Mayo,
That's what I thought too! However, that is not what we've experienced.
I will admit that water probably has a lot to do with the "non-performance" of some of these plants.
We do not have an irrigation system, and the plants have rain as their primary water source.
Supplementary watering (i.e. garden hose) only happens when there has been no rain for somewhere around 10 days.
So perhaps those that struggle in full sun are not as drought tolerant as those that don't.
Perhaps someone can contribute on the water needs of different cultivars.
Giles
