Q: I have a pesky weed which I don’t have any idea what it is or where it came from. I feel like it showed up because of my poor soil. I have dug up this weed, made sure no roots were left, brought in some good rich dirt, then sprinkled Preen all around. This weed still came back to haunt me! Could you please identify it and give me some suggestions to control it?
A: Charlie has crept into your landscape: “Creeping Charlie” to be exact: Glechoma hederacea. This common weed is also called ground ivy and gill-over-the-ground. Notice that it is pleasantly fragrant when you squeeze it. This, and its square stems, tell you it is a member of the mint family.
Since Creeping Charlie is a perennial plant, your pre-emergent Preen had little effect on it.
A better strategy is to lightly spray every four weeks with one of the broadleaf weed killers (click for sources). You should have good control after three applications.
If the weed is in centipedegrass or St. Augustine, spray very lightly to avoid hurting your turfgrass. Follow the label exactly.
You could also choose to simply pull it up but this will take several months of vigilant pulling to gain control.