Many weeds and groundcovers that colonize grass lawns where they are unwanted can be controlled or eradicated by depriving the invading groundcover of the sunlight its needs to survive. Raising the mowing height on your lawn mower allows the desired grass blades to grow longer, creating shade conditions at soil level. Shade prevents the groundcover from carrying out photosynthesis and producing nutrients for its growth. This can often be enough to kill the groundcover plant or stunt its growth so severely that the desired lawn species can gain control over the contested soil space and crowd out the groundcover species.
Determine the type of grass cultivar you have growing and understand the proper range of blade length your type of grass is best suited to. You can exceed it to help choke out groundcover, but you should know the optimal range for ongoing care.
Sharpen your lawn mower blades at least once a year. Longer grass blades are more flexible, which in turn means that it takes a sharper blade to slice through cleanly without tearing or ripping, which can damage the healthy grass and invite disease. Remember, the goal is to keep the desired grass healthy as well as long enough to shade the groundcover.
Raise your lawn mower blade height to the top end or just beyond the desired mowing height for your species of grass. If you are growing St. Augustine grass, it prefers to be mowed to a height of between one and three inches. Raising the mowing blade to three and a half or four inches will help provide more shade to the groundcover without doing harm to the St. Augustine grass.