WEED CONTROL

Herbicide technology has revolutionized our ability to manage a variety of common turfgrass weeds. However, these granular, powder and liquid marvels are simply tools in an overall strategy for integrated weed management. Incorrect herbicide use or, worse yet, poor fundamental turf management, offsets the benefits these tools can offer us. With that in mind, plan herbicide use and other weed management techniques with a weed identification reference in one hand, a calendar in the other and an herbicide label in your back pocket. The integrated approach to managing the annual onslaught of weeds means first outsmarting the weeds and then using tools like herbicides to finish them off.

What does it mean to outsmart weeds?

Newspaper reporters are taught to ask key questions such as who, what, where, when, why and how. Determining the answers to these types of questions for weeds is the fundamental premise of integrated weed management and the key to outsmarting weeds. Weeds thrive when turfgrass is struggling. This can be due to climate but also can be due to controllable factors such as poor drainage and soil compaction.

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Weeds that thrive under controllable circumstances are known as indicator weeds. For example, prostrate knotweed, spurge and goosegrass all can outperform turf in compacted soils and point to compaction as a possible problem when they occur persistently. Nutsedge and annual bluegrass are often concentrated in moist or poorly drained areas. Remediation of problem sites is a necessary long-term approach to weed management and can help prevent unnecessary future herbicide applications.

Weed life cycles

Outsmarting weeds also requires knowing both the weeds themselves and their life cycles. Weeds can follow one of four life cycles: summer annual, winter annual, biennial or perennial.

  • Biennials are relatively rare in both nature and in turfgrass systems, so they do not warrant much discussion here, beyond the simple fact that the life cycle occurs over a two-year period.

  • Annuals may not always begin and end their life cycles in the same calendar year but always within a 12-month period. Summer annuals, as the name implies, enjoy their most active growth during summer, following a spring germination period and preceding a fall seed dispersal and death. Common summer annual weeds in turf include crabgrass, goosegrass, foxtails, spurge and prostrate knotweed. Winter annuals characteristically germinate in fall, overwinter as juvenile plants and complete their life cycles the following spring. Common winter annual weeds in turf include annual bluegrass, wild garlic, chickweed and henbit.

  • Perennial weeds can persist for several years under good growing conditions and thus represent the greatest challenge to effective control. This persistence depends on storage structures that help perennials survive through stressful times. Examples of such structures are taproots, bulbs or nutlets, rhizomes and stolons. Common perennial weeds in turf include bermudagrass, orchardgrass, quackgrass, dandelion, clover and nutsedge. As I'll discuss below, control of these different weeds is highly dependent on both life cycles and the specific biology of weeds within each life-cycle group.

Weed control strategies: broadleaf weeds

Selective broadleaf herbicides include the phenoxy group (e.g. 2,4-D, dichlorprop, MCPA, MCPP), the benzoic acid group (e.g. dicamba) and the pyridinecarboxylic acid group (e.g. clopyralid, triclopyr). A more recent introduction to the turf market is quinclorac; and the newest broadleaf herbicide, carfentrazone, is formulated as a mixture with phenoxy chemicals.

In established turf, these herbicides do not normally harm turfgrass plants. However, seedling turfgrass plants have less green tissue and are less efficient at breaking the same herbicides down, making them more sensitive. Check the label for post-application seeding intervals, which may range from 2 weeks to a month or more. Also, most products require you to wait until new turf has been mowed three times before using them, though a few can be used somewhat sooner.

Life cycle plays an important role in determining the ideal time to apply broadleaf herbicides. Perennials such as dandelion or clover are at their weakest in fall, when food reserves are low following summer. These weeds experience a growth resurgence in fall, and send new reserves to storage structures in preparation for winter. If you apply herbicides in fall, you can take advantage of this flow of reserves to the roots to help the herbicide reach these critical storage areas.

This strategy works only if no new planting is set for the fall. When turfgrass is seeded in fall, wait until after the new turf has been mowed three times, be it in mid-to-late fall or the following spring. Spring applications may be somewhat less effective because perennial broadleaf weeds are sending resources to the shoots for flower production, so their underground parts might survive.

Annual broadleaf weeds are most commonly controlled with post-emergence herbicides. That is not to say that pre-emergents aren't available for controlling annual broadleaf weeds. Where problems with winter annuals are apparent, it may make sense to use a pre-emergent such as isoxaben. However, post-emergent products are generally cost-effective and usually are applied anyway for broadleaf perennials, so it makes sense to rely on them for annuals as well. Post-emergents also interfere less with fall overseeding.

Post-emergence control of annual broadleaf weeds is best when they are young and before they produce flowers. Once annuals begin flowering — in spring for winter annuals or in late summer/early fall for summer annuals — herbicides are less effective because resource allocation is no longer to leaves and roots. Further, their life cycles are nearly complete, and the application is only hastening what is already about to occur (the end of the weeds' life).

Post-emergence broadleaf herbicides are most effective when spray-applied. Sprays provide better coverage of target weeds and surfactants can improve the coverage even more. Granules require moist foliage to stick to the leaves of target weeds and so may not provide adequate control if conditions are not ideal. However, granules are a less-risky way to apply broadleaf herbicides when sensitive broadleaf ornamentals are near the treated area and drift is of some concern. They also can be applied with a spreader, which is very efficient for large areas. Numerous products exist for broadleaf weed control; always consult the herbicide label for proper rates and other application directions.

Weed control strategies: annual grasses

Selectively controlling one type of grass growing within another is challenging. Thus, even though a few post-emergence options such as MSMA and quinclorac are available, pre-emergence control has become the standard for annual grasses.

Strictly speaking, pre-emergence herbicides do not influence germination but rather kill young grass plants as they try to emerge through the soil surface. Most pre-emergence herbicides inhibit both root and shoot growth of plants that encounter them in soil. This makes them highly effective on developing weeds but also possibly detrimental to newly established turfgrasses. Mow newly seeded turfgrasses at least three times before applying pre-emergence herbicides to assure safety.

Siduron and quinclorac are exceptions because they can be used in conjunction with seeding of most cool-season turfgrasses. Oxadiazon is another exception. It can be used to control weeds during sod or sprig establishment.

Except for dithiopyr, which has some early post-emergence activity on annual grasses, pre-emergence herbicides are ineffective on annual grasses once they've emerged, so proper timing is critical. Factors in nature (phenological indicators) such as spring tree flowering are often used to time pre-emergence applications, but soil temperature is the most consistent gauge. Crabgrass germinates once soil temperatures consistently reach 55°F in spring, while annual bluegrass germinates once soil temperatures drop to this range in late summer or early fall.

Because of the variable climates we experience, soil temperatures — and, therefore, proper application timing for summer annual grass control — range from February in the southern Transition Zone to May in northern states. For winter annual grasses such as annual bluegrass, the application window ranges from August in northern states to early fall in the southern Transition Zone.

Summer annual grass control often requires a repeat application later in the growing season because most pre-emergence herbicides don't persist in soil for the entire growing season. Dithiopyr and prodiamine have relatively long soil residual and often will persist through the growing season with only a single application. Immediately water in any pre-emergence herbicide to move the material into the rootzone where it becomes active.

Control of perennial grasses

Three approaches are available for control of perennial grasses: mechanical removal, nonselective herbicides and selective herbicides.

Mechanical removal of perennial grasses may provide a temporary solution but storage structures such as rhizomes allow these weeds to come back. Plus, physical removal is labor-intensive if the weeds are numerous.

Nonselective herbicides are effective against perennial grasses but present an aesthetic risk to the turf, except when used to control cool-season grasses in dormant warm-season turf. Choice of nonselective herbicide is important with perennial grasses. For best long-term results, choose herbicides with systemic activity such as glyphosate or glufosinate. Time applications when the target weeds are growing actively and allow time for a second application if it is necessary to achieve better kill.

Selective herbicide control of perennial grasses in turf is the most challenging option because of the biological similarity between the weeds and the turf. Timing of selective herbicide applications should also focus on when the weeds, rather than the turf, are growing actively. This approach is most promising for controlling volunteer warm-season grasses in cool-season turf. Herbicides such as fenoxaprop or fluazifop may selectively suppress warm-season perennials (such as bermudagrass), but getting a complete kill is usually unrealistic. Cool-season turfgrass tolerance to these herbicides should always be confirmed by checking the label for proper rates and timings. Control of perennial grasses in managed turf will usually require an ongoing effort, regardless of what approach you select.

While weeds present unique management and budgetary challenges, do not forget to focus first on the turf and then on the weeds themselves. A healthy turf, be it a fairway, lawn or athletic field, is the cornerstone of weed control and a springboard for planning herbicide applications.

Sensitivity to anti-chemical public sentiment also begins with attention to proper cultural practices. Whether it means dealing with club members, homeowner clients or concerned parents, public relations are now a regular part of the job. Avoid viewing and using herbicides as the sole solution to weed problems.

Remembering these fundamentals will make managing turf weeds more efficient and help you maintain a quality rapport with those who look to you for expertise and professional discretion.

Dr. Matthew J. Fagerness is professor of turfgrass science at Kansas State University (Manhattan, Kan.).

All products mentioned in this article are shown in the “Turfgrass Chemical Update,” beginning on page 22. It provides a complete listing of all herbicides registered for turf use, including extensive label and supplier information.

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© 2014 Penton Media Inc.

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