Weeds can be controlled by using physical methods (i.e., cultivation, mowing, mulching, hand-pulling, selecting a weed-free growing medium, and planting cover crops) or chemicals (fumigants and herbicides). Because of the tremendous variation in times and frequency of seed germination, reproductive and survival structures, growth habits, and growth rates of weeds, all of these methods will be needed at one time or another.
Cultivation is a common method of controlling weeds in nursery and landscape plantings. Tractor-mounted cultivators and rototillers or hand-held hoes and walk-behind rototillers may be used. Though cultivation is an effective way of controlling weeds, several problems are associated with it. Cultivating is time consuming because to maintain a satisfactory level of control, it must be repeated throughout the season. Weeds must be cultivated before they grow too large, and immediately after a cultivation, additional weed seeds may germinate. Moreover, cultivation may break apart underground structures of perennial weeds, such as roots (mugwort), rhizomes (quackgrass), or clusters of nutlets (nutsedge), causing weeds to propagate.
Cultivation also can result in the loss of soil structure. Repeated passes over the field with a tractor-mounted cultivator can cause soil compaction. Repeated tillage with a light rototiller or just one pass with a heavy rototiller can destroy the structure of the upper part of the soil. Cultivation should be done with light equipment and frequently enough to prevent the development of weeds larger than 12 inches.
When cultivating, equipment operators must avoid damaging the roots and trunks of ornamental plants, especially when using a tractor-mounted cultivator or rototiller. The rotary cultivator has been developed for cultivating between rows of trees. It is mounted on the three-point hitch of a tractor and connected to the power takeoff system (PTO). It extends from the side of the tractor and has a retractor bar that leads the way. Each time the bar hits a tree, it gently guides the rotary hoe around the tree. Specialized cultivators and weed "brushes" are available for use in seed and liner beds and in other situations involving narrow row spaces.
Despite the drawbacks of cultivation, it is a useful way to control weeds in herbicide-sensitive crops or clean up a field prior to applying a preemergence herbicide. Cultivation is commonly used in landscape plantings because the large variety of plants growing in a small area limits the use of herbicides.
Mowing is commonly used to control weeds in fields of nursery stock or landscapes. However, this method is time consuming, provides only short-term control of weeds, and must be repeated throughout the growing season. Damage to crop plants may occur when equipment operators run into them.
Mulches may be applied to limit weed growth around high-value crops in a nursery or in landscape plantings. Mulching can effectively control weeds from seeds that germinate at or near the soil surface. Organic mulches, such as wood chips, shredded bark, or other plant residues, can be applied to weed-free soil soon after planting. To be most effective, they should be applied at least 2 inches, but not more than 4 inches, deep. A thicker layer of mulch may be harmful to landscape ornamentals and may provide habitat in which weed seeds may germinate and survive entirely within the mulch layer. It is important to avoid covering the stems and leaves of plants. Perennial weeds growing from underground structures are capable of growing through organic mulches. They must be controlled with postemergence, translocated herbicides or must be pulled repeatedly. Using a preemergence herbicide in combination with organic mulches results in much improved weed control. The section on preemergence herbicides provides information on which herbicides should be placed under mulches and which should be placed on top.
Inorganic mulches include plastics, gravel, and on a very limited scale, ground rubber tires. The primary inorganic mulches used are black polyethylene film and many types of geotextile weed barriers that allow air and water exchange through holes punched or woven into the fabric. Only black films should be used. Clear or white films allow weeds to grow under them. Black film increases soil temperature and, like the other mulches, maintains high moisture levels at the surface of the soil.
Solid black plastic has been used for years to prevent weed growth in fields and landscapes. The promoters of the weed barriers claim that solid black plastic is harmful to plants because it does not allow for air and water exchange. In theory this is true, but no one has been able to document any harmful effects. After several years of trying the newer weed barriers, many landscapers are returning to the use of solid black plastic. It seems some weeds can grow through weed barriers, while others take root in the organic mulches that are normally placed on top of them. The roots then penetrate the holes in the weed barrier and become extremely difficult to remove. The recommendation now is to use solid black plastic under a mulch on relatively flat surfaces. On slopes, where mulches would slide off solid black plastic, use a weed barrier that has some texture to hold the mulch in place.
Gravel and ground tires should always be used on top of plastic so that they will not blend with the soil during freeze/thaw cycles.
Hand pulling of weeds is a method that everyone involved in weed control would like to see become obsolete, but it probably never will. Total weed control is difficult to achieve but is necessary in certain high-value areas such as container-grown nursery stock, seedbeds, or highly visible landscape sites.
Hand pulling is used to eliminate weeds that escape preventive measures.
By selecting a growing medium that is naturally and consistently weed-free, growers can avoid many weed problems in container-grown plants. Vermiculite and perlite are weed-free because of the way in which they are produced. Most peat contains very few, if any, weed seeds. Organic matter that has been properly composted will be weed-free because the high temperatures reached during composting kill weeds and their seeds. Composted or aged bark, sawdust, leaves, or sewage sludge may be used.
Steam pasteurization is an effective way of controlling weeds and most soil-borne insects and pathogens. Media should be heated to between 160 degrees and 180 degrees F and held for 30 minutes. Steamed media should be covered or used within about a week because it can be reinfested while stored. Portable steam generators are available for steaming outdoor beds.
Controlling weed growth around growing areas and preventing weeds from going to seed limit the potential for reinfestation of cleared areas. These tasks require time and attention to matters that seem to have little short-term impact but, in the long run, are very worthwhile.
It is possible to achieve almost total weed control in fields through cultivation and the proper selection and use of herbicides. However, clean cultivation and the broadcast application of herbicides can create many aesthetic, environmental, and soil and crop management problems. Soil erosion, pesticide and fertilizer runoff, and impassable fields after rainfall are all problems associated with total weed control. These problems can be overcome with a minimum output of labor and money by properly using a controlled vegetative cover.
Cover crops should not be grown within fields with less than 5 feet between rows, because any established cover crop will be too competitive with the nursery stock. In this situation, ornamentals should be grown in blocks surrounded by strips or roads of cover crop. The distance between strips will be determined by the slope of the site. The more slope there is, the closer together the strips of cover must be.
Cover crops can be grown in plantings with more than 5 feet between rows. A vegetation-free strip 2 to 5 feet wide should be maintained in the row of ornamentals, with the cover crop established in the remaining space between rows. In shade trees, the strip of cover may be 8 feet wide, while in smaller trees or shrubs, the cover may be only 2 feet wide. While 2 feet of cover is not enough to operate equipment on, it is enough to stop erosion. The cover must be kept away from crop plants with herbicides for the same reasons weeds must be controlled.
Allowing volunteer vegetation to grow between tree rows is simple and inexpensive in the short run but has several disadvantages. Volunteer vegetation is composed of annual and perennial weeds that require frequent mowing during the growing season. Many annual weeds die with the first frost and fail to prevent soil erosion in late fall and spring. Perennial weeds that spread by roots, rhizomes, or vine-like stems may invade the tree rows.
Planting a permanent cover crop between tree rows can reduce mowing and minimize weed problems. Cover crops control erosion, limit runoff of pesticides and fertilizer into streams and groundwater, and reduce the amount of herbicide that needs to be applied to a field. If the recommendation for a chemical is 4 quarts per acre and 36-inch strips are sprayed in rows located 72 inches apart, only one-half of the total ground area will be covered, so only 2 quarts of the chemical will be required per acre of nursery stock. The sod is easy to mow because the herbicide does the trimming around the trees. Sod provides a firm, uniform working surface year-round for equipment and laborers. Equipment as well as most nursery practices can be used under wet conditions if there is sod in the field. The sod also helps protect and build soil by limiting compaction and adding organic matter to the soil.
For years tree fruit growers and some nurserymen have planted perennial grass cover crops between tree rows. Orchardgrass, Kentucky 31 tall fescue, and pasture-type perennial ryegrass are most commonly used and provide many benefits. However, orchardgrass, Kentucky 31 tall fescue, and pasture-type perennial ryegrass all have a rapid growth rate and require frequent mowing during the growing season.
A low maintenance cover should grow slowly or low for minimum maintenance, have a low fertility requirement, tolerate a variety of soil conditions, not limit crop growth, and be aesthetically attractive. The fine fescues have these characteristics. Unlike tall fescue, which has a relatively broad, flat leaf, the fine fescues have leaves that are tightly folded and narrow. Though similar in many ways, there are several distinctions between fine fescue species.
The best fine fescues to use as cover crops are hard fescue and red fescue. Red fescue establishes faster than hard fescue and forms a denser sod because it produces short rhizomes. Hard fescue grows lower than red fescue.
Many fine fescue cultivars are now available with endophytes. Endophytes are beneficial fungi that live within the grass plant and deter insect feeding. Choose an endophyte enhanced variety if available.
The fine fescues can be weedy during establishment because they have small seedlings and slow (hard and sheep) to medium (red and chewings) lateral spread. Establishment is especially poor during hot summer months. The best time to establish fine fescues is in late summer to early fall or in early spring. Apply 20 to 40 pounds of seed per acre to soil that has been loosened by rototilling or cultivation. Adjusting soil pH and fertility levels according to soil test results will speed establishment of the cover crop.
Because fine fescues do not hold up to heavy traffic as well as tall fescue, tall fescue should be used for grass roadways in and around a nursery.
Chemicals used to control weeds are classified in several ways: