Yeah, I'm questioning the idea now.

What I might do for a splash of extra colour this summer is cut a piece of membrane to the diameter of the soil surface, with a slit to fit around the stem, then plant lobelia on top of that, so that when it dies off at the end of the summer I can lift the whole section off and throw it away...

Ceanothus likes it dry, apparently, would the water that drains through from this top layer (when watered) be enough to keep it healthy?