Weed management should be an integral part of any turf or landscape management plan. It should begin before planting to provide the best growth conditions for seedlings and transplants and continue throughout the life of the planting. Obviously, though, weed management is going to be very different in different sites in terms of weed problems, management objectives, and control options.

Effective weed management doesn’t happen by chance. It requires that you:

• Have a thorough knowledge of the weed problem,
• Prepare a well-designed plan using all appropriate weed control strategies to deal with that problem,
• Implement the plan appropriately, and
• Evaluate the effectiveness of the plan and adjust it accordingly.

Managers who use this approach to weed management are going to have the best success.

The Weed Problem

For many people, there’s little interest in identifying weeds, but a lot of interest in controlling them. However, effective weed control must begin with accurate weed identification and some assessment of the magnitude of the problem. Your county Extension agent can help in identifying weeds. Once you’ve accurately assessed a weed problem, you can select from a variety of effective control options. Knowing whether you are dealing with an annual, biennial, or perennial weed is essential in determining which control methods will be most effective.

Annual Weeds

Annual weeds are generally the easiest to control. However, they are abundant and are prolific seed producers. For example, a single redroot pigweed plant can produce over 100,000 seeds. While many seeds germinate the following year, some remain dormant in the soil for many years. Thus, seeds that were produced many years before can maintain an annual weed threat.

Generally, infestations of annual weeds are comprised of 2 to 5 species, with 1 or 2 species predominating. Do not focus control efforts solely on the dominant species, however. The less‑dominant weeds may thrive in the absence of competing weed species.

Biennial Weeds

Infestations of biennial weeds are comprised of both first- and second-year plants. First-year plants, which are usually easier to control, are often inconspicuous and so frequently escape treatment. Second-year plants, though more noticeable, are often much less susceptible to herbicide treatment. Musk and plumeless thistles, for example, are quite easily controlled with 2,4-D in the rosette stage but become quite tolerant to the herbicide after their flower stalks begin to elongate. Mowing to remove the seed-bearing flower stalks gradually reduces the future biennial weed
population.

Perennial Weeds

Perennial weeds are usually the most persistent and difficult to control. Perennial weeds occur as relatively isolated plants, in patches, or can be uniform and dense across an extensive area. They are a problem in turf and in both annual and perennial ornamental plantings.

A few relatively isolated perennial weeds often fail to cause much concern. But they spread rapidly — a single, healthy quackgrass rhizome can produce over 400 feet of rhizomes in less than a year — so it’s important to make maps, noting areas infested with particularly troublesome perennial weeds. Designate infested areas for specific perennial weed control treatment.

Mixed-Weed Infestations

Often, weed problems do not fall into convenient categories. A weed infestation may be comprised of annual, biennial, and perennial weeds. Selection of the most effective management program depends on accurate identification and noting the relative incidence of problem weeds at a particular site. Depending on the types of weeds present, you may have to use several different control methods, perhaps applied at different times of the year, to achieve adequate control.

Predicting A Weed Proglem

Weeds that are favored by management practices tend to pose the most serious problems. Obviously, such practices are chosen to meet the growth requirements of the desirable plants. However, weeds that have growth requirements and growth habits similar to those plants will be favored as well and are likely to emerge. For example, grasses have low growing points and are not controlled by mowing; thus, grass weeds can be major problems in turf. Perennial plantings will often have problems with winter annuals, whereas spring-seeded plantings will have infestations
of summer annuals.

By recognizing these relationships and observing what is happening at a site already, you can predict future weed problems. This will allow you to assess what level of control is necessary and to set up a weed management plan even before planting or the next growing season.

Some weed problems can be anticipated and largely avoided. When establishing turf, for example, the most effective measure for controlling quackgrass precedes sodding or seeding. If the site contains quackgrass which can spread rapidly, apply a herbicide to kill the weed before tilling the soil. There is no way to kill quackgrass selectively after turf is established.

Weed Maps. A valuable tool for controlling weeds is a weed map. Normally, seeds produced by annual weeds during the previous year will have the greatest impact during the current growing season, although dormant seeds may create problems in later years. If you have noted on your weed map where annual weeds have gone to seed in previous years, you can anticipate where problems are likely to occur. Using a preemergence herbicide on these potential trouble spots, for example, could head off some problems (e.g., crabgrass in lawns). Likewise, mapping perennial weeds will help you plan your control strategy for these recurring weeds.

Weeds as Indicators. Many weeds, including dandelions and quackgrass, are distributed very widely over a broad range of environmental conditions. Other weeds, though are very specific in their site requirements and, therefore, provide clues about environmental conditions at a particular site. In other words, weeds are often the result, rather than the cause, of the poor performance of ornamental plants. Consider the following examples:

• Annual bluegrass grows well in compacted, wet soils, and/or moderately shaded, wet soils.
• Knotweed and pineapple weed are found in compacted soils, such as on athletic fields. These weeds often grow in association with each other, with knotweed germinating in March, about 60 days before the last spring frosts.
• Ground ivy, chickweeds, violets, and moss are very shade tolerant. These species often grow in areas that are too shady for turfgrass.
• Rough bluegrass, heal all, horsetail, and violets are associated with wet, shady areas.
• Prostrate spurge is associated with high soil temperatures and hot locations.
• Sandbur, carpetweed, silvery cinquefoil, and mossy stonecrop are associated with dry, infertile, and, frequently, acidic soils.

Planning Your Weed Management Program

Once you’ve determined the weed problem, you can determine what level of control is appropriate, both economically and aesthetically. For certain ornamental plantings, absolute weed control is the goal. But for many other plantings, complete weed control is not only unnecessary but also impractical. Using turf as an example, weed control is much more important on golf greens than along roadsides (refer to the chart in Appendix F “Principles of Pest Management”). Keep in mind that the weed management goals (i.e., the level of control deemed appropriate) are often subjective and will vary from one planting to another and from one customer to another.

Once you know both the weed problem and the weed management goals, you can customize a weed management program for a site. Determine which prevention, cultural, and mechanical weed control practices best fit into the site management plan. Acquaint yourself with the properties and capabilities of the various herbicides that you might use. Both herbicide choice and rate are influenced by the weed problem, soil characteristics, plant growth stage, and future plans for the site. Special rules related to environmental protection may also limit herbicide choice
and rate. Site management practices will also limit your choice of weed control strategies.

Methods of Weed Control

Chemical and nonchemical control measures have a place in a weed management program. Which ones you use will depend on such factors as the site, the desirable plants, the weeds to be controlled, the seriousness of the weed problem, the cost effectiveness of different options, health and environmental concerns, and prevailing social attitudes. The growth stage of the desirable plants is also very important. For example, a plant may be more sensitive to a herbicide when it is a seedling or when buds are forming; the pesticide label will provide information on plant sensitivity. Plants may also be susceptible to injury from nonchemical control efforts. Yew and arborvitae, for example, are very sensitive to stem injury from hoeing when they are young.

Prevention

Keep in mind that it is often easier and less costly to keep weeds out of a site than it is to control them once they become established. In addition, watch for new weeds while scouting weeds. The earlier a troublesome new weed is found, the better the chance of preventing its spread to the rest of the site.

Weeds often get their start in a site from a few seeds that were accidentally planted along with turf or ornamental seed. You can prevent such introductions by planting only tested and tagged seed. Certified seed is high quality and is free of noxious weed seeds.

Weed infestations may get their start in mulches, manure, or newly added soil. Recognize the potential for this problem and choose these materials carefully.

Mechanical Control

Mechanical control can be very helpful in controlling weeds. Examples of mechanical control include tillage, cultivation, mowing and others that we discuss below.

Tillage for lawn establishment, hoeing, and cultivating are among the mechanical practices that can fit into turf and landscape management.

Tillage. Tillage prior to planting (e.g., establishing a new lawn) controls weeds by burying the weeds, severing the shoots from the roots, or uprooting plants so they desiccate. Tilling may, however, bring long-buried weed seeds to the surface, where they can then germinate. Small annuals and biennials can be killed by burying, but this won’t work for most perennials (except seedlings) unless done repeatedly. For best results, till when the soil is dry so roots will have little chance of becoming reestablished.

Cultivation.

Though not applicable to all ornamental plantings, cultivation is an effective means of controlling many annual grasses and broadleaf weeds, if it is done frequently. Because most annual weeds germinate within the top 2 inches of soil, cultivation should be limited to this narrow zone. Deeper cultivation can damage roots of desirable plants and can bring buried weed seeds to the soil surface where they will germinate.

Hoeing, Pulling and Cutting. Hoeing and hand pulling weeds are still effective and highly selective methods of controlling weeds in small areas (e.g., small gardens, isolated weed patches). Ideally, remove weeds while they are still small (less than 4 inches) and be sure to remove the entire root system of perennial weeds. In small areas, annual broadleaf weeds can be controlled by cutting them by hand at the soil surface. This approach will not work for grasses because they grow from points at or below the soil surface. Perennials also have growing points at or below the soil surface, so control would only be achieved if you cut them often enough to deplete their underground reserves.

Mowing. Mowing is very effective in maintaining turf and helping it withstand weed competition. The grass will grow back because its growing point is below the cut surface, but many broadleaves will be controlled. Even grass weeds, such as quackgrass, may be weakened by repeated mowing and will be less able to compete with the desired grass species.

Set cutting levels as high as possible because tall grass produces dense shade that prevents seeds from germinating. Do not remove more than 30% of the turfgrass leaf surface during any one cutting otherwise, “mowing shock” may result, allowing weeds to invade.

Mowing is effective only against tall weeds. It helps reduce the weeds’ ability to compete with the crop and also prevents seed production. It is feasible, but difficult, to control certain tall perennials by mowing. You would need to mow frequently to deplete the weeds’ food reserves to the point that the plants cannot grow back. Mowing is the dominant cultural practice used in turf. Mowing stimulates tillering, which increases turf density and can reduce weed invasion. Turfgrass will benefit from being maintained at an optimal height of cut. Mowing height and frequency have a significant impact on turf growth. Often, the ideal height for grass health and vigor is not the ideal height for a particular use, such as golf. So, mowing height is often a compromise.

Mulching. Mulching is a means of weed control but is not appropriate for all plantings. Mulches control annuals and biennials by preventing the seedlings from getting light. Perennials are more difficult to control with mulches because they may grow through the mulch (depending on the material) or beyond the edge of the mulch.

Organic mulches (e.g., sawdust, shredded bark) should be applied at a minimum depth of 2 inches on weed-free soil. Be aware that straw and hay mulches often contain weed seeds that could create a future weed problem. Be sure the mulch is kept away from the trunks of trees and shrubs to prevent problems with rodents or diseases.

Inorganic mulches (e.g., sand, gravel, black polyethylene film) provide a weed resistant surface and allow water penetration (except in the case of plastic film that is not perforated). More than one mulch can be used, as when gravel is put on top of black plastic.

Burning. Though not a very popular method of control, gas burners are used occasionally to remove vegetation from sand traps on golf courses.

Cultural Control

Although weeds compete with turf and ornamentals, remember that the reverse is also true: these plants compete with weeds. By using the best management practices, you can ensure that the desirable plants grow so well that they either shade out smaller weeds or are vigorous competitors. In essence, anything that helps a desirable plant will help it compete with weeds.

Several practices help the desirable plants gain a competitive advantage over weeds. For example, planting immediately after the tillage for lawn establishment, which destroys any weeds that had germinated or emerged, gives the grass an even start with the weeds. Weeds that emerge before the grass are much more competitive than weeds that emerge afterward.

Ornamental plants vary in their ability to withstand competition from weeds. Plants that germinate and grow rapidly deter weed growth. Conversely, plants that establish themselves slowly allow weeds to gain a stubborn foothold. Through planning and weed surveillance, you can determine the weed control methods that are most effective and appropriate for your situation.

Plant ornamental plants at the right time and spacing and then manage the plants to stimulate rapid, vigorous growth. This will keep weeds at a disadvantage.

The Weed Management Program in Action

Having a well-thought-out weed management plan that fits well with your total plant management plan should be very reassuring as you approach the busy growing season. It may not be the perfect plan, but it certainly will be a lot more reliable than the hit-or-miss weed control used by some.

Review your plan as spring nears. Most importantly, stick to the plan. Too often in the rush of spring, managers sacrifice weed control or plant safety to save a little time. If your plan appeared the best route to follow earlier, it’s probably still the most effective way to handle the weed problem. Slight modification due to unforeseen circumstances, such as equipment breakdown may sometimes be necessary. But don’t let hurried, poor judgment jeopardize your weed management plan.

Protecting Sensitive Areas

Review the herbicide use plan as it relates to adjacent sensitive areas that might be adversely affected by the herbicide application. Consider neighboring sites as well as the one you’re treating. Remember, for example, that many garden plants are especially sensitive to 2,4-D. Don’t use this herbicide adjacent to gardens where sensitive plants are growing. Worrying about adjacent plants is especially important in turf and landscapes, where plants of different sensitivities are grown close together. Review the chapter “Overspray and Drift” for tips on how to prevent off target movement of pesticides.

Think about the presence of lakes, streams, or ponds that are near sites you will treat or if setbacks exist. Leave an untreated buffer strip adjacent to bodies of water to catch any runoff from the site that might contain herbicide. Both direct application and runoff water containing herbicide can be harmful to established vegetation on grass waterways and terraces. Keep the applied herbicide in the site you are treating for the greatest environmental safety and for efficiency of the herbicide treatment.

Monitoring the Weed Management Program

You’ve done the best possible job of assessing the weed problems, considered all practical strategies of weed control, and developed and implemented a management plan to the best of your ability. Now you need to monitor the plan to see if it was as good as you thought. Do you need to adjust or modify the plan to better suit your needs?

Weed Scouting

Monitor the success of your weed management program plan as the growing season progresses. Watch for seedling weeds that escape and decide whether a follow-up treatment is necessary to control them. Don’t let escapes get beyond the stage for effective control. Mark the location of escaping patches of perennials on a weed map so that you can consider them when planning future weed management programs. Finally, note any new, unfamiliar weeds and have them identified.

Minimize Plant Injury

If your herbicide choice and rate were appropriately matched to soil conditions or plant growth stage and if you took steps to minimize drift, herbicide injury shouldn’t be a problem. However, conditions like heavy rainfall or unusually cold or hot weather following herbicide application sometimes causes even relatively safe herbicides to cause plant damage.

Weeds Adapt

Weeds have a tremendous capacity to adapt to both cultural and chemical weed control practices. Mowing favors low‑growing weeds that spread vegetatively or produce seed on short flower stalks that escape the blades of the mower. With selective herbicides, there are always some weed species that are not controlled. The initial escapes may not even be noticed, but they will reproduce and multiply. Continued use of the same herbicide treatment results in a proliferation of those initial escapes until eventually you have a different weed problem. This is an example of a weed shift.

Refining Your Plan

With assessing, planning, implementing, and scouting, you have all of the ingredients for a successful weed management program. Your plan may not work perfectly each year and may require some changes. Modify your plan to deal with escaping weeds. Although weeds will adapt to control efforts, continued refinements of your plan should ensure effective weed management in the future.