|Exploding Myths about Clematis
There are but two. One states that a clematis likes its feet in the shade and its head in the sun. It is not the shade that the roots require, it is the water that lies in the shade. If clematis are given adequate watering and adequate mulching they will flower in full sun. A second myth states that clematis enjoy an alkaline soil which comes from the observation that they flourish in chalk soil. It is not the alkalinity, however, that encourages the clematis, rather it is that chalk holds deposits of water and this is what the plant is after. Given water it will flourish in any soil as long as there are no extremes of acidity or alkalinity.
See: Howells, J. The Rose and The Clematis as Good Companions. P.43.