Crabgrass Management

The summer of 2003 provided many challenges for crabgrass control in turf. Complaints about poor crabgrass control throughout southeast Pennsylvania began early in August. Normally when poor crabgrass control is reported, questions arise about the preemergence product selected, time of application in the spring, rate applied, proper calibration of application equipment, any cultural practices that may have broken the herbicide barrier, etc. This year, breakthroughs occurred where everything was done correctly. These breakthroughs in control resulted in patches of fairly large, unsightly crabgrass during the month of August. Crabgrass plants 6-8 inches in diameter, with the first seedheads just forming, were reported in early August.

This maturity crabgrass plant in late summer suggests that control from the preemergence product broke down, not that the application was made too late in the spring. (If that were the case, the crabgrass plants would be seeding and very large earlier in the summer).

The combined effects of weather conditions suitable for extended crabgrass germination and the normal dissipation of preemergence herbicides contributed to this summer's crabgrass breakthroughs. Think of it as "the perfect storm" for crabgrass. Very warm and wet conditions throughout July facilitated crabgrass germination. Crabgrass normally stops germinating by mid-July, but even so, the wet weather allowed ample germination to the very end of crabgrass's natural germination window. The length of residual control provided by preemergence crabgrass herbicides varies, depending on the product and the rate applied. Even products with the longest residual control, including pendimethalin, dithiopyr, and prodiamine, were at the end of their intended period of control in July. Most years, dry conditions during the midsummer help to reduce crabgrass germination pressure. This year, the crabgrass pressure was still very strong because of warm, wet weather through the summer. The continued pressure combined with the natural decline in herbicide protection to lead to crabgrass "breakthroughs".

Unfortunately, there is no way to predict this combination of weather events when the preemergence herbicide applications are made. Most years, the use of preemergence herbicides (especially the longer-residual products mentioned above) provides ample crabgrass control. The additional expense of making split applications or using a postemergence herbicide to clean up crabgrass breakthroughs is often excessive. The postemergence herbicides Acclaim Extra or Drive can be used, depending on the size of the crabgrass. Drive is useful where large crabgrass plants are intolerable or will impede planned renovations.

Since crabgrass is a warm season annual, it will be killed at the first frost. In many lawn situations, it is acceptable to do nothing to control breakthroughs that become evident late in the summer. Keep in mind that the seed produced this year will result in increased potential for crabgrass problems next year. It may be advisable to plan on using a preemergence crabgrass herbicide the following spring to control heavy populations. It is not necessary to apply preemergence crabgrass controls every year. If control during the season was acceptable, treatments may be skipped until the population returns to unacceptable levels.

It's important to keep in mind that preemergence crabgrass control products cannot be used if overseeding is planned since the herbicide will inhibit germination of the desirable grass as well as the crabgrass. (The length of time that must elapse from preemergent herbicide application to seeding varies according to the product selected - the label will have the details). One exception to this is if the crabgrass herbicide Tupersan (siduron) is used. Tupersan can be applied at seeding to inhibit crabgrass germination in the seedbed. It does not work well in the extreme southeast part of the state, though, and is quite expensive.

Evaluating Crabgrass Control

When evaluating any crabgrass management plan, it's important to look at the big picture. Before blaming the preemergence crabgrass herbicide, remember to question the other factors that can contribute to the success or failure of your crabgrass management program. Some of the factors influencing crabgrass control are:

• Crabgrass pressure: Was there a lot of seed in the soil just waiting to germinate? Areas that have been untreated in past years can build up quite a reservoir of seed. Certain areas are more likely to have crabgrass pressure, such as the edges of driveways and walks.
• Weather conditions: Were conditions favorable for crabgrass germination over an extended period of time? This summer, warm and continued wet weather allowed crabgrass germination into mid July. During "normal" years, the lack of rainfall during midsummer months helps reduce the amount of crabgrass that will germinate.
• Product selection and rate: What preemergence product was selected and what rate was applied? The longest lasting products in our area are Barricade, Dimension, and Ronstar. For turf with high crabgrass pressure, the higher label rates or sequential applications are recommended.
• Was the proper rate applied? Calibration is very important and often overlooked. Was the sprayer or spreader carefully calibrated to deliver the correct amount of product? Proper measurements of the area to be treated are also important.
• When was the application made? Preemergence products need to be applied about two weeks before expected crabgrass germination to allow time for the product to be "activated" or moved by rain into the top ¼ inch of soil where it will do its job for the season. Dimension provides some early postemergence (as well as preemergence) activity on crabgrass, a good choice if the application is made later than desired.
• Cultural considerations. Crabgrass needs light to germinate, so any problem that results in open areas in the turf (disease, insects, mower scalping, poor fertility, etc.) will enhance crabgrass infestations. Maintaining short turf favors crabgrass - higher mower heights (in the 2 ½ - to 3-inch range) can help shade the soil surface and provide competition for crabgrass. The best competition for crabgrass is a thick, healthy lawn.

Preemergence Crabgrass Herbicides

There are many extremely effective preemergence crabgrass control products to choose from. Some of the products that have been used for a number of years include the active ingredients benefin (Balan), bensulide (Betasan), benefin+trifluralin (Team), pendimethalin (Pre-M), and oxadiazon (Ronstar), among others. These products are applied in the early spring before crabgrass emergence. Two newer products, Barricade and Dimension, can be applied either in the late fall (November) or early spring to provide long-lasting preemergence control of crabgrass. A description them follows:

Barricade (prodiamine) Novartis
Barricade is a selective preemergence herbicide for control of annual grass and certain broadleaf weeds in established turf. It is also labeled for use in established landscape, perennial and wildflower plantings. Barricade is most effective when activated by at least a half inch of rain prior to weed seed germination and within 14 days of application. It can be applied late fall or early spring. Do not apply to overseeded turf within 60 days after seeding or until second mowing. Barricade may inhibit germination of newly seeded or overseeded turf for up to six months after application depending on rate.

Dimension 1EC (dithiopyr) Rohm and Haas
Dimension is a selective preemergence herbicide used for control of crabgrass and certain other annual grasses and broadleaf weeds. It also has early postemergence activity on crabgrass. Dimension is labeled for use on Kentucky bluegrass, fine fescue, perennial ryegrass, tall fescue, creeping bentgrass, and Zoysia grass. The product will control germinating and emerged crabgrass that has not reached the tillering stage of growth. Re-seeding or overseeding within three months of application may inhibit establishment of desirable turf. Dimension provides a wide "window of opportunity" for crabgrass treatment. Dimension can be applied as a fall treatment for control of crabgrass germination during the following spring, or as an early spring application.

Postemergence Crabgrass Herbicides

There are postemergence herbicides that can be applied after crabgrass emergence. A description follows:

MSMA (methanearsonate)
MSMA is a selective, postemergence herbicide for control of some annual grasses and broadleaf weeds in bluegrass and zoysiagrass turf. Injury will occur to fescues and bentgrass, and discoloration of lawns is common after application. Apply to well established, actively growing turfgrass when temperatures are between 80-90 degrees. Mow turf to 1-1.5 inches before treatment. Two or more treatments at 14 day intervals may be necessary.

Acclaim Extra (fenoxaprop-ethyl)
Acclaim Extra is a selective, systemic, postemergence herbicide for control of annual grasses in turf. Acclaim Extra controls only labeled grasses which are emerged at the time of spraying. Young, actively growing grasses are more easily controlled than larger grassy weeds. Kentucky bluegrass seedlings must have tillered and have been emerged for over one month before application. Do not apply Acclaim Extra to seedling fine fescue, tall fescue, or perennial rye less than four weeks old. Tank mixing with broadleaf herbicides like MCPP or 2,4D or applying Acclaim within five days before or 21 days after the broadleaf application will reduce Acclaim Extra activity.

Drive 75 DF (quinclorac)
Drive is a selective herbicide with pre- and postemergence activity on certain grasses and broadleaf weeds. Drive is root and shoot absorbed and has soil residual properties. Crabgrass is most sensitive prior to the two tiller stage and also at mature stages (a weak control period exists between these stages). Clover, dandelions and speedwells are controlled by postemergence applications and are also controlled from seed. Optimum timing in southeast Pennsylvania may be in early June to coincide with crabgrass development and clover/dandelion treatments.

Another application window would be late summer at the time of overseeding. Drive may be applied at the time of seeding or overseeding, however, it should not be applied within four weeks after emergence of Kentucky bluegrass or perennial ryegrass.

Drive can be tank mixed with preemergence or postemergence herbicides to broaden the spectrum of weed control. For instance, goosegrass is not controlled by Drive, so addition of a preemergence herbicide would be suggested if goosegrass was a problem. To broaden the spectrum of broadleaf weeds, addition of standard broadleaf herbicides is recommended. Follow all restrictions of the tank mixed products concerning overseeding, etc.