A: The weed you brought to the office is called Japanese clover, Lespedeza striata, or Common Lespedeza. It is a prostrate, freely-branched summer annual. The leaves are very tiny and they form a dense mat. Japanese clover produces small purple or pink flowers, which are actually quite striking up close but the plant is basically too small to have much ornamental value. Flowering occurs during July – October and it reproduces by seed only.
Common Lespedeza can found in fields, pastures, open woods, stream banks, roadsides, railroads, waste ground, disturbed and cultivated sites. This plant is utilized for horse and cattle forage. It was brought to this country around 1904 and has spread rapidly.
It is difficult to kill a mature, seed producing plant once it has reached its maturity. When I say maturity I mean it has reached the stage of its life cycle where it is capable of producing seed. You might consider pulling it up prior to it releasing seeds and/or use a pre-emergent herbicide next spring to possibly prevent the dormant seeds from germinating. Of course, our major focus should be taking care of the lawn so it is strong by proper fertilization, watering and mowing. Check out the University of Florida website on residential St. Augustinegrass, http://edis.ifas.ufl.edu/LH010