Know Your Post-Emergence Options: Weed Strategies

And even the best maintained turf area may need a postemergence herbicide treatment to keep it free of these troubling species that we call weeds.

ANNUAL GRASSES. The annual grasses are species that die each year and must re-grow from seed. There are both summer annual and winter annual grasses that infest turf. Summer annuals are those that germinate in the spring and die in the fall, such as crabgrass, goosegrass and foxtail.

Winter annuals germinate in late summer or fall, live through the winter as a mature species, produce seed in the spring and die. Annual bluegrass (Poa annua) is the most common winter annual grass found in turf.

The best way to control summer annual grasses is to apply a preemergence herbicide that will kill the germinating seed with little damage to the mature turf, but perfect timing is not always possible. Customers will sign up for service after the crabgrass has germinated in the spring, or annuals may germinate after the preemergence application has lost its effectiveness.

ANNUAL CONTROLS. The organic arsenicals Methar 30™ (DSMA) and Daconate 6™ (MSMA) are the oldest herbicides used for postemergence annual grass control in turf. These products can be effective, but can be difficult to use due to inconsistent control and dependence on specific environmental conditions. They also pose a risk of turf phytotoxicity. There is minimal residual preemergence control with the organic arsenicals, and weeds may re-germinate if a separate preemergence material is not also applied. Effectiveness varies by region and these materials should be evaluated on test areas before treating a large turf area.

Acclaim Extra™ (fenoxaprop-ethyl), released last year, is a reformulation of the postemergence annual grass control product Acclaim that was introduced in the 1980s. The new formulation now contains more of the active ingredient. It is more consistent in its control of annual grasses than the arsenicals, and phytotoxicity is not as great a problem. It is not as sensitive to weather conditions as the arsenicals but it is less effective when applied following dry conditions. Combining Acclaim Extra with phenoxy herbicides, or applying it after a phenoxy has been applied also results in poor crabgrass control. Acclaim has no preemergence activity and germination may occur in the treated area if it is not combined with a preemergence material.

Dimension™ (dithiopyr) is the first herbicide to have both preemergence and postemergence activity on annual grass weeds. Its postemergence activity is limited to the early stages of weed development, and its effectiveness varies in different parts of the country. In the Midwest, we have found it to be a very good material for early postemergence control. Its preemergence activity is excellent and will usually provide season-long control even in wetter years when other herbicides begin to break down.

Quinclorac, which will likely be called Drive™ when released, is an experimental product for postemergence control of crabgrass and certain broadleaf species (particularly white clover). It has limited preemergence activity. In my research, I have observed it to provide a very rapid knockdown of weeds. Like Acclaim Extra, quinclorac would have to be applied with a preemergence material to prevent late germination.

BROADLEAFS. Both annual and perennial broadleaf weeds are common problems in cool-season turf. It is possible to control some annuals with preemergence herbicides, but their germination timing is often such that postemergence treatments make more sense. Perennials like dandelion, plantain and clover require postemergence treatments.

Table 1 lists postemergence herbicides for perennial broadleaf weed control in turf, and they are the basic ingredients found in single ingredient and combination herbicide products.

Many of these herbicides are available in either the ester or the amine formulation. The ester formulation is more volatile, but it is better able to penetrate the cuticle of weeds and is usually more effective at dealing with the difficult to control species. The amines are less volatile and are thereby safer in urban environments where susceptible, non-target species are nearby.

There are low-volatile esters available for some of these active ingredients. These materials have improved effectiveness over the amines but with less volatility than older esters. Low-volatile esters are preferable in the fall when the risk to non-target species is minimal. In the spring, or when highly susceptible species surround the treated area, the amines are the best choice.

BROADLEAF PRODUCTS. Dicam-ba is a very effective herbicide for the selective control of broadleaf weeds in turf. It is particularly effective against some of the more difficult to control species like thistles, buckhorn plantain and ground ivy. Dicamba is usually combined with other materials such as 2,4-D, MCPP or MCPA.

Dicamba is usually sold in the ester formulation. Its primary limitation is its volatility, which limits its use in urban environments where highly susceptible species, like tomatoes and grapes, can easily be damaged.

The newest addition to this category is Vanquish™. This is an amine formulation of the same active ingredient in dicamba. Our research in 1995 with this product at the Iowa State University turf research area (Bingaman, Christians, and Gardner, 1996) showed it to be very effective on broadleaf weeds. It was also quite effective when combined with triclopyr, a pyridinoxy acid herbicide that will be discussed later.

The phenoxy herbicides, around since World War II, are inexpensive and effective. There are some variations in weed control among the different materials, however. While 2,4-D is very effective against dandelion, plantain and other common weeds, it is less effective against white clover, violets and some of the other more difficult to control species. MCPP is much more effective against clover and some of these other species. The MCPA and 2,4-DP each have their own variations in species effectiveness as well.

An effective strategy is to formulate combination products with the phenoxy herbicides to take advantage of their variable characteristics. MCPP and 2,4-D are commonly combined to broaden the control spectrum. Another widely used group of combination products contain 2,4-D, MCPP, and dicamba. These products are marketed under a variety of commercial names including Trimec,™ Triplet,™ Trexan™ and others. Other combination products formed by combining the phenoxy herbicides include Tripower,™ which is a combination of MCPA, MCPP, and dicamba, and Dissolve,™ which combines 2,4-D, 2,4-DP and MCPP.

Later research into alternatives to phenoxy herbicides led to the discovery of the pyradines. Triclopyr was the first of these materials to reach the turf market. It provides excellent control of white clover, violets and some of the other hard to kill broadleaves. It is somewhat weak on dandelions, however. Triclopyr was first released in a combination product with 2,4-D called Turflon-D,™ This is a very effective combination on a wide variety of weeds. Turflon-D is an ester formulation, and there is some risk of non-target damage.

Later, an amine formulation of this combination product called Turflon II™ Amine was released. The list of combination products containing triclopyr has recently been expanded with the release of Cool Power,™ a combination of triclopyr, MCPA and dicamba and Horsepower™ which contains triclopyr, MCPA and dicamba in an amine formulation.

Clopyralid, which is closely related to triclopyr, was released in the turf market in 1990. It is generally not used alone but is combined with triclopyr and marketed under the trade name Confront.™ This product is 33 percent triclopyr and 12.1 percent clopyralid.

Confront™ is very effective on many difficult to control broadleaf weeds, including clover, violet, spurge, and yellow woodsorrel (Oxalis). It is also good for sensitive sites because it doesn’t drift to non-target plants.

Methar 30 is a registered trademark of W. A. Cleary, Dayton, N.J. Daconate 6 is a registered trademark of ISK Biotech Corp., Mentor, Ohio. Acclaim Extra is a registered trademark of AgrEvo, Wilmington, Del. Dimension is a registered trademark of Rohm & Haas, Philadelphia, Pa. Drive is a registered trademark of BASF Corp., Research Triangle Park, N.C. Vanquish is a registered trademark of Novartis, Greensboro, N.C. Trimec is a registered trademark of PBI Gordon, Kansas City, Mo. Tri-Power, Horsepower, Cool Power and Dissolve are registered trademarks of Riverdale Chemical, Glenwood, Ill. Confront, Turflon-D and Turflon II Amine are registered trademarks of DowElanco, Indianapolis, Ind.

Mention or absence of any product is not meant to be either an endorsement or criticism of any product. Always read and follow label instructions.