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Feeling crabby about crabgrass?

Do you have crabgrass in your lawn? You may have it and don’t even know it until you see large brown spots in your grass during the winter. It is a common summer annual weed and unless you manage it early, you need to tolerate it.

I used to have a large butterfly garden and I remember pulling out crabgrass constantly in the early spring. I would curse it as I crawled on my knees, although when it filled in the bare areas of the lawn, I didn’t mind because at least it was green and it blended in well since it was grass.

Biology of Crabgrass

There are five crabgrass species in Florida – India crabgrass, blanket crabgrass, southern crabgrass, tropical crabgrass, and smooth crabgrass.

smooth crabgrass

Smooth crabgrass is prevalent in north Florida. Compared to southern crabgrass, the grass blade is not hairy. Photo credit: Denise DeBusk, UF/IFAS Extension Alachua County.

Crabgrass has thick blades that can be 1-2 inches long and their seedheads can have two to six “finger-like” spiked branches.

Once you have this weed, it is difficult to manage. One reason is because each crabgrass plant can produce up to 700 tillers and over 200,000 seeds. Another reason is for St. Augustinegrass, there is not a selective postemergent herbicide that can be sprayed on it after it comes out of the ground. This is because the postemergent herbicide may kill crabgrass but it could also kill your St. Augustinegrass.

Crabgrass germinates in the early spring when the soil temperatures are greater than 50°-55°F. Crabgrass will grow in areas where the turfgrass stand is weak. Due to its biology, the best way to prevent or reduce crabgrass is to have healthy turf.

Control Options for Crabgrass

  • Mow your turf at the recommended height (Table 1). If you have crabgrass, remove the clippings when the seedheads are present.
  • Apply the proper amount of fertilizer at the correct time. You should do a soil test to determine your nutrient needs and lime requirements.
  • Apply preemergence herbicides before the crabgrass germinates. For north Florida, this is around Mar 1. For central Florida, it is Feb 15. You should make a follow-up application 60 days after the initial treatment. The best preemergent herbicide has pendimethalin as the active ingredient. Atrazine can be effective as well.
  • Raise your crabgrass tolerance threshold – the whole “as long as it is green” philosophy. Know that you may have some crabgrass and just manage it if it gets out of control. If you can’t stand your lawn browning out in the winter from the weeds dying, then you can spread annual ryegrass, a winter annual, until it comes back. For an even more Florida-friendly approach, you can replace it with drought-tolerant groundcovers or perennial plants, especially if you already have problems with turf.
  • See Crabgrass Management Biology and Management in Turf for more control options.

    mowing heights

    From Mowing Your Florida Lawn:

Precautions for Using Preemergent Herbicides

  • Avoid applications near the root zones of palms.
  • Avoid using preemergents on new sod or plugs until they are well-rooted (at least 60 days after installation)
  • Don’t purchase a “weed and feed” product for this since the time you should apply the herbicide is before when you should fertilize so the fertilizer would be ineffective and possibly go into the environment.

If weeds have taken over your yard and you choose to rip it out and resod, you can apply glyphosate and renovate your lawn with sod during the rainy season.

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