St. Augustine grass is a widely used grass in the southern United States. It is a warm-season grass, which means it prefers warm and humid weather. It is a tough species of grass that requires little maintenance. St. Augustine grass can spread and choke out other nearby plants. However, it isn't that complicated to keep St. Augustine grass from spreading into unwanted places. With a few tools and materials, you can keep the grass where you want it---in your yard.
Dig a narrow trench approximately 3 inches deep along the perimeter of your garden, or wherever you want to keep your St. Augustine grass from spreading. Keep the sod intact so you can place it back after you fill in the trench later.
Take your garden edging and set it in the trench. If you are using the standard 4-inch garden edging, then you should have 3 inches below the ground level and 1 inch above.
Fill the trench back in and put the sod back into place. Your St. Augustine grass should not be able to spread under or over your garden edging easily.