Crabgrass is a common lawn weed that generally appears in late May in the North and as early as February in the South. The leaves of crabgrass are wider than typical grass blades, and the stems haphazardly fan out in a low profile that seems to elude mower blades. The best way to remove crabgrass is by using a pre-emergent herbicide, also known as a crabgrass preventer. If you have a lawn, you need to know what type to use, when, and how to apply a crabgrass preventer.
Because crabgrass produces so many seeds, it cannot be controlled in a single herbicide application. Begin to apply crab grass preventer in early spring to prevent crabgrass, chickweed, poa annua, and other common lawn weeds from germinating, before they start to spread. Apply before the third or fourth mowing to protect all season long, and commit to a multi-year plan of prevention and control to rid your lawn of the unsightly weed.
|When to Apply Crabgrass Preventer|
|Best month for pre-emergent?||Northern states IN May and Southern states in February/March|
|Best soil temperature?||Consistently 55 to 60 degrees Fahrenheit when crabgrass is germinating|
|When is it too early?||A month before your last freeze date or chemicals can break down too early|
|When is it too late?||Late spring; crabgrass has taken over|
|Best time for post-emergent?||Several applications late spring into summer, final application between August and October|
Types of Crabgrass Preventers
A pre-emergent herbicide, or crabgrass preventer, stops the seeds left from the last growing season from germinating. Here are types of pre-emergent crabgrass preventers:
- Liquid: A liquid preventer work efficiently to cover a large lawn. It is also effective for killing seeds in tight cracks in patios, driveways, and sidewalks. A liquid solution may be concentrated and require mixing and a sprayer.
- Granular: Applied with a spreader and then watered in, granular formulas work well for small lawns. However, granular forms do take longer to work than liquid.
- Synthetic Herbicide: Uses chemical herbicides to eliminate crabgrass.
- Organic Herbicide: If you worry about any effects of using chemicals, there are organic methods that will kill crabgrass such as corn gluten or vinegar. These work much more slowly and must be applied at exactly the right time and temperature. Using vinegar to kill crabgrass can also kill the grass you're trying to cultivate
- Selective Herbicide: This product typically has chemical ingredients that target only the weeds you choose to kill, but will not harm other plants or grasses. Use selective products with ingredients, such as dithiopyr, benefin, and oxadiazon if you want to only kill crabgrass.
- Non-selective Herbicide: This chemical product typically kills all types of weeds, but it can also affect grasses. This is the product to use if you want to kill more than crabgrass.
When to Apply
Getting the timing right is critical to successfully controlling this weed. The ideal time will vary depending on where you live and soil temperature. How much sun and shade your lawn receives affects the soil's temperature. Here are general guidelines:
- Northern states: Apply at the beginning of May
- Southern states: Apply potentially as early as February into early March
Many gardeners follow the rule of applying crabgrass killer in the early spring shortly after the flowering period of the forsythia bushes. However, many lawn companies prefer to apply crabgrass killer just as the forsythia bushes are beginning to flower. To get a more accurate time on when to apply crabgrass killer, use a soil thermometer to take a reading a couple of inches down into the lawn.
Depending on where you live, begin to take readings with a soil thermometer in early February to make sure you don't miss the best time for application. You can apply a crabgrass preventer when the soil temperature reaches 55 to 60 degrees Fahrenheit for a few days in a row because that's when the majority of crabgrass seeds begin to germinate. Use a soil temperature map by typing in your exact location to get data on soil temperatures in your area.
How to Use a Soil Thermometer
Using a soil thermometer correctly to take this reading is simple. You will need a clean soil thermometer, screwdriver, tape measure, and a marker or duct tape. Follow these basic steps but always read the product label for exact directions:
- Measure two inches from the tip of a screwdriver toward the handle.
- Mark this measurement with a magic marker (or use duct tape).
- Plunge the screwdriver into the ground until this two-inch mark is at ground level. This gives a pilot hole so that you do not risk breaking your soil thermometer.
- Insert the soil thermometer into the pilot hole to get your reading (follow instructions for how long the thermometer needs to stay in the soil before withdrawing it for reading).
- Use the thermometer in both shady and sunny spots around the lawn.
How to Apply Crabgrass Preventers
Always read the instructions on the product for exact directions. Here are a few tips:
- Do not apply the product to grass that is already wet.
- Apply granular formulas right before rain is forecast so that the rain can help water the preventer into the soil.
- Do not apply a preventer if the forecast calls for drenching downpours that can dilute the formulas.
- Do not seed your lawn at the same time you are applying a crabgrass preventer. Disturbed soil can make the herbicide less effective.
If you miss the window of opportunity for the application of a pre-emergent herbicide in early spring, use a post-emergent crabgrass killer.
For small clumps of crabgrass growing through cracks in your sidewalk or driveway, simply remove the weeds by hand or with a sharp hoe.
Biology and Management of Crabgrass. University of Massachusetts Amherst Center for Agriculture, Food, and Environment
Weed Control in Home Lawns. University of Georgia Extension