Clematis vines love full sun, but the roots need the soil to stay cool and retain moisture (but still drain so the roots do not sit in a bog). The roots need to be protected from the direct rays of sunlight year-round.
A layer of organic mulch 2 to 3 inches deep will help conserve moisture, block sunlight and absorb the day’s heat without transferring it to the roots. Low-growing perennial shrubs will block sunlight year-round and keep the roots cool. Brick pavers, stones or boulders strategically placed block sunlight, keep the roots cool and prevent moisture loss from the soil.
Using all three elements adds interest to the base of the clematis vine. Try mixing a base planting of perennials with a decorative boulder. The boulder should be on the south to southwest side of the clematis, or the side that gets the most direct sun and heat. Mulch the entire area to conserve moisture throughout the clematis root zone and beyond.
Shallow-rooted perennials do not compete with clematis as much for nutrients and water. The Ohio State University Extension recommends Artemisia "Silver Mound," hardy perennial geraniums, creeping phlox (Phlox subulata), coralbells (Heuchera spp) and candytuft (Heuchera sanguinea).