Just about every book and website tells you keep Clematis roots cool. Planting them so that the roots are shaded is a gardening myth! Clematis roots don’t need to be any cooler than any of your other perennials.
Keep Clematis Roots Cool
Commercial growers in California grow clematis in black pots sitting on the ground. They grow just fine. If heat was a problem, they would not use black pots, and they would shade the clematis in some way.
More and more clematis are grown in containers – providing a very warm root zone.
The first thing to understand is that all plant roots like to be cool. On a hot day you only have to dig down a very short distance to find cool soil – that is what plant roots like.
Clematis tend to be planted deeper than most plants so that clematis wilt does not kill the plant. In northern climates the crown should be planted several inches below the surface of the soil. That means the roots are even deeper, and so they naturally get the cool soil they prefer.
A lot of references suggest planting other plants right at the base of clematis to keep them cool. This is not a good idea. The other plants keep the crown too wet which can lead to rot. More importantly, the other plants can compete with the clematis for root space which is never a good thing.
Plant your clematis deep, and give them space. Mulch around the plant will be beneficial.
1) Debunking the Tallest Tale About Clematis: https://secure.clematisqueen.com/content/debunking-tallest-tale-about-clematis
2) Photo Source: Dietmut Teijgeman-Hansen