Crabgrass – A Real Pain in the Grass!

Loading player for /content/dam/ext_vt_edu/topics/lawn-garden/turfgrass/turfandgardentips/tips/precrabgrass_control_2009.mp3...

Crabgrass is the number one weed in lawns and the best time to treat is usually before it is seen. This podcast details the best management practices in developing a crabgrass control program.

The number one weed problem in managed turfgrasses is crabgrass (Digitaria sp.). As days get longer in late winter/early spring and soil temperatures warm, crabgrass seed are primed for germination. When soil temperatures reach 55o F for 2 to 3 days, germination is possible and can continue throughout the spring and summer. No matter how well crabgrass has been controlled in previous years, there is still a tremendous seed bank in the soil and any weak spots in the lawn are likely sites to be invaded by this fast growing summer annual. Crabgrass’ rapid emergence and extremely fast growth rate allow it to get a jump start on both cool and warm-season grasses alike. Since crabgrass is a warm-season grass, it is particularly competitive against cool-season grasses (fescues, bluegrasses, ryegrasses etc.). Given that it is such a tough competitor, what are the best management practices to battle this pest?

A thick, healthy turf is the best method of crabgrass control. Crabgrass requires sunlight to germinate, a requirement that we can use to our advantage in the lawn. Dr. Shawn Askew, Extension Turfgrass Weed Scientist, advocates that the lawn itself controls far more weeds than any chemical ever applied because the most weed control is gained by maintaining a dense turf canopy. If the lawn is reasonably dense entering the spring growing season, crabgrass pressure (and that of any other spring germinating weed) is minimal. It is still feasible to use a standard preemergent (PRE) herbicide (a product that controls germinating plant seedlings), but it is unlikely your lawn will be swamped with weeds. If your turf stand is very sparse, then it is almost guaranteed that you will have significant weed pressure. So there is now a decision to be made—do you apply a PRE herbicide to control weeds OR do you apply new grass seed to fill in the gaps. It is very important to understand that for most standard PRE herbicides available to homeowners, there is no selectivity in control between weed or grass seed. The only exception in a PRE herbicide that can be applied for crabgrass control at seeding of cool-season grasses is Tupersan (siduron). Crabgrass can also be controlled with a specific early postemergent (POST) herbicide such as Drive (quinclorac), and this product has safety on several cool- and warm-season grasses. Many of these approaches are best left (and sometimes they are only available) to professional lawn care operators. Remember – the label is the law when it comes to pesticides. This protects you, the environment, and your lawn.

What PRE herbicides are readily available to homeowners? There are many trade names of products on the market, so while it takes a little extra effort to look for the complicated common chemical names, it’s the safest way to identify the product you are looking for. The following chemicals can most often be found at stores that deal with specialty products for lawns and landscapes, and at least a few of them will likely be available at your big-box retailers. A recent survey of both specialty lawn and garden centers and big box retailers indicated these active ingredients were available: benefin, benefin + trifluralin, dithiopyr, pendimethalin, and prodiamine. Many of these products are formulated on a fertilizer carrier that supplies spring fertility as well. Using them according to label directions should supply the necessary level of active ingredient for weed control, and provide all the fertilizer the grass needs for spring greening. There is also an organic PRE crabgrass control product on the market as well: corn gluten meal (CGM). CGM works by releasing a protein that slows development of weed seedling roots leaving seedlings vulnerable to drought. In periods of extended rainfall CGM will fail to control weeds and its length of activity is very short-lived (perhaps a few weeks) as compared to standard synthetic chemistry which may last 120 days. CGM is not selective to turfgrass seedlings and will control them just as well as crabgrass. CGM works best in cooler climates and in lawns that have good turf density in the spring. Labeled rates of CGM treatments will also deliver approximately 1 pound of water insoluble nitrogen per 1000 sq ft, making CGM another offering in the group of products known as “weed and feed” materials. Repeat applications of CGM for extended weed control will actually supply N that greatly exceeds recommended levels for spring growth. In a transition zone state such as Virginia, CGM has provided acceptable levels of crabgrass control when used as the first spring PRE treatment on moderate density lawns, and where season-long weed control is desired, the second PRE application at 4 weeks after the CGM treatment can be made with a low label rate of a standard synthetic PRE herbicide. For this second PRE treatment, avoid weed and feed products (or at least select those that are very low in N) and try to apply the PRE herbicide alone. This combined approach with CGM and a standard PRE herbicide feeds the plant and reduces the total amount of synthetic chemistry applied.

Timing of PRE applications? Mother Nature provides a valuable visual tool in the landscape that typically allows us to optimize the timing of PRE herbicides for homeowners: the forsythia. Its blooming can never be taken as an absolute signal of pending crabgrass emergence as Mother Nature is not perfect. However, it works in most years and Dr. Askew’s research has found that the time when forsythia starts to drop its blooms is when PRE herbicides need to be in place in order to achieve optimum crabgrass control. Don’t be alarmed if your lawn care operator has applied earlier in the season; this is simply because with the number of lawns they have to treat, there is no way all applications can be made according to forsythia bloom. The standard PRE herbicides they are using have soil activity for 6-8 weeks that will address a broad window of crabgrass germination potential. Having the PRE products in place ensures no escapes in crabgrass control by missing the appropriate application timing.

Post-treatment considerations? One thing required for all PRE herbicide applications is to water the product into the soil with either a suitable rainfall or irrigation event. The only way the product works is if it gets into the top of the soil profile to form a chemical barrier that germinating seedlings penetrate. Appropriate moisture is critical to optimize herbicide efficacy. And remember to keep all products on the turf and off hardscapes. This is the easiest way to protect our water resources.

This podcast was developed by Mike Goatley and Shawn Askew, Extension Turfgrass Specialists at Virginia Tech. Commercial products are named in this publication for informational purposes only. Virginia Cooperative Extension does not endorse these products and does not intend discrimination against other products which also may be suitable.