Transcript of:
Spring Crabgrass Control in Virginia’s Lawns
Mike Goatley
Have you been overwhelmed with crabgrass and other summer weeds in your lawn in recent years and vow to not fight this very aggressive weed all summer long? Then mid-March through mid-April presents the ideal time to preventatively treat for crabgrass across Virginia (the earlier dates suitable for the Tidewater and Piedmont regions, and the later dates for the Valley and Ridge). The key here is “preventative” control with what is known as a preemergent herbicide; that is, a herbicide that kills tiny seedlings as they germinate and emerge from the soil. Warming soil temperatures, longer days, and spring rains are all priming crabgrass to germinate. Timing is everything—these products are ineffective if applied to emerged crabgrass.
Are there any handy environmental signals available to us on which to base PRE herbicide applications? You bet— Dr. Askew’s rule of thumb is to apply a PRE sometime between 100% Forsythia bloom (the brilliant yellow-flowered shrub seen throughout Virginia in the spring) and 50% bloom drop. This period is highly correlated to soil temperatures that promote crabgrass seed germination.
What chemicals are available? The table in the accompanying handout and attached VCE publications detail some of the most likely PRE herbicide active ingredients you are probably going to find at a lawn and garden center. The broadest spectrum of weed control will likely be gained by applications of prodiamine, pendimethalin, dithiopyr, and oxadiazon. You might also find some older active ingredients such as benefin, bensulide, or oryzalin. Used according to label directions, each of these materials is effective in crabgrass control. And remember that you can always find the latest use rates and recommendations of herbicides in the lawn if you use the Pest Management Guide that is available at the Virginia Tech website (just search for “PMG” on the VT Webpage, www.vt.edu).
What does a PRE herbicide application mean regarding spring seedings of desirable turfgrasses? It generally means DON’T SEED! Most PRE herbicides will not distinguish between weed or turfgrass seed and will effectively control any of them for up to 3 months after the application. Two notable exceptions that you might explore (and these products are often only available through licensed lawn care applicators) are siduron, a PRE herbicide that can be safely applied ONLY TO COOL-SEASON turfgrasses at spring seeding for crabgrass control, and quinclorac, a PRE herbicide that has a pretty flexible window of application timing for many cool AND WARM-SEASON turfgrasses depending on the species. These are the only exceptions to herbicides that can be applied at seeding, and even they have special use precautions to ensure they work as desired.
What about “weed and feed” products? These are clearly some of the most popular PRE weed control/fertilizer products for homeowners because your taking care of two lawn management steps in one pass (weeding and fertilizing). The concept is pretty straightforward: the herbicide is impregnated on a fertilizer carrier. These products are usually well formulated for a reasonable application of N fertilizer to cool-season grasses in the spring (typically they provide around 1 lb of N/1000 sq ft total), especially if they contain 15-30% slow release N by weight. Ideally, no N-containing fertilizer would be applied to warm-season grasses until after complete greening has occurred, but this is not possible with these products if one is going to benefit from the PRE crabgrass herbicide. Therefore, try to select products that contain significant slow-release N so that you don’t get a lot of spring greening that could be lost by a late freeze.
What will ensure optimum PRE herbicide activity? First, as uniform of an application as you can make will improve control. Most products available to homeowners are granular in form and require a properly calibrated spreader to ensure uniform, accurate delivery. Many licensed professionals will use liquid applications, and these work well because they are using water as their carrier and as a diluting agent to ensure thorough coverage. Both types of treatment require irrigation within 24-48 hours of application to activate the chemical in the soil, so try to time the treatment with a pending rainfall or irrigation event. And remember, keep any of these products OFF the hardscapes; if you get it in your lawn it will stay put and will not pollute our water sources.
Any natural organic sources available for weed control? The answer is yes and the active ingredient is actually very effective in PRE weed control, but unfortunately it is very short-lived in the soil. The product is corn gluten meal. At its recommended application rates it also will be providing approximately 1 lb N/1000 sq ft. Research is currently being conducted to develop programs that might marry some of the desirable aspects of a natural weed control product like corn gluten meal with the predictability in response of our synthetic chemicals.
Finally—is the use of a herbicide absolutely essential to gain crabgrass control? The answer depends on what you are looking for in a lawn. For some folks, “green is good”… they’ll accept just about any kind of plant material in the lawn. The one problem with crabgrass is that it is so aggressive that it can rapidly outcompete many of our turfgrasses, particularly the cool-season grasses. Therefore, if a crabgrass-free lawn is truly the goal, a spring PRE treatment is likely required. However, as Dr. Askew always details in his presentations, the absolute best weed control is achieved by maintaining a healthy, dense turf. Best management practices such as proper mowing, nutrition, and irrigation go a long way in providing a weed free lawn with minimal chemical inputs. Montgomery Co. Extension Agent Barry Robinson has had a crabgrass-free lawn for many years and he does not use a PRE herbicide. Barry’s approach is to mow his cool-season turf as high as his cutting deck will go and utilize the competition from the turf to keep the crabgrass in check.
Has this information been helpful to you? Do you have other suggestions or comments regarding this or other podcasts that would serve your needs? If so, please contact us. And always remember that your local Virginia Cooperative Extension office is ready to assist you in attaining an environmentally friendly lawn—let’s keep our lawns green and our water clean!
Are there any handy environmental signals available to us on which to base PRE herbicide applications? You bet— Dr. Askew’s rule of thumb is to apply a PRE sometime between 100% Forsythia bloom (the brilliant yellow-flowered shrub seen throughout Virginia in the spring) and 50% bloom drop. This period is highly correlated to soil temperatures that promote crabgrass seed germination.
What chemicals are available? The table in the accompanying handout and attached VCE publications detail some of the most likely PRE herbicide active ingredients you are probably going to find at a lawn and garden center. The broadest spectrum of weed control will likely be gained by applications of prodiamine, pendimethalin, dithiopyr, and oxadiazon. You might also find some older active ingredients such as benefin, bensulide, or oryzalin. Used according to label directions, each of these materials is effective in crabgrass control. And remember that you can always find the latest use rates and recommendations of herbicides in the lawn if you use the Pest Management Guide that is available at the Virginia Tech website (just search for “PMG” on the VT Webpage, www.vt.edu).
What does a PRE herbicide application mean regarding spring seedings of desirable turfgrasses? It generally means DON’T SEED! Most PRE herbicides will not distinguish between weed or turfgrass seed and will effectively control any of them for up to 3 months after the application. Two notable exceptions that you might explore (and these products are often only available through licensed lawn care applicators) are siduron, a PRE herbicide that can be safely applied ONLY TO COOL-SEASON turfgrasses at spring seeding for crabgrass control, and quinclorac, a PRE herbicide that has a pretty flexible window of application timing for many cool AND WARM-SEASON turfgrasses depending on the species. These are the only exceptions to herbicides that can be applied at seeding, and even they have special use precautions to ensure they work as desired.
What about “weed and feed” products? These are clearly some of the most popular PRE weed control/fertilizer products for homeowners because your taking care of two lawn management steps in one pass (weeding and fertilizing). The concept is pretty straightforward: the herbicide is impregnated on a fertilizer carrier. These products are usually well formulated for a reasonable application of N fertilizer to cool-season grasses in the spring (typically they provide around 1 lb of N/1000 sq ft total), especially if they contain 15-30% slow release N by weight. Ideally, no N-containing fertilizer would be applied to warm-season grasses until after complete greening has occurred, but this is not possible with these products if one is going to benefit from the PRE crabgrass herbicide. Therefore, try to select products that contain significant slow-release N so that you don’t get a lot of spring greening that could be lost by a late freeze.
What will ensure optimum PRE herbicide activity? First, as uniform of an application as you can make will improve control. Most products available to homeowners are granular in form and require a properly calibrated spreader to ensure uniform, accurate delivery. Many licensed professionals will use liquid applications, and these work well because they are using water as their carrier and as a diluting agent to ensure thorough coverage. Both types of treatment require irrigation within 24-48 hours of application to activate the chemical in the soil, so try to time the treatment with a pending rainfall or irrigation event. And remember, keep any of these products OFF the hardscapes; if you get it in your lawn it will stay put and will not pollute our water sources.
Any natural organic sources available for weed control? The answer is yes and the active ingredient is actually very effective in PRE weed control, but unfortunately it is very short-lived in the soil. The product is corn gluten meal. At its recommended application rates it also will be providing approximately 1 lb N/1000 sq ft. Research is currently being conducted to develop programs that might marry some of the desirable aspects of a natural weed control product like corn gluten meal with the predictability in response of our synthetic chemicals.
Finally—is the use of a herbicide absolutely essential to gain crabgrass control? The answer depends on what you are looking for in a lawn. For some folks, “green is good”… they’ll accept just about any kind of plant material in the lawn. The one problem with crabgrass is that it is so aggressive that it can rapidly outcompete many of our turfgrasses, particularly the cool-season grasses. Therefore, if a crabgrass-free lawn is truly the goal, a spring PRE treatment is likely required. However, as Dr. Askew always details in his presentations, the absolute best weed control is achieved by maintaining a healthy, dense turf. Best management practices such as proper mowing, nutrition, and irrigation go a long way in providing a weed free lawn with minimal chemical inputs. Montgomery Co. Extension Agent Barry Robinson has had a crabgrass-free lawn for many years and he does not use a PRE herbicide. Barry’s approach is to mow his cool-season turf as high as his cutting deck will go and utilize the competition from the turf to keep the crabgrass in check.
Has this information been helpful to you? Do you have other suggestions or comments regarding this or other podcasts that would serve your needs? If so, please contact us. And always remember that your local Virginia Cooperative Extension office is ready to assist you in attaining an environmentally friendly lawn—let’s keep our lawns green and our water clean!