I Seem To Recall That My Father, Who Was A Keen Gardener, Advised Placing A Slate On Top Of The Roots When Planting Clematis. Tru

Answer 1

This is, simply, to shade the roots from the sun. Clematis must have a cool root zone in order to flourish.

Here is a collection of articles to give you more information on the plant and its care:

https://www.gardeningknowhow.com/category/ornamental/vines/clematis