4 seeds. When the weeds in one experiment were reduced by 90%, the seed production was only reduced by 10%.

You could survey the seeds produced by the Shepherd’s purse growing round your school.

Some produce seeds when the plants are very young. Most crop plants only produce seeds when the plant is mature. Most weeds start producing seeds when still very young and continue to produce seeds over a long life cycle. Shepherd’s purse produces seeds throughout the year.

How old is a Shepherd’s purse plant when it first produces seed?

Some weeds produce seeds which can live for a long time and keep coming up year after year. After Shepherds purse seeds had been buried for 16 years, still 47% of the seeds were able to grow. Greater plantain seeds were still 84% viable after 21 years. Black Nightshade was 83% viable after 39 years! This means that there are often huge numbers of weed seeds in agricultural land – one scientist counted 30,000 seeds in one square metre of land under cereals. Remember the old saying “One year’s seeding means seven year’s weeding.”

Some seeds remain dormant until conditions linked to crop cultivation are experienced. Poppy seeds require a large fluctuation in temperature from day to night – like the conditions which occur on the soil surface after tilling in spring.

You could collect Wild oat seeds and try to find out what conditionsare required to break their dormancy.

Some weeds have rapid seedling growth and establishment. This gives the weed a head start on the crop.

Some weeds do very well in extreme conditions like the high release of nitrate in autumn, followed by the low nitrate of winter.

Some fruits are well adapted for dispersal so they can colonise new areas – like the “parachutes” of members of the dandelion family.

How slowly does a dandelion seed fall?

How does that compare with a seed which has only half a parachute, or none at all?

Some weeds can grow from small parts of the parent plant. Ask any gardener about the problems they have getting rid of Ground Elder or Couch grass.

How deep can you plant a piece of Couch grass and it can still grow to the surface? (Don’t do this in the Biology Department garden!! Perhaps lengths of drain-pipe would be best to use.)

Some weeds produce plenty of viable seeds even when they are self fertilised. Young dandelion florets first try to catch pollen from a different flower, but if that fails, they use pollen from the same flower to produce a viable seed.

Some weeds are exceptionally good at competing with other plants. You know how well Goose grass (or Cleavers) sticks to clothes when you throw it at a friend – the little hooks on the plant enable it to climb up its neighbours and get more than its fair share of the sunlight.

Anne Bebbington comments:-

  • Many of the plants that we think of as weeds are not too fussy about where they grow. They don’t need special growing conditions.
  • They can often spread rapidly using horizontally growing shoots, underground stems or even growing from broken off bits of plant.
  • They also often produce large numbers of seeds and have very successful ways of spreading their seeds around. This means that wherever there there is a bit of space there is a good chance that a seed will find it.
  • Once the seeds have germinated many weed seedlings show fast strong growth so that they are able to be more successful than the plants growing around them. To help these cultivated plants farmers and gardners have to remove the weeds.
  • The Field Studies Council’s fold-out chart ‘A guide to fruits and seed dispersal’ has lots of information about how fruits and seeds are spread around. You will find more information about this here.

Science & Plants for Schools – free teaching resources for science and plant biology, for primary, secondary and A-level

What makes a plant a weed

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  • Penn State Extension – Introduction to Weeds: What Are Weeds and Why Do We Care?
  • eOrganic – An Ecological Understanding of Weeds

Weed, general term for any plant growing where it is not wanted. Ever since humans first attempted the cultivation of plants, they have had to fight the invasion by weeds into areas chosen for crops. Some unwanted plants later were found to have virtues not originally suspected and so were removed from the category of weeds and taken under cultivation. Other cultivated plants, when transplanted to new climates, escaped cultivation and became weeds or invasive species. The category of weeds thus is ever changing, and the term is a relative one.

Weeds interfere with a variety of human activities, and many methods have been developed to suppress or eliminate them. These methods vary with the nature of the weed itself, the means at hand for disposal, and the relation of the method to the environment. Usually for financial and ecological reasons, methods used on a golf course or a public park cannot be applied on rangeland or in the forest. Herbicide chemicals sprayed on a roadside to eliminate unsightly weeds that constitute a fire or traffic hazard are not proper for use on cropland. Mulching, which is used to suppress weeds in a home garden, is not feasible on large farms. Weed control, in any event, has become a highly specialized activity. Universities and agricultural colleges teach courses in weed control, and industry provides the necessary technology. In agriculture, weed control is essential for maintaining high levels of crop production.

The many reasons for controlling weeds become more complex with the increasing development of technology. Plants become weeds as a function of time and place. Tall weeds on roadsides presumably were not problematic prior to the invention of the automobile. However, with cars and increasing numbers of drivers on roads, tall weeds became dangerous, potentially obscuring drivers’ visibility, particularly at intersections. Sharp-edged grasses are nominal nuisances in a cow pasture; when the area is converted to a golf course or a public park, they become an actual nuisance. Poison oak (Toxicodendron diversilobum) is rather a pleasant shrub on a sunny hillside in the open country; in a camp ground it is a definite health hazard. Such examples could be given ad infinitum to cover every aspect of agriculture, forestry, highway, waterway and public land management, arboretum, park and golf-course care, and home landscape maintenance.

Weeds compete with crop plants for water, light, and nutrients. Weeds of rangelands and pastures may be unpalatable to animals, or even poisonous; they may cause injuries, as with lodging of foxtails (Alopecurus species) in horses’ mouths; they may lower values of animal products, as in the cases of cockleburs (Xanthium species) in wool; they may add to the burden of animal care, as when horses graze in sticky tarweeds (Madia species). Many weeds are hosts of plant disease organisms. Examples are prickly lettuce (Lactuca scariola) and sow thistle (Sonchus species) that serve as hosts for downy mildew; wild mustards (Brassica species) that host clubroot of cabbage; and saltbrush (Atriplex species) and Russian thistle, in which curly top virus overwinters, to be carried to sugar beets by leafhoppers. Many weeds are hosts of insect pests, and a number are invasive species.

Modern weed control can be classified as mechanical, chemical, or biological.

Mechanical control

Mechanical weed control began when humans first pulled weeds from their cereal crops and attempted to grow single plant species, free from all plant competition. That was the start of monoculture, a method that since has come to dominate agriculture. Contrary to the principles of ecology, farmers throughout the world grow the major food, fibre, and forage crops in a monoculture because experience has shown that the highly improved modern crop species give their highest yield under this system.

From hand pulling, humans devised simple tools such as the spud, the knife, and the hoe to eliminate weeds. For thousands of years, from the Egyptian culture to the Renaissance, those simple methods were used. The first efforts to turn away from simple hand methods and mechanize the arduous task of weed control began in 17th-century England. Since then there has been continuous improvement of agricultural tools used to destroy weeds and of cultural methods employed to minimize weed growth. The principal virtue of cultivation of row crops is the control of weeds. Any method of weed control that minimizes tillage tends to conserve soil structure and maintain fertility.

In addition to tillage, other mechanical methods of weed control involve burning, grazing, and the use of ducks or geese in certain crops (in cotton and mint especially). All of those methods have drawbacks: there is the arduous, painful nature of hand weeding; the repetitious and often harmful nature of clean tillage with machinery; the slow, fuel-consuming nature of burning; and the costly requirement of livestock or fowl for the biological grazing methods. Tillage, still the most widely used method of row-crop weed control, has been greatly improved by development of precision seeding and close preadjustment of tiller tools, allowing the passage of weed knives within an inch or less of the young crop plants. Despite these improvements it is known that weed knives injure crop roots, especially late in the tillage season. Additionally, tillage tools can spread perennial weeds rapidly, bringing about rapid infestation of whole fields.

Such methods as crop rotation, use of smother crops, use of weedfree seed, mulching and covering, and cleaning of machinery to prevent spread of weed seeds are also classified as mechanical.

Weed, general term for any plant growing where it is not wanted. Weeds interfere with a variety of human activities, and many methods have been developed to suppress or eliminate weeds. In agriculture, weed control is essential for maintaining high levels of crop production.