When to apply a crabgrass preventer to your lawn

June 19, 2015

If your garden is otherwise a lush, green oasis, crabgrass can really put a damper on things. Here's how to get rid of it.

When to apply a crabgrass preventer to your lawn

Line of defence

The best weapon you have against this annual weed is crabgrass preemergence herbicide (also called crabgrass preventer).

  • You apply this product in the spring before the crabgrass seed sprouts. This granular herbicide works by creating a chemical barrier at the surface of the soil.
  • As the seeds begin germination, they take in the herbicide and die.
  • This sounds easy enough, but where you're likely to get it wrong is in the when. Apply too early and microorganisms and natural processes in the soil break down the herbicide. By the time it's needed, much of the product has lost its potency.
  • Apply too late and you've missed the early stage of germination when the herbicide works.
  • There is nothing in preemergence herbicide that kills crabgrass seedlings once they've sprouted.
  • You're just pushing around that spreader for the exercise, not to mention wasting expensive products.

Can you prevent it?

As far as prevention goes, you're out of luck until next spring. So how do you select the exact instant for application? Don't depend on the calendar. Pay attention to your grass instead.

  • Fill up the spreader with preemergence granules between the second and third mowing of the year. T
  • he window is short, only about a week and a half, when the soil hits the ideal temperature, about 11°C (52°F).
  • If you've had a particularly bad crabgrass problem, you're not done for the season. Chances are that the crabgrass will germinate and spring up later in the summer.
  • Preemergence herbicides have a life of about 50 days (check the label; product life spans vary). Once that chemical barrier breaks down, dormant crabgrass seeds, which can remain viable for years, may germinate into seedlings.
  • Or if your yard butts up against property that has a thriving crabgrass crop, you can bet that thousands of seeds will blow into your lawn, just when your herbicide is calling it quits.
  • You don't need to reapply the preemergence herbicide to your whole yard, but hit areas again where crabgrass thrives, like right next to driveways and walking paths.
  • If you're going to err on timing your crabgrass preemergence application, err on the early side. While you'll lose some effectiveness, you'll still probably kill lots of crabgrass. If you go late, you're likely to miss early sprouts.
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