Digitaria (crabgrass' botanical name) is a warm-season annual that reproduces by seed. It grows in clumps with stems that radiate out from the center and resemble crab legs. This opportunistic grass is the nemesis of homeowners wanting to cultivate a manicured lawn, perennial flower beds, and meticulous walkways. It often appears in the spring and proliferates in the hot, dry conditions of the summer if it's not eradicated first.
If crabgrass is a problem in your yard, luckily, you have more than one arrow in your quiver to remove it. The most natural way is to cultivate a healthy lawn and outcompete the pesky weed. Pulling the weed in the spring, providing your lawn with the proper nutrients, and then reseeding where necessary should get the job done.
For a tenacious crabgrass infestation, you can use post-emergent herbicides that kill crabgrass well after it has germinated. However, post-emergent herbicides tend to be effective only on young plants. You must be vigilant in detecting them and then act promptly when applying the herbicide. Pre-eminent herbicides, on the other hand, kill crabgrass before seedlings emerge, making it a more effective way to get the job done and allowing you time in the season to cultivate a lawn where there once was weeds.
When to Kill Crabgrass
It helps to familiarize yourself with the plant's life cycle to effectively eradicate crabgrass. When spring soil temperatures (at a depth of 2 to 3 inches) reach 55 to 60 degrees Fahrenheit, crabgrass seeds start to germinate. From mid-summer through fall, the plant produces more seeds in an attempt to increase its numbers. Once the first frost arrives, the plants themselves (but not the seeds) are killed.
When organic methods are used, gardeners must wait until the seedlings are big enough to hand pull, roots and all. While crabgrass doesn't spread via its roots, it's still essential to remove the whole plant in the spring before amending the soil and reseeding.
Pre-emergent herbicides (also called "crabgrass preventers") come in either a granular or a liquid form and kill crabgrass seedlings right as they germinate. Think of pre-emergent herbicides as forming an invisible shield across the soil's surface that stops emerging crabgrass dead in its tracks.
Equipment / Tools
- Gardening gloves
- Long-sleeved shirt and pants
- Pitchfork or weeding tool
- Seed and fertilizer spreader or weed sprayer
- Organic compost
- Grass seed
- Pre-emergent herbicide
Killing Crabgrass Organically
You don't really need chemicals for crabgrass control. While it's easy to kill crabgrass with herbicides, the best way to remove it and control it is by first pulling it, and then cultivating healthy new grass.
Soak the Soil
Water the problem area with a sprinkler or garden hose, then wait about 30 minutes to allow the water to soak in.
Pull Out the Crabgrass
Using a pitchfork or weeding tool, remove young crabgrass when it's big enough to rip out, roots and all. If you're working with a large patch, this could take a while.
Add organic compost to the area in question. Work the soil so that it is thoroughly incorporated.
Apply Grass Seed
Seed the area with grass seed, and then place a layer of weed-free straw over the top to maintain moisture and protect the seeds from being eaten by birds.
Keep the Soil Moist
Water the area multiple times daily, depending on moisture conditions. Make sure the soil is consistently moist until the seeds sprout.
Switch to Deep Watering
When the grass is long enough to mow, start irrigating the lawn more deeply and less frequently. Crabgrass is a notoriously shallow-rooted weed; if you water too frequently and for short duration, it will help it thrive.
Mow the New Grass
To maintain your new patch of grass, mow it high. This means leaving grass at a height of 2 1/2 to 3-inches tall. Doing so will allow the lawn grass to "protect its own turf," depriving crabgrass seeds of the light they need to germinate.
Weed as Needed
Throughout the season, pull any remaining crabgrass plants that sprout up. By next year, your yard should be free of it.
Killing Crabgrass With Pre-emergent Herbicides
To successfully kill crabgrass with pre-emergent herbicides, timing is of the essence. Apply pre-emergent herbicides before germination, which usually coincides with the blooming of lilac bushes. Once you kill the weed from its roots, you can then reseed the area after a waiting period.
In early spring, check the weather and select a time to treat when a steady rain is in the forecast. Water actually activates pre-emergent herbicides.
Put on gloves and protect yourself with long pants and a long-sleeved shirt. Measure the lawn area and refer to package directions to mix the proper herbicide application amount.
Apply the Fertilizer
Spray or sprinkle the herbicide over the area in question. Make sure to apply it thoroughly and uniformly, taking care not to miss a spot.
Reseed the Area
Wait two to four months before reseeding the problem area, and then follow irrigation and mowing recommendations to prevent weeds.
Tips for Killing Crabgrass
- In the fall, fertilize or compost more heavily. This will help beat back the weeds and provide nutrients for your lawn. Also, don't let bare spots remain uncovered for long or opportunistic crabgrass will take root. Fill in the bare spots each fall by overseeding.
- Re-apply pre-emergent herbicides if you question your product's coverage. Because crabgrass seedlings do not all germinate at once, re-application can kill some of the later-germinating seedlings.
- Make sure not to aerate the lawn after applying pre-emergent herbicides. The act of doing so will deactivate the barrier and allow weeds to sprout.
- You can also avoid chemicals altogether by applying an organic pre-emergent herbicide, like corn gluten, instead. This natural product will suppress crabgrass germination, while also acting as a fertilizer for your lawn.
Working With Pre-Emergent Herbicides
There are many different types of pre-emergent herbicides for killing crabgrass. Dimension (with the active ingredient dithiopyr) is safe to use on most lawn grasses and provides long-lasting coverage—an important consideration since not all crabgrass seeds germinate at once. Dimension also displays some effectiveness as a post-emergent herbicide.
Tupersan (with the active ingredient siduron) will not damage germinating grass seed. In fact, its active ingredient is often combined with starter fertilizers for new lawns. This makes Tupersan a convenient option for those starting new lawns from seed.