March into the battle against crabgrass
As the weather warms, outdoor warriors prepare for battle: us vs. weeds.
Spring lawn care for cool-season fescue lawns does not typically include seeding or fertilizing. However, spring is the right time to prevent and control the gardener’s arch nemesis: stealthy crabgrass.
It’s important to understand your adversary. Crabgrass is a warm-season summer annual grass that germinates right before the first growth period of lawn grasses in the spring. Its seeds lurk in the soil until the conditions are right (i.e., soil temperature above 55°F and adequate moisture).
Because of its early spring lead, crabgrass has a competitive advantage against cool-season lawn grasses (e.g., fescue, bluegrass and ryegrass).
Crabgrass has a horizontal growth pattern (unlike turfgrass, which grows vertically). It is so vigorous that it chokes out lush fescue turfgrass.
The best tactic is to be preventative, rather than try to fight crabgrass once it has taken hold. The key to success is to apply preemergent herbicide before the weeds emerge.
Virginia Tech’s turfgrass weed scientist, Dr. Sean Askew, determined that preemergent herbicides should be applied when forsythia starts to drop its bright yellow blossoms.
Preemergent products also combat other annual grass foes such as barnyardgrass and goosegrass, as well as a few annual broadleaf weeds.
What are best herbicides?
For crabgrass prevention, look for products that contain one or more of these active ingredients: benefin, benefin plus trifluralin, dithiopyr, pendimethalin or prodiamine.
These chemicals are not specific; therefore, desirable grass seed cannot be sown when preemergent herbicides are applied.
However, there is one preemergent herbicide that can be safely applied prior to or at seeding of cool-season turfgrass in the spring: siduron (i.e., TupersanTM). It’s available to lawn care professionals, but homeowners may have difficulty finding the product in stores. It is available on websites.
You may find “weed and feed” products available for sale in garden centers and big-box stores. But be cautious of adding fertilizer (specifically nitrogen) to your lawn in the spring.
Here in Virginia, nitrogen can overstimulate the shoot growth of cool-season turfgrasses at the expense of the root system. This added fertility in spring can injure tall fescue and lead to decline in the turf during the summer months.
If you choose a crabgrass preventer that also contains fertilizer, choose a product with slow-release nitrogen, which should be indicated on the label.
An organic alternative, corn gluten meal (CGM), works against crabgrass by releasing a protein that slows the development of weed seedling roots, leaving seedlings vulnerable to drought. Therefore, CGM is not effective in periods of extended rainfall.
Some other weaknesses with CGM are that it lasts only a few weeks, thereby requiring two to three applications. As with most chemical preemergents, it cannot be used when seeding.
It should also be considered a weed and feed product. If you want to try CGM, perhaps use it once in early spring, and add a half rate of synthetic preemergence crabgrass herbicide, making sure that the herbicide does not contain any fertilizer.
Once you have purchased the appropriate preemergent herbicide, use a broadcast spreader to distribute the granules.
It is necessary to water preemergent herbicides soon after application. Most products must receive at least one-quarter inch of water within 48 hours of application to keep the herbicide from decomposing due to the effects of the sun. Hopefully, Mother Nature will supply rain this spring, but a sprinkler is the next best method.
Preemergent herbicides are typically active in the soil for six to eight weeks. Some lawn care strategists recommend a second application around Memorial Day. If it’s hot and dry by late spring, the second round may not be necessary.
Don’t mow too low
Although crabgrass suppression is a battle, most weed problems in Virginia can be attributed to a single mistake: mowing the lawn too short. Keep your mower set at a minimum height of three inches.
For long-term success, the best strategy to control weeds in turf is to nurture a healthy, dense lawn by following correct cultural practices.
Remember to conduct a soil test every three years to determine your nutrient needs, and diagnose insect and disease problems correctly before treating. Your local Cooperative Extension office can help.
Note: Always follow herbicide label directions and obey label warnings. Do not assume that more product is better; often more product may cause turf or environmental damage.
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.
Lela Martin is a Master Gardener with the Chesterfield County office of the Virginia Cooperative Extension.