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Should You Kill Crabgrass Now?

August 28, 2015

What to do with Crabgrass in your lawn in late August: Kill it or do nothing since the frost will kill it in a month or two?

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. (click photo to enlarge)

First a little bit of background. Crabgrass is an “annual” weed, which means it germinates in spring, then in summer the plants produce seed and then it dies in fall with the first frost. Spring applied crabgrass preventers will stop most of the seeds from growing, however generally not all of them. Just when your lawn is thinned from fungus disease or insects, a stray crabgrass seed germinates in the weakened areas. The crabgrass then hunkers down flat where your lawnmower can’t reach and spreads to choke out your remaining good grass. Each plant produces seeds… lots of seeds! These seeds then germinate next spring so even more crabgrass plants can threaten to win the battle for your lawn.

If you decide to wait till fall to let the frost kill your crabgrass, you will miss the chance to kill it before it produces more seeds before it is killed by frost. So I would suggest killing it now before you get more crabgrass seeds for next year.

If you want to kill crabgrass without killing your good grass, spray it now with Ortho Weed B Gon MAX plus Crabgrass Control. You can use this spray on any lawn type except St. Augustine, Centipede and Bahia. This spray works best when you go after younger crabgrass plants. For larger crabgrass plants you may need to repeat your spray in 2 to 3 weeks. If your crabgrass is very thick with very little good grass and you are planning to seed, you may want to use Roundup to kill everything. The big reason is with the Ortho Weed B Gon MAX you will need to wait 4 weeks after spraying to plant grass seed, however with Roundup you will only to need to wait a week.

Dallisgrass is oftern confused with Crabgrass. Dallisgrass seed head on left and Crabgrass seed head on right.

Dallisgrass is often confused with Crabgrass. Dallisgrass seed head on left and Crabgrass seed head on right.  (click photo to enlarge)

One note:

One weed that gets confused with Crabgrass this time of year is Dallisgrass. Dallisgrass is a perennial grass that is common in the south and surrounding states to the north. Since it is a perennial grass, it comes back every year from roots so a spring applied preventer to take care of annual grasses that grow from seed each year does not work. Often the only choice is to kill this grass with a weed killer like Roundup. The downside is that Roundup will kill all vegetation in the area that you spray, however you can seed that area a week later. You may still have Dallisgrass next year because the plants produced seed all summer. The seed heads provide the best way to identify Crabgrass and Dallisgrass.

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12 Comments
  1. Oh oh I have both weeds you stated. I am using Crabgrass spray bottle. Thank you

    Sent from my iPhone

    >

  2. Hi lenleone1
    Hope you solve your weed problem. Let me know if you have more questions.

  3. Rob permalink

    Hi Ashton, I live in Massachusetts and despite all of my efforts for the last few years, my lawn is now completely crabgrass. Is it best to just spray vegetation killer and start over? If so, is it too late in the fall to do try to re-plant a new lawn?

    • Hi Rob
      Sorry about your crabgrass problem. A month ago I would have suggested you kill the vegetation and replant, however based on your location I would say it is too late. Even if you killed the crabgrass you are a week away from seeding and therefore you are likely bumping up against weather that will slow the establishment of your new lawn. Here is a link to a blog posting that may help you figure out why your crabgrass prevention has failed: https://tipsfromashton.wordpress.com/2015/08/19/why-didnt-your-crabgrass-preventer-work/
      You may find that you have more good grass than you think once the frost zaps your crabgrass. If you think you have good grass hiding amongst your crabgrass, feed your lawn now and again in 6 weeks to help your good grass fill in the thin areas that the crabgrass will be vacating.

  4. Rob permalink

    Hi Ashton, Thanks for the advice. What product works best for feeding now and then again in 6 weeks. In the spring, we generally get decent grass, but by the time the hot weather comes in July, the crabgrass takes over and overwhelms the good grass. What product should I put down in the spring to help prevent the summer crabgrass? is that the Scott’s with the halts?

    Thanks again!

  5. Hi Rob
    You can feed now with Turf Builder and follow up with Turf Builder WinterGurad in mid November. Next spring feed with Turf Builder Halts Crabgrass Preventer by April 20. This crabgrass preventer will not allow you to put down grass seed. If you need to seed you should use Scotts Starter Lawn Food plus Weed Preventer (the crabgrass preventer does not last as long, however it is the only choice if you plan on seeding in spring).

  6. Rob permalink

    Hi Ashton, Thanks again for your advice last fall. So far, we’ve seen some really nice grass come up. I followed your advice and put down the Winterguard last fall and then followed up in the spring with the Turf Builder with Halts. Our yard is now a mix of really nice, thick grass, and bare spots. We de-thatched all of the areas where the dead crabgrass was leaving just soil. My fear is even though we treated the entire yard with Halts, that the crabgrass will come back in the bare spots. While I know we can’t re-seed, could we top off the bare spots with compost and lay down some sod? Or will the Halts harm the sod?

    • Hi Rob
      The waiting period for sodding after putting down Halts is the same as seeding: 4 months. Typically sod will take longer to root to the existing soil after putting down a pre-emergent. You may be able to dilute the weed prevention barrier by mixing some compost into the existing soil before sodding as you suggested. If you want to try this on some of your most visible problem areas let me know how it works out for you.

  7. Rob permalink

    OK, thanks. I don’t want to risk putting the sod down. Do I just leave the bare spots as is and then reseed in the fall?

    • Hi Rob
      I think that is what I would do. You may get some late germinating crabgrass toward the end of summer as the crabgrass preventer nears it’s 4 month time frame especially if there is no competition from your good grass.

  8. Rob permalink

    Hi Ashton, So far everything you said in this string is working, so thank you for the advice! Now that all the snow has melted, the lawn continues to be a mix of grass and bare spots. I did some reseeding in the fall and am seeing that grass. Since we didn’t get any crabgrass in the bare spots, would you suggest re-seeding now or put down another round of Turf Builder with Halts to ensure that the crabgrass won’t make a comeback. Thanks again! I remain hopeful that the lawn has turned a corner.

    • Hi Rob
      Glad everything is working out so far. If your bare spots are only a few inches in diameter, I suggest you go with Turf Builder with Halts and allow your existing grass to fill-in the thin areas from spreading roots. If the bare areas are larger, then you could seed those spots with EZ Seed and use a crabgrass preventer that will not harm your new grass seed. This crabgrass preventer for seeding (Starter Lawn Food plus Weed Preventer) does not have as long of a crabgrass preventing residual as Turf Builder with Halts, however it is the only one that can be used when seeding.

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