I have some clematis in a large pot and was thinking of planting lavender next to it to shade the roots. Will the the lavender's root system compete with the clematis's?
How deep are a lavender's roots
You may want to look at http://www.purplehazelavender.com/humidity.htm that discusses transplants and lavender's root ball.
Seems that lavender does not like alot of water as compared to clematis, although that is just in my experience in Zone 8a. I have salvia growing about 11/2' -2 ' away from my clematis....ofcourse that is not in a pot.
I got this info from Texas Gardening: temperatures and cold hardiness/heat tolerance