I suspect this is fast becoming the biggest nemesis we have experienced with lawn weeds in quite some time.
Virginia Buttonweed is hard to control, and there are only a handful of products that seem to work on it. Calls come in to GardenLine each weekend describing a low-growing, vine-like weed with tiny white flowers that look like propellers. (Take a look at these pictures.)
One potential control for Virginia Buttonweed is Image herbicide. It is 3.30 percent ammonium salt of imazaquin, but seems to only work April-June. The warmer it gets, the less effective Image seems to get. Ironically, during the cool autumn and winter monthsyou should avoid Image because it can decimate a St. Augustine lawn. In fact, the Image herbicide technical data specifically states,that it should not be used during the "slow growth season" which, for us, is the fall and winter.
The only other proven control is a broadleaf-weed killer that contains a combination of either 2-4D and Dicamba or 2-4D and Trimec. Not all Broadleaf Weed Killers contain either of these combinations. Many broadleaf weed killers on the market are simply 2-4D, Trimec or Dicamba alone. Two products sold in this area that contain one of the combinations are Fertilome Weed Out and Bonide Weed Beater for Southern Lawns.Another is Trimec Plus, a combination of Dicamba and 2-4D, but it's not as readily available in the market place.
These products should be used with extreme caution in the hotter months of the year - July, August and September. During the fall and winter months, you should test them with spot treatments. You should also pull up as much weed as you can. In the springtime, it is fair game, but it takes two applications during the growing season to be effective. But that's all ... more than two applications can also deteriorate the turf.
If you test in the fall and don't see much control, pull up what you can and wait for the spring and early summer for new growth.