Are you asking about this plant?
Florida Betony (Stachys floridana)
The good news is that it's a native (so shouldn't be labeled as a weed,) a super nectar plant for bees/butterflies/hummingbirds, and disappears during the hot part of the year. It's welcome in the lawn (especially since it's green now while grasses are dormant) but I don't want it in beds either. :
http://plants.usda.gov/core/pr...
The bad news is that it's quite a spreader, and very weedY where unwanted. Not easy to evict at ALL. One could find and remove the rhizomes & tubers. Easier said than done since the rhizomes creep in all sorts of directions for surprisingly long distances and break so easily. I've dug up rhizomes before and was trying to find one, just ONE, to show DH why I couldn't just get rid of that stuff by pulling it up "by the roots" like most other unwanted plants, but couldn't, and abandoned plans to try to evict by only shovel at that point.
If you're fastidious about pulling up new growth, you can tire it out eventually. I managed to do that to a patch of it in my Mom's yard, but I think realizing what it was the 2nd year it showed up helped, may not have had time to form big rhizomes yet.
If you have huge patches, or tree roots that prevent thorough dig-searching, you may need to smother for a year (or more? IDK how long yet.) I have HUGE patches of it too, some are where I'd planned to make new beds. I'm going to have to smother all around a tree or abandon plans to have a bed there. That will be tricky since there are big, exposed roots that probably shouldn't be buried.
Want to compare "before" pics, then our progress during this battle? I'll take some pics today of what I realized I'm dealing with.
What have you done so far that hasn't worked?