Ground-covering perennials are ideal for covering bare spots in the garden, filling in challenging areas such as on slopes and around trees, and choking out weeds. Direct Gardening offers a wide selection of ground-covering plants including flowering ground covers and those grown primarily for their foliage. Some popular perennial ground covers include sedum, ajuga, border lilies, creeping phlox, hostas and daylilies. When buying ground-covering perennials for sale online, make sure the plants will grow in your hardiness zone. To determine how many ground cover plants to buy, determine how much space you have to fill and what the plant's mature height and spread is. Other factors to consider are their sunlight requirements, water needs, soil requirements, foliage and flower color, and bloom times.