I saw your pix on the lawn forum. I tried something this year that worked for some of my weeds. It is a three part process so it might not work for a large lawn, but here it is.

I had oxalis in full shade.

1. Sprayed with soapy sugary water as a sticking agent. 1 tsp of dish soap and 2 ounces of molasses to a quart of water.
2. Dusted the wet leaves with baking soda and allowed it to dry. Load a sock with baking soda and tap the side to let the dust fall out onto the weed. It does no damage to the sock so you can use any sock.
3. Sprayed with strong vinegar.

Even in full shade the oxalis melted away. Here are the pictures. My apologies for the picture quality. I set up the tripod and did not realize how bad they would look until it was done. The target weed is on the left side of the tree trunk. In the first one I had done steps 1 and 2 the previous day with no ill effect.

Second one was taken immediately after spraying with vinegar. You can see the foaming on the leaves.

Third was taken after 5 minutes. You can see the yellow flower petals wilting a little. The rest of the leaves were doing the same but you can't really tell.

Fourth was taken at 30 minutes. Wilt is evident.

This one was taken an hour later. The plant has wilted away just like I would expect on a hot, sunny day.

Here's the first one again for direct before and after comparison.

Again, this was in full shade (north side of tree) on a day in the high 60s.

dchall, will this method work for weeds in St. Augustine grass? For some reason the weeds are real bad this year.

Thanks - Mary

The vinegar will kill the St Augustine but the baking soda will not. That was the reason for trying the baking soda...to see if other weeds besides crabgrass would be killed. So far only crabgrass is sensitive to it. Could be bermuda is also sensitive to baking soda, but that was found in a separate experiment.