You found this article because you are looking at how to get rid of Doveweed and all I want to say is "I'm so sorry you have a doveweed problem because I'm right there with you!"
Doveweed is one of the toughest weeds to deal with here in Florida. I've had some very public battles with it.
Where Doveweed Comes From
Doveweed is a summer annual weed which means seeds that are in the soil germinate in spring, the plants grow all summer, maturing and dropping more seeds, then they die when temps cool off in the winter.
Most of you are really seeing it now in later August because prior, it looked alot like your St Augustine grass. In fact, I've heard from many folks over the years that they think the doveweed looks enough like their St Augustine that they just live with it. This, however, is a mistake. Doveweed will take over large spots in your lawn, thinning it out year after year.
Getting Rid of Doveweed
If you have doveweed in your lawn right now, I recommend you get after it right away. The more mature it gets, the harder it is to eradicate.
You will want to use a post-emergent herbicide and the best one I know is called Celsius. You will mix this into a pump sprayer (1 teaspoon per gallon), along with a surfactant. 1 gallon of spray mix covers 1,000 sq ft of lawn space. You will probably just have to spot spray. Coat the leaves of the doveweed until they are shiny wet like you see in this photo.
It may take a couple of applications to wipe it out - just follow the label on the intervals.
You can use Celsius in St Augustine grass lawns, bermuda, centipede and zoysia. It has no temperature restrictions so it can be used in the heat of summer when the doveweed is raging.
If you have a major problem this year, I highly recommend that next year you invest in Pennant Magnum. This pre-emergent will suppress doveweed and even prevent a good portion of it. I've been using it now for a few years and have seen the good results from it.
The jug seems expensive, and it is up front, but the use rate is very low so that jug will last the typical DIYer many many years. It's best to get the application down in spring prior to soil temps reaching 60F.
If you utilize these two strategies together, you can eliminate a doveweed problem in a couple years. Just be diligent!