That is a weed called doveweed, a warm season annual weed that begins germinating when the soil has warmed in late spring. It loves wet conditions, so a good practice is to not overwater but rather to water with a good soaking on an infrequent basis. Mowing St. Augustine turf high (3 ½” or more) is also helpful in slowing the spread of this weed.

Atrazine is an herbicide that does a pretty good job of both preventing seeds from establishing and controlling newly sprouted weed plants, because it is absorbed both by emerging seedlings and the roots of established doveweed plants. Atrazine is most effective when applied right before doveweed begins to emerge, in late spring. Celsius is a herbicide that can help control existing doveweed plants after it is too late to prevent them with a pre-emergence herbicide, but should be used when temperatures are not much above 90 degrees if possible, to avoid slowing the growth of the St. Augustine lawn.

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Dennis,

Yes it is costly. Unfortunately not much works on this weed and the Celsius goes a long way and is good for a number of broadleaf weed problems.

A second option would be products containing Trimec that are formulated for southern lawns. These can be damaging to a St. Augustine lawn when daytime temps are above the mid 80's, which significantly narrows the window of opportunity, and will likely require more than one application.

One thing to keep in mind is that this annual weed will be dying back with the arrival of cold weather and will return when things warm up next spring, so treating this late may not be very helpful.

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