Broadleaf weed control

It is a usually a necessary evil to perform some postemergence broadleaf weed control in the spring whether you are a professional lawn care operator or a golf course superintendent. However, this has fostered the misconception that spring is the ideal time to control broadleaf weeds. Before determining your weed control strategy, it is important to correctly identify the weeds on a site, and then consider whether they are predominately annuals or perennials. Identification of the weed and knowledge of its life cycle is necessary in order to best determine what control strategies to use. For example, many common and most difficult-to-control broadleaf weeds are perennials and it makes more agronomic sense to apply postemergence materials, such as 2,4-D and triclopyr, in the fall for the control of these weeds (See Table 1, page 40).

For a list of common broadleaf weeds, see “Chemical Update: Turfgrass Herbicides,” Grounds Maintenance, January 2003. The species listed easily account for 90 percent of the perennial broadleaf weed problems in many areas of the country. Fall-applied postemergence herbicides should be your first choice to control perennial broadleaf weeds. If making a spring application, be aware that control of underground structures may not be complete. The weed may regenerate from these structures, necessitating a second herbicide application.

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While fall applications of broadleaf herbicides will control annual broadleaf weeds, you can achieve more effective control of these weeds either with preemergence herbicides, or, if necessary, postemergence materials applied early in the weed's life cycle. Control of annual broadleaf weeds with a postemergence material in the fall is only warranted if heavy cover of these weeds are preventing the turfgrass from filling in bare spots in the lawn.

RECENT DEVELOPMENTS

There are many herbicides available to combat broadleaf weeds (see “Chemical Update: Turfgrass Herbicides,” Grounds Maintenance, January 2003). While you can buy some of the herbicides individually, control is most often accomplished with a three-way combination of the herbicides. 2,4-D, MCPA, and triclopyr are generally more effective on dandelions, while 2,4-DP, MCPP, and clopyralid are more effective on clovers. Dicamba is better for difficult broadleaves such as thistles. Usually a three-way herbicide will have 1 or 2 compounds from each grouping (e.g. 2,4-D, MCPP, and dicamba).

PBI Gordon now markets broadleaf herbicide combinations with a herbicide called carfentrazone, which is reported to result in faster burn down of the targeted weeds. Carfentrazone acts to disrupt cell membranes, which results in rapid death of tissues and browning of the affected plant. Initial results indicate that while overall control with these products is no better than with other combination products, faster burn down of broadleaf weeds can be achieved. New to the market this year, FMC is offering carfentrazone as “stand-alone” product. However, it is not meant for use by itself. The intent is to use the stand-alone carfentrazone product tank-mixed with another broadleaf herbicide in order to improve the burn-down speed.

Table 1. The advantages of fall-applied post-emergence herbicides
Herbicide translocation In the fall, the weed translocates carbohydrates into the taproot for winter. Fall applied broadleaf herbicides are much more effective because they are readily translocated into the root system, resulting in death of the root system as well as the leaf tissue
Control in spring results in bare patches filled in by crabgrass Control in the fall is not only more effective, but also gives the turf time to fill in the bare spot without competition from crabgrass and other annual weeds. While postemergence broadleaf herbicides will not control crabgrass, often the best control of annual grasses is a dense stand of turf.
Less risk of damage to ornamentals In the fall, most annual ornamental plants and vegetables have reached maturity and leaves of trees and shrubs are beginning to turn color and fall off the plant, resulting in reduced risk of drift injury to these plants by herbicides.
Winter annuals Henbit and common chickweed, are beginning to germinate in mid- to late-fall and can be effectively control if herbicide application is done after they germinate.

For several years after their introduction, the pyridinoxy herbicides triclopyr and clopyralid were only available in Turflon II, which contained triclopyr and 2,4-D, and Confront, which contained triclopyr and clopyralid. However, in the last few years, several new products have been registered which contain triclopyr or clopyralid in a three-way herbicide combination. These new combinations offer increased choices and broader spectrum of weed control. Remember that the products containing clopyralid will no longer be available for residential lawn applications.

In addition, several new combination products are now available without 2,4-D. MCPA has been around for many years and has a very similar mode of action and spectrum of control. It is, however, a little more expensive to manufacture. However, with some of the environmental concerns that have been raised about 2,4-D, it has become necessary to register products that do not have 2,4-D in them. These products will generally work the same as three-ways with 2,4-D and can therefore be used in areas where 2,4-D usage is no longer desired or permitted.

Two other products are not new to the market, but are labeled for control of certain broadleaf weeds. Quinclorac, the active ingredient in the crabgrass postemergent Drive, has shown good activity against dandelion and clover and is labeled for this use. The product is not the best choice for overall long-term control. However, in certain situations it may be a useful addition to your weed control program. If you are using Drive to control crabgrass postemergently during the summer you can also target the dandelions and you may get good suppression. But, be aware that an application of a three-way broadleaf herbicide in the fall will probably be necessary to achieve complete control.

The other herbicide is chlorsulfuron. It has been on the market for many years primarily for the control of tall fescue in Kentucky bluegrass turf. However, it is labeled for the control of wild violets, chickweed, and purslane and several other weeds. Check the label for specifics, as this product is not to be used on tall fescue or ryegrass, and there are other restrictions.

RESIDENTIAL USE OF CLOPYRALID DISCONTINUED

Dow AgroSciences issued a press release last summer to announce that they were discontinuing residential turf uses of clopyralid. Clopyralid had been on the market for about 15 years and is a very useful herbicide for the control of many lawn weeds, particularly the clovers. The concern with clopyralid is that low concentrations of residues can persist for considerable periods in composted turfgrass clippings. The concentrations found were in the part per billion range (1 ten-millionth of 1 percent). While clopyralid is active on only a few plant families, one of them, the potato family, is particularly sensitive even to part per billion concentrations. Ironically, it includes such things as potato, tomato, and peppers, our most common garden crops. When clopyralid-containing clippings were composted, most of the clopyralid residues would break down. But, when the compost was used for vegetable gardening, the residues that remained caused damage to tomato and pepper crops. As a result of environmental concerns that arose from this issue, the product was discontinued on residential turf.

Clopyralid can still be used on golf courses. And, it appears that clopyralid can still be used on commercial properties including multi-family residential developments, provided that professional lawn care applicators notify commercial property managers not to compost clippings. It is very important that steps be taken so that clippings from theses sites not end up in compost. Also, since the regulations on commercial landscape have not been clearly established, you should call the appropriate regulatory agencies in your state for clarification on what uses of clopyralid are and are not permitted in the landscape industry. Dow AgroSciences is actively researching the development of a replacement compound, floroxypyr, which should have a label in turf in the near future.

SOME FINAL NOTES CONCERNING BROADLEAF WEED CONTROL

It is very important to select the right herbicide and the most appropriate formulation in order to get the best possible control. Consult the label to determine whether the addition of a surfactant is warranted. Remember that many of the broadleaf herbicides are available as both amine and ester formulations. Ester formulations are more effective than amine formulations, especially as temperatures decline. Esters, however, are more volatile and you must exercise more care around ornamentals with these materials when temperatures are above 60 degrees. Remember that postemergence herbicides are most effective if applied during sunny weather with no rainfall within 24 hours of application.

How late you can apply these materials depends on weather conditions. In general, if the plant tissue is losing quality due to frost or cold temperatures, it is probably too late to get enough herbicide into the root system for effective control. However, depending on weather conditions and location, you can achieve effective control with these materials through applications as late as the second week of December. Herbicides applied in the very late fall have little effect two or three weeks after application. But, when the same areas are revisited in the spring, control can, in fact, approach 100 percent.

Dr. David Gardner is professor of horticulture ad crop science at The Ohio State University (Columbus, Ohio).

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