Weed - biology.

Weed - biology. I INTRODUCTION Weed, term applied to any plant that grows where it is not wanted. A weed is usually characterized by rapid growth, and it typically replaces other, more desirable plants. Some plants, such as crabgrass, are considered weeds everywhere they grow, but many plants are considered weeds in some regions and not in others. For example, the shrub lantana is raised as an ornamental plant in many parts of the United States, but has become a serious pest in Hawaii. Although most weeds damage cultivated plants by competing with them for sunlight, water, and mineral nutrients, some weeds are parasites that grow directly on other plants, and thus either weaken or kill them. Many weeds are also hosts for disease-causing organisms. For example, some of the fungal diseases that infect food crops spend part of their life cycle on a weed that typically grows near the crop. In addition to harming cultivated plants, many weeds, such as Canada thistle, can poison livestock if eaten. Although livestock poisoning is not a serious problem in the eastern United States, the western states, having many poisonous species, report a considerable loss each year. For example, the wild onion, although not poisonous, spoils the flavor of milk produced by cows that consume this weed. Although generally harmful or undesirable, weeds can also provide benefits for agriculture. Many weeds do serve as safe and nutritious forage for grazing animals. Weeds prevent or retard soil erosion in open pastures and stabilize the thin, fragile soils of tropical farmlands. They conserve water in semi-arid and Mediterranean climates by enhancing water storage, increasing shade, and breaking up soil with their root growth. This prevents a condition called hard cap, in which rain puddles that set on sun-baked soil rapidly evaporate without ever penetrating the ground. Some weeds may provide a source of food. The tender shoots of weeds such as dock, pokeweeds, field mustard, and dandelion are considered as tasty and nutritious as garden greens. Archaeologists have uncovered evidence that prehistoric Native Americans cultivated weeds, such as knotweed and little barley, alongside traditional crops, such as corn, beans, and squash. In addition to their agricultural uses, many weeds, such as tansy, chicory, and smartweed, have medicinal properties and are used extensively in homeopathic and naturopathic medicine. II TYPES OF WEEDS Weeds, like many plants, are usually divided into three categories: annuals, biennials, and perennials. This division is based on how long a single generation lives. Annuals complete an entire life cycle in one growing season. They germinate, flower, and fruit between the spring and winter of the same year. The seeds, however, may live buried in the soil as long as 70 years. Repeated plowing and cultivating may be necessary to control annual weeds. Lamb's-quarters and chickweed are common annual weeds. Biennials live through two growing seasons, producing only greenery the first year, and bearing flowers and fruits in the second. Biennial plants usually have strong root systems that send up new shoots when the original tops are removed. The bull thistle is an example of a biennial weed. Perennial weeds are often the most troublesome weeds to control because they have strong, well-developed underground parts and produce large numbers of seeds. Perennials live for at least three years, flowering and producing fruit each year after reaching maturity. Among the most widespread perennial weeds are bindweed, sorrel, and leafy spurge. III METHODS OF CONTROL There are various methods of weed control, ranging from simple pulling-by-hand to elaborate chemical weed killers. Generally, more than one method is necessary because of the resilience of the weed plant or its seeds. Pulling weeds by hand is effective, but it should be done only when the soil is moist so that the underground roots are removed along with the tops. Although tiresome and time-consuming, hand weeding is the most effective control for weeds growing in rock gardens or between rows in vegetable gardens. Removing weeds using a hoe is effective, but only when weeds are not well established and the weather is dry. In moist weather many weeds revive after hoeing and continue growing. Repeated hoeing at regular intervals is the best direct approach for hoe weeding the home garden in moist conditions. Another effective method of weed control is mulching, which consists of applying a layer of leaves, straw, or other organic mulching material over the weeds. Mulching prevents weeds from germinating and growing and adds nutrients to the soil. For shallow-rooted plants, such as azaleas, blueberries, and rhododendrons, mulching is necessary because hoeing injures their root systems. Chemical weed killers, or herbicides, are usually divided into three major groups based on their uses. One group, known as soil sterilants, consists of chemicals that sterilize the soil and clear the ground of all plant growth. These chemicals are often used in such locations as footpaths and tennis courts, and may be effective for weeks or even months. The second group of chemical weed killers, called selective weed killers, is especially effective for controlling broad-leaved lawn weeds, such as dandelions, plantains, and sorrels. The third group of chemicals, nonselective weed killers, are chemicals that kill all kinds of weeds but do not sterilize the soil. For example, poison ivy can be eliminated from an area with a nonselective weed killer, and immediately afterward desirable plants may be grown in the same soil. Although these chemicals are effective and may boost agricultural production, they pose risks to workers that use them and are a significant source of water and soil pollution. Alternative approaches to weed control, such as altering plowing and cultivation practices, are being encouraged by environmental organizations to minimize the use of herbicides in crop farming, particularly in developing countries that use large quantities of the chemicals. No matter what type of weed killer, it is extremely important to closely follow the manufacturer's instructions, especially with respect to dilution, time of application, and evenness of distribution. Even with these precautions, weed killers may be dangerous, and their use is restricted by federal, state, and local regulations. Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.