Q: I have pennywort in my St Augustine lawn. How can I control it?
A: Like its weedy cousin, creeping Charlie, pennywort, also known as dollarweed, is tough to eradicate. Any of the three-way broadleaf weed killers (click for sources) will give moderate control but products that contain carfentrazone (click for sources) or quinclorac (click for sources) are better. Be sure to read and follow product ingredient labels. This will also inspire you to read and understand which lawn grasses these two chemicals can be used on.
Remember too that pennywort loves damp soil. A dry summer or fixing the damp soil area will control the weed on its own. It is easy to pull.