weed(redirected from Weed Science)
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See T. J. Muzik, Weed Biology and Control (1970); R. E. Wilkinson and H. E. Jaques, How to Know the Weeds (2d ed. 1973).
a plant whose growth in specific areas is undesirable because it worsens conditions for the growth of crops, decreases the yield and quality of crops, or is useless or poisonous. The term “weed” is a relative one. For example, the sunflower is an extremely valuable oil crop, but when it sheds its seeds in a standing crop and the seeds sprout the following spring, it may be considered a weed (a contaminant) in relation to the other plants (usually winter wheat) that were sown later. Weeds damage plantings (fruit and berry plantings), shelterbelts and afforested areas, meadows, roads, streets, drainage and irrigation canals, rivers, lakes, reservoirs and their shores, stadiums, and right-of-ways for electric power transmission lines, gas pipelines, and petroleum pipelines. They are distributed over the entire globe. Of the several thousand known species, approximately 1,500 occur in the USSR.
Damage from weeds is diverse. Weeds choke crops by absorbing large quantities of water and nutrient matter from the soil, by discharging harmful substances from their roots into the soil, and by depriving other plants of light. All this decreases the crop yield or, in many cases, leads to destruction of the crop. Climbing weeds, such as field bindweed and black bindweed, cause lodging of agricultural crops, which makes harvesting difficult and leads to severe crop losses. Tall-stemmed and succulent weeds (sow thistles, thistles, and goosefoots) clog the working elements of harvesting machines, thus lowering productivity. The harvested material contains succulent parts of weeds, which significantly increase the moisture content and temperature of the grain and seeds and lower the quality. Grain contaminated with seeds of poisonous weeds, such as Heliotropium lasiocarpum, may poison humans and farm animals. Weeds in hayfields and pastures crowd out valuable grasses, thus lowering the yields and their nutritional value. Animals whose feed contains poisonous weeds may become intoxicated. The primary foci of reproduction of many crop pests and diseases are often weeds. Weeds lower the water-carrying capacity of irrigation canals and promote silting. Weeds on construction sites and rights-of-way for petroleum and gas pipelines increase fire hazards.
Weeds are classified according to their reproduction, dispersal, and regeneration; their place of growth and specialization; and their taxonomic position. Weeds are divided into the following three biological types according to their reproduction, dispersal, and regeneration: seed annuals and biennials, vegetative annuals and biennials, and perennials.
Seed annuals and biennials reproduce, disperse, and regenerate by means of seeds, fruits, and aggregate fruits or seed lobes; they bear fruit once in a lifetime and then die.
Annual weeds—those weeds that live for one year—are divided into four subgroups: early spring, late spring, winter, and wintering. Early spring annuals (wild oat, Cannabis ruderalis, goosefoot, Atriplex, ryegrass) contaminate early spring crops, such as wheat and barley, especially heavily. Late spring annuals, which include bristle grass, barnyard grass, and amaranth, are specialized contaminants of millet and other late spring crops. Many of the early and, particularly, late spring annuals, including Atriplex, goosefoot, and amaranth, are also post-harvest weeds; that is, they proliferate after the harvesting of crops, producing a harvest of seeds. Winter annuals (annual bromes) most often contaminate winter crops; they readily live through the winter and bear fruit the following year. In contrast to winter types, wintering annuals (shepherd’s purse, field pennycress) can grow and bear fruit normally even when shoots are formed in the spring.
In biennial weeds (melilot, Onorpodum and many other composites), the vegetative organs are formed from the seeds in the first year, and the plants bear fruit and die in the second year.
Parasitic and hemiparasitic weeds form a category somewhat apart from the above types. Parasitic weeds attach themselves by suction to stems (dodder on clover, medic, and many other plants) or to roots (Orobanche on medic, tobacco, and tomatoes). Hemiparasitic weeds include Rhinanthus vernalis, which attacks rye, and Odontites serótina, which attacks cultivated and wild grasses.
Vegetative annuals and biennials reproduce, disperse, and regenerate by seeds and vegetatively. The lifespan of each plant usually does not exceed two years. These plants are categorized as tuberous (Lathyrus tuberosus, marsh woundwort) and bulbous (perennial violets, Sisymbrium polymorphum). There are winter, wintering, spring, and biennial subgroups.
Perennial weeds have long-lived underground organs; each year they form shoots, which exist for one to two years and die off after bearing fruit. There are four groups of perennial weeds: taproot weeds (absinthe, Rumex confertus), caespitose weeds (certain sedges and grasses), sucker-producing weeds (creeping thistle, field bindweed, field sow thistle), and rhizome weeds, including shallow-rooted types (common quack grass, Leymus, Hierochloë) and deep-rooted types (horsetail, licorice, Sophora). Closely related to the rhizome group is an independent group of weeds with sprouts and tendrils that spread and root above ground (white clover and creeping crowfoot).
Weeds are very fertile. A knowledge of their reproduction, dispersal, and regeneration makes it possible to differentiate various measures for eliminating each biological type and group.
Weeds are classified according to their place of growth and specialization as (1) segetal weeds (growing in fields of grain), (2) weeds of natural lands, (3) ruderals (growing in debris, wastelands, and overgrown areas), and (4) weeds of special areas.
Segetal weeds may contaminate winter crops, early spring and vegetable crops, late spring crops, perennial herbs, fallow and uncultivated lands (long fallow, boundaries, and field paths), or fruit and berry crops and parks. In each of the subgroups there are specialized weeds that usually grow only in plantings of a single crop or even a single variety; examples are darnel (which grows in spring wheat), Fagopyrum tataricum Gaerth and black bindweed (in buckwheat), field pea (in peas), and a special type of wild oat (in the Pobeda variety of oat).
Weeds of natural lands and special areas include the following subgroups: hay and pasture weeds (poisonous, harmful, unproductive, barely edible, and inedible); forest weeds; weeds of areas with disrupted natural herbaceous covering; river, water, and shore weeds; and weeds of special areas, such as airfields and sports fields. Ruderal weeds grow in places where refuse is thrown, on wastelands, near dwellings, and along roads. Some are poisonous (Hyoscyamus), others are thorny (cocklebur), and still others are stinging (nettle). Weeds of special areas are difficult to discover in crops or to distinguish from seed material because they resemble the crops they contaminate in seed size and external appearance. Quarantine weeds are a separate category.
Classification according to taxonomic position has great significance in the chemical control of weeds. The differences between monocotyledonous and dicotyledonous weeds make it possible to destroy dicotyledonous weeds in plantings of monocotyledonous crops and to destroy monocotyledonous weeds in dicotyledonous crops.
The principal weed-control measures are directed against the most harmful characteristic weeds contaminating a given area. Other weeds may also be dangerous species, but they are found in comparatively small numbers; measures are taken to prevent their proliferation. There are three principal, or simple, types of weed contamination: (1) by weeds with root suckers, (2) by weeds with rhizomes, and (3) by annuals and biennials. There are also four compound types, in which the characteristic species of weeds are represented by two or three of the above groups. The degree of weed contamination may be slight (single weeds), moderate (up to one-fourth of the herbage of crops), severe (approximately the same amount of weeds as crops), or very severe (weeds clearly predominate). A system of weed control that takes into account the type of contamination is worked out for each field and plot. Weed control involves the use of sound land-cultivation methods (crop rotation and proper timing of plowing, planting, and harvesting), the use of chemical herbicides, and the implementation of measures to control the purity of seed material and to protect against quarantine weeds.
REFERENCESSornye rasteniia SSSR, vols. 1–4. Leningrad, 1934–35.
Kazakevich, L. I., and B. M. Smirnov. Kak ochistit’ polia ot sorniakov. Saratov, 1950.
Dobrokhotov, V. N. Semena sornykh rastenii. Moscow, 1961.
Mal’tsev, A. I. Sornaia rastitel’nost’ SSSR i mery bor’by s nei, 4th ed. Moscow-Leningrad, 1962.
Kott, S. A. Sornye rasteniia i bor’ba s nimi. Moscow, 1969.
Smirnov, B. M. Bor’ba s sorniakami v Povolzh’e. Saratov, 1975.
B. M. SMIRNOV