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Michael A. Schnelle
Extension Horticulture Specialist
A weed is a plant that interferes with the intended use of an area (any plant out of place). In landscape plantings, weeds pose unwanted competition (water and nutrient) and detract from the appearance of the site. In nurseries, they are troublesome in both container- and field-grown operations.
Managing weeds in a landscape is a challenge. Weeds often occur when desired plants are not thriving due to unfavorable cultural practices. There are several ways in which weeds injure plants or affect land use. Primarily, weeds:
Identification: It is essential to differentiate whether a weed is a broadleaf or a grass. Broadleaf weeds have broad leaves with netted veins, while grasses have long, thin leaves with parallel veins. This difference is extremely important because management procedures are very different. Knowing a weed's life cycle is also important in choosing a control method. Annual weeds germinate, grow, flower, set seed, and die in the same year and are relatively easy to control. Biennial weeds require two years to complete their life cycle. In the first season, the seed germinates and the plant assumes a compact or rosette growth habit. The plant overwinters, and, in the second growing season, the stems elongate, flower, set seed, and die. Perennial weeds are the most difficult weeds to control. They live for two or more years and may be either herbaceous (die to the ground each winter) or woody (have persistent stems).
Collecting Specimens for Identification
If you are unable to identify the weed, find an authority who may be familiar with the weed. If this is not possible, you may want to collect a sample to send to your Cooperative Extension agent. No matter how much experience you have had, some problems will require further investigation, and you will want to collect specimens. Directions for sampling and shipping are also available from your Extension agent.
Susceptible Stages of Weeds
Most plants produce viable seed, whether they are annuals, biennials, or perennials. One plant may deposit thousands of seeds on the soil surface. Freezing and thawing buries these seeds in the soil, where many remain viable for years. Managing weed populations before they produce seed is an important step in a comprehensive weed control program.
Weeds are most susceptible to herbicide treatments when they are seedlings. Emerging seedlings can be controlled with soil-applied herbicides, and small emerged weeds can be controlled with foliar herbicides. The meristem of many emerged seedling grasses is protected below ground from contact herbicide treatment or mowing, while most emerged broadleaf weeds have exposed meristems that are vulnerable to properly applied mechanical or herbicide treatments. Annuals, biennials, and perennials (woody or herbaceous) all have about the same susceptibility during germination. After germination, the life cycle of the plant is crucial to the timing of herbicide applications.
Whether a seedling is a broadleaf or narrowleaf plant also affects its susceptibility. Annual weeds become progressively more resistant to herbicides as they approach maturity; treatments should be made as early as possible. Summer annuals flower from middle to late summer, and winter annuals flower in early spring. Herbicides should be applied to annual weeds before they flower and produce seeds.
Biennial weeds that are past the seedling stage are most susceptible to postemergence herbicide treatments when they are in the rosette stage, either before frost the first year or early in the second year before the plant puts up a flower stalk (bolts) to complete its life cycle.
Most perennial weeds perpetuate themselves from seed, but many also reproduce vegetatively by means of rhizomes, bulbs, tubers, stolons, or runners. These organs, sometimes called propagules, have buds that produce new shoots. Propagules also store carbohydrates to supply energy to the shoots. These nutrients move from the storage organs to the new shoots until leaves develop sufficiently to intercept sunlight and begin producing food. Carbohydrates that are not needed for the growth and development of plant shoots, buds, flowers, and fruits are translocated to the storage organs for later use.
Whether herbaceous or woody, perennials are most readily controlled as seedlings, while they are still succulent and before propagules form. Once the propagules have developed (as early as four weeks after germination for some herbaceous perennials), the plant will usually survive even if the above ground portion is destroyed by mowing, cutting, freezing, or contact herbicide application. New shoots develop from untreated or uninjured buds using the food that is stored in the propagules.
If perennials are to be controlled after the seedling stage, the food reserves must be depleted, the storage organs must be destroyed by freezing, desiccation, ortillage, or a herbicide that interferes with plant growth must be applied. This situation occurs in woody perennials after the leaves have fully expanded in late spring and in herbaceous perennials when plants are in the bud-to-bloom stage. Easy-to-control perennial weeds can be eliminated by repeated cultivation to deplete the food reserves of the propagules. However, in most cases, cutting up propagules such as rhizomes, tillers, or roots helps the plants multiply. Control of top growth with contact herbicides or mowingwill not kill most perennial weeds.
Treating perennials with a translocated systemic herbicide may be the most efficient method of controlling these plants.
Weed Management Programs
Efficient and successful weed management programs must use an integrated approach. Non-herbicide control methods include proper production and maintenance, hand or mechanical elimination of weeds (pulling, hoeing, rototilling, and disking), and mulching. An integrated approach can sometimes eliminate the need for herbicide applications. The best weed control programs usually integrate all methods.
Non-herbicide Management
Many cultural practices used in growing woody plants are methods of managing weeds. Herbicide control is impossible for some plants because the appropriate herbicides are not registered, cannot be used selectively where many plant species are grown together, or are injurious to the species. In these cases, the grower or landscape manager must depend on non-herbicide weed control practices. This is a common occurrence for specialty ornamentals.
If you can choose your planting area, select a site that is free of perennial weeds, such as nutsedge, bermudagrass, quackgrass, and field bindweed. Growers of field-grown ornamentals should look for land that is free of perennial weeds.
Sometimes you can eliminate perennial weeds by growing other crops for one or more growing seasons. Cultivating (removing weeds mechanically), whether by rototilling, disking, hoeing, weeding by hand, or digging with a shovel, is a very effective way of controlling young weeds. The best control is achieved if the weeds are small (in the two- to four-leaf stage) and if the soil surface is allowed to dry after cultivation.
Proper management encourages desirable plants and discourages weeds. It involves preparing the seedbed properly, planting desirable plants at the right time, using transplants rather than seed, using adapted species, and applying the proper amount of water and fertilizer at the correct time. Insect and disease management also must be practiced to maintain tightly knit, vigorously growing plants. If the soil is allowed to become bare and exposed to the sun through improper management, weeds will germinate and grow. But, if desirable species are properly managed, they can shade or crowd out many weed species. Most annual weeds will not become established in well-managed ornamental ground cover plantings.
Mulches also help control weeds because they prevent sunlight from reaching weed seeds. They also keep the soil surface cool, reduce loss of soil moisture through evaporation, and prevent large fluctuations in soil temperature. Refer toTable 7-1 for a partial list of acceptable mulch materials. Bark chips, compost, and black plastic are currently very common materials. Even better than using black plastic is using the new fabric weed barriers. They keep out weeds, but allow for better gaseous and water exchange than does black plastic.
Mulches normally are put down after the plants are in place. If plastic is put under organic mulches, it should be punctured to allow for air and water drainage; if plastic is not used, the organic mulch should be at least three to five inches deep. Organic mulches are especially helpful under long-season annual or perennial crops or under woody trees and shrubs. When using organic mulches, it may be necessary to apply a nitrogen-containing fertilizer late in the season, since materials high in organic content may greatly reduce the nitrogen available to plants. Organic mulches also increase soil filth over time by breaking down and mixing into the soil surface.
To reduce the quantity of weed seeds in the soil, you must cultivate or mow frequently enough to prevent weeds from producing seed. Studies have shown that if seed production is stopped, 20 to 30 percent of the weed population in the soil can be removed each year. If weeds have been allowed to seed for many years, however, it takes two to three years to realize a significant reduction in new germination.
Cultivation is one way to keep weeds from producing seed. Shallow cultivation is the best method, since deep cultivation, such as spading, can bring buried seed to the surface, where it may germinate. Deep cultivation can also damage roots. Remember, the roots of a tree or shrub that function in water and nutrient uptake are located in the top few inches of the soil (deeper roots function mainly in keeping the plant stable and erect). When cultivating the soil, be careful not to introduce weeds by adding weed-infested topsoil, compost, or manure, or by using purchased plant materials that contain weed seeds.
Mowing and cutting also are excellent methods of reducing weeds, especially when they are three or more inches tall. Mow or cut after the soil surface is dry but before weeds set seed. Although mowing seldom eliminates weeds, it discourages tall weeds and encourages shorter weeds (mowing often causes a shift in population to low-growing species that can tolerate mowing).
Pesticide Management
Herbicides can be used to eliminate or reduce weed populations. However, if they are used, they should be integrated with non-herbicide practices whenever possible. The efficacy of herbicide control depends upon what type of herbicide is used, where it is placed, when it is applied, and what environmental conditions are prevailing at the time of application.
Herbicides can be classified into three categories, depending on when they are used. Preplant herbicides are used before planting, preemergence herbicides are used after planting but before weed seedlings emerge, and postemergence herbicides are used after planting and after weeds have emerged.
Preplant herbicides can be fumigants applied to or injected into the soil. Fumigants are toxic to all living plant tissue and should not be used near roots of plants that will remain in the landscape. Rototill and prepare the soil for planting before applying fumigants. Cover the area to be fumigated with a plastic tarp to keep fumes in the soil. After removing the tarp from the fumigated area, allow one week (and heed label recommendations) for the soil to be completely aerated before planting. Repeated tilling speeds up aeration.
Fumigants are extremely toxic to humans and should be handled with great care. Fumigation is common in production nurseries, but it is seldom used in landscape beds because of the toxicity problems.
Preemergent herbicides are applied to the soil and are active on germinating seedlings. These are the herbicides most commonly used on ornamentals. Selective preemergent herbicides kill or injure only certain plant species. They are used in plantings of woody ornamentals, ground covers, bedding plants, lawns, and vegetable gardens.
A preemergent herbicide may or may not need to be incorporated into the soil to be effective. If it is the kind that requires incorporation, it should be applied as a preplant herbicide. Incorporation may be achieved mechanically by rototilling, disking, or hoeing. It can also be achieved by irrigation, in which case the water movement through the soil profile carriesthe herbicide downward.
Avoid using nonselective, preemergent herbicides near trees, shrubs, or lawns. Valuable landscape plantings can be lost through careless use of these herbicides. Postemergent herbicides include contact and systemic types. Contact herbicides kill the green parts of the plant that they touch. Some herbicides, such as paraquat and glyphosate, can also damage trees whose bark is thin or green. Quite often, plants treated with contact herbicides produce new sprouts from the roots. Therefore, if you want to completely eliminate a weed, contact herbicides may not give long-lasting results. Systemic herbicides enter one part of the plant and are translocated throughout the plant, killing the entire organism. These herbicides may enter the foliage, stems, bark, or roots of the plant.
Whenever you use herbicides, follow label directions. The label is the best source of information concerning the product. If you need further information, contact your County Cooperative Extension agent, nursery worker, or the manufacturer.
An integrated pest management approach to weed control employs the use of common sense cultural practices with the use of chemical controls whenever necessary. High profile locations often must be at an aesthetic level of zero tolerance for weeds. In such cases, herbicides will undoubtedly be necessary to realize a completely weed-free area without the use of costly hand labor.