You cleared a nice swatch of land in preparation for a nice, green ground cover. Now you need to decide how many plants to buy. Realize that plants need at least two years to establish and grow strongly. The spacing between plants affects how quickly a newly planted area becomes a lush carpeted ground cover. Use mulch to suppress weeds around the new plantings.
Determine what ground cover plant you wish to grow. Whatever plant species you choose, note the expected mature width of a plant. For example, if you want to grow creeping phlox (Phlox stolonifera), its mature spread is about 24 inches. A creeping stonecrop (Sedum spp.) grows only 6 to 12 inches wide. Jot down these data on a note pad to make estimations and inform your decisions later.
Measure the area to be planted with ground cover plants with a measuring tape. For rectangular areas, multiply length times the width of the bed to determine area. For circular beds, measure the radius of the circle, the distance from the circle's center to the edge, and multiply it by itself and then multiply that product by 3.14. For example, a 16-foot diameter circle with radius of 8 feet yields 8 x 8 x 3.14 = 200.96 square feet.
Consult a plant spacing table, such as that provided by Wilson Brothers Nursery, to determine the number of plants needed to cover the planting bed.
Look at mature plant width size noted for your preferred ground cover plant from Step 1. The mature width of the plant determines the spacing distance required now. For example, if the creeping phlox gets 24 inches wide, plants should be spaced 24 inches apart, center-to-center in the ground. This allows each plant to grow 12 inches toward another, maturing when their tips just touch each other.
Consider slightly closer plant spacing for faster coverage by the plants. In the case of the creeping phlox, perhaps spacing them 12 to 18 inches on center from each other will make a denser-looking ground cover.
Compare the plant spacing options against the plant spacing table, as in Step 3. Note how placing plants more closely increases the number of plants and costs required for the area.