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Why Didn’t Your Crabgrass Preventer Work?

August 19, 2015

In late summer some folks see crabgrass in their lawn even though they put down a preventer last spring. Their question: “Why didn’t the product work?”

First of all it is good to understand a little bit about how crabgrass grows. Crabgrass is an annual weed that germinates in spring when soil warms up, thrives in summer when it produces seed, and then dies when it frosts in fall. Crabgrass weed preventers kill the new plants as they sprout, not before they sprout. A dormant crabgrass seed can lay in the soil until the conditions are right for growth. If grass is mowed tall (2-1/2 to 3 inches) and the lawn is thick because it has been well fed, crabgrass has a hard time germinating. If the crabgrass preventer is active when it tries to sprout, the preventer will kill it. Crabgrass seeds germinate like waves hitting a beach. The first crabgrass sprouts next to a sidewalk or on a south slope where soil warms first in early spring. The next crabgrass to germinate is in a thin lawn where the sunlight and warmth helps the seed get started. The last crabgrass germinates in mid to late summer in newly exposed soil as grass thins from disease or insects or as the soil surface is broken from weeds being pulled exposing crabgrass seeds to the warm surface.

Crabgrass germinated in this area where the grass was thinned from a lawn fungus problem.  It is easier to kill before it gets big.

Crabgrass germinated in this area where the grass was thinned from a lawn fungus problem. It is easier to kill before it gets big.

Here are the top five reasons that crabgrass preventers may not work as well as expected:

First, if the granules were spread too light or areas were missed in the lawn, prevention will not last as long as expected.

Second, if the product was put down too late, you may see crabgrass growth near a sidewalk or on a south slope where the soil warmed faster in early spring and the seeds started to sprout before application.

Third, if the soil was disturbed after spreading by aeration, dethatching, or pulling weeds, crabgrass seeds will sprout as they are exposed and the crabgrass barrier has been disturbed or broken.

Fourth, the granules will be less effective if the lawn does not receive around a quarter to a half inch of water within 2 to 3 days of application. If there is heavy rainfall to create flooding and soil movement or if the spring/summer rainfall is significantly above normal, the prevention will not be as strong.

And Fifth, if your grass was weakened because it was mowed too short or there was drought in late summer or your lawn got a fungus or insect problem that thinned the grass, crabgrass could germinate if the crabgrass preventer was no longer active. (Crabgrass preventers generally last from 6 weeks to 4 months depending on the product used. Scotts Halts products last up to 4 months.)

There are two reasons Scotts Halts and Scotts Turf Builder with Halts work better than other brands. These products contain one of the top rated ingredients for preventing crabgrass and the granules are smaller to provide better coverage of each square inch of lawn surface. Competitive products that do not work as well use larger particles and a lower rated, cheaper ingredient to prevent crabgrass.

If you have crabgrass in your lawn now, you can spray it with Ortho Weed B Gon MAX plus Crabgrass Killer.

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  1. How do I reach you?

    Sent from my iPhone


    • Hi lenleone1
      I can help you figure out what is going on with your lawn. Give me your location and some info about your lawn condition or problems and what products you have applied this year. I will answer your questions right here on this blog. The side benefit to others is they will be able to understand your unique situation and will see my response and this may help them with their own lawn.


  3. Hi lenleone1
    Feed with Turf Builder WinterGuard about 6 weeks after your Weed and Feed. You can also still spray any stray weeds in about a month if you did not get them with the Weed and Feed application.

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