Great idea - I have had great success growing the purple Jackmani clematis over large rhododendrons. This vine thrives in moisture-retentive but well drained soil - just add a shovelful of compost to your regular soil, loosening the soil in a one foot deep and two feet wide hole and set a young clematis vine into the soft soil. Clematis is one vine you can plant a few inches deeper than it was growing in the container. Plant this vine in the spring and add a tablespoon of Osmocote around the roots every April. This way the clematis will enjoy a slow release of fertilizer - too much plant food encourages aphid and other problems on clematis. Although I have had great luck with Jackmani clematis, other varieties to consider are The President,(also purple) Ramona (blue), Henryi (white) and the pink clematis anemone.