It looks like a baby mimosa tree and has little pea like seeds on the underside of the weed.

Need help getting this weed under control.

Some areas have grown up to over a foot tall and I was able to cut those with shears. But the problem is in an area where the lawn has not been kept up (to the side of a garage where only the dog seems to venture) this stuff is like a carpet on the ground. The mower will not hit it, and it would be next impossible to cut all of it out.

Any ideas on what to use to rid this area of the weed yet still allow for the St. Augustine (that is still there) to live would be greatly appreciated.