Dollar weed is a warm-season, perennial weed that can infest St. Augustine grass when the turf becomes week or is persistently over-watered and wet. Correcting cultural practices of irrigation and mowing are the first steps to controlling dollar weed and can be followed by the use of atrazine herbicides, if needed, according to Clemson University.
Reduce the amount of water being applied to your St. Augustine lawn each week. Keep it moist under the thatch but not consistently wet. Stick as close as possible to 1 inch of water per week applied in a single, deep watering session. More frequent and ample watering will drive more dollar weed growth.
Mow your St. Augustine lawn to its proper blade height for optimal health, which is between 1 and 3 inches. When growing St. Augustine in shade conditions, keep the grass at 3 inches for optimal performance.
Spray young dollar weed plants in the spring and fall with an herbicide containing the active ingredient atrazine. Select a product designed for St. Augustine and/or centipede grasses. Apply according to product label directions and only use on established lawns that are in their active growth cycle.