Understanding Weeds to Understand Weed Control
Correct identification of the problem weed gives access to information about its life cycle. Weeds fit into three fairly distinct categories — grasses, sedges or broad leaves. Based on their life cycles, weeds are of three types — annual, biennial or perennial. In most cases, weeds may exhibit two or more life habits.
Annuals grow from seed, mature and produce seed for the next generation in one year or less. Annuals are best controlled when they are very small seedlings, but at all costs, should be removed before they set seed.
Biennials require two years to complete their life cycle. These plants grow from seed that germinates in the spring and develops heavy roots and compact rosettes or clusters of leaves the first summer. Biennials remain dormant through the winter, then in the second summer they mature, produce seed and die.
Perennial plants live more than two years and may live indefinitely.
Annuals and biennials should be controlled early in the growing season, not only to destroy plants of the current generation, but to prevent seed formation for the next.
Perennial plants may grow from seed, but many also produce storage structures such as bulbs, tubers, rhizomes (underground stems) or stolons (above-ground stems) from which plants can develop. Seed is the primary method of introducing these weeds to new areas; however, perennial weeds are often spread during soil preparation and cultivation.
Weed control practices on perennials must deal with below-ground structures as well as the above-ground portions of the plant.
In perennial weeds, the period when root reserves have been maximally depleted and carbohydrates are beginning to move back down to form new underground structures occurs when the plant has reached one-fourth of its height or is at the early flower bud stage. These are the ideal time for control measures, such as clean cultivation, close mowing or foliar applied herbicides.
Source: Erhardt, W., Weeds and Weed Control; Commercial Vegetable News, Special Newsletter No. 9, April 1992.