Recently I applied Bayer Southern Weed Killer which contains the active ingredient 2,4-D. From what I've read this herbicide is safe for St. Augustine if applied as directed. I have noticed that since applying the herbicide that my St. Augustine is yellowing and is becoming very brittle. I am able to easily rake piles of green grass. See that attached picture.

I'm wondering if this is due to the 2,4-D or because of something else. The only other thing that I can think of is that last week I mowed the yard while it was wet. I didn't like the idea of mowing wet but it needed to be done between rain events. Maybe this is just the clippings that were not mulched.

http://i11.photobucket.com/albums/a199/rack04/IMG_0253.jpg

Generally, 2,4-D is OK on St Augustine (it is labeled for it), but you can't over-apply and you can't apply when it is very hot, and you can't apply to a stressed lawn. In your photo, I see minor yellowing leaves which look like herbicide damage and minor grey leaf spot, neither of which would cause all that green grass to break off from the runners. However, yellow and brittle as you describe would be herbicide damage.

I would stop raking the green grass. Repair of herbicide damage on St Augustine can be fast or slow, depending on how much you applied, but it will eventually recover. Water it a little more often than you were doing previously. You didn't do this by mowing when wet.

Thanks for the information. DFW is supposed to get some heavy rain this week so hopefully this will help the situation. I suspect the temperature was the culprit. I attempted to apply the herbicide when the weather forecast showed lower temperatures. Typically I water 1 1/2 to 2 inches a week but with all the rain we've had lately I haven't needed to water in 3 weeks.

I have worked very hard this growing season to repair the neglect that this yard has seen in previous years. The original homeowner that bought the house in 1954 died in 2008. The house has been vacant since then and the lawn has seen it's fair share of neglect. I just hope all my hard work hasn't been wasted.

If you hadn't watered in 3 weeks it may have been stress plus temp. I don't know how much rain you had in that time but if you didn't have 3-4 inches soaked in, that probably was a contributing factor. Warm season grasses are generally quite sensitive to stuff like this. In fact, a number of 'southern lawn weed killers' are the same formulation as 'northern lawn weed killers', just at lower concentrations of active ingredient(s), as companies know that homeowners aren't always on top of things like this, and they try to minimize potential damage of overapplication by settling for a lower kill percentage when correctly applied. They should do a better job of labeling, but they are afraid of losing sales. "Apply only to lush, healthy lawns" probably wouldn't go over well on a weed killer.