Clematis is not picky with regard to soil pH; slightly acidic would be best, so around 5.8 to 6.5 will do. The main cultural requirement of clematis is that their roots remain on the cool side and evenly moist (but in well drained soil). Although the roots do best shaded, the foliage needs full sun for most of the day. A 1-2 inch layer of mulch will help to keep the roots cool and evenly moist.