Answer: Crab grass and St. augustine are so similar that there is not a product out that will eliminate the crabgrass without harming the St. augustine. Here is your dilemma -- you have to remove all live crab grass to keep the lawn weed-free in the new year. This may mean pulling or digging it out. If you dig it out, fill in the bare spots with sod as soon as possible. Skip the plugs that do not completely cover the soil to block out germinating weed seeds.
You can also spot-kill any green-growing crab grass with the selective herbicide Agralawn Crabgrass Killer available at local independent garden centers. Or you can remove everything with a nonselective herbicide like Roundup or Finale. These areas should also be filled with sod when the weedy dead grass is removed.
Around mid-February, many gardeners like to apply pre-emergence crabgrass preventers found at local garden centers. Remember, these only prevent seed germination, and growing crab grass will not be affected. Also, they cannot be used immediately after sodding or plugging because they might prevent the good grass from rooting down.
Wish there were an easier solution!
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