It probably wasn't the tannin in the roots, it was probably a water issue. I have many azaleas under my oaks, but I keep them watered. Or that azaleas do better in morning sun, too much sun stresses them.

Thanks for the info, creatrix. I don't think water was the issue with the azaleas as I was pretty careful about that. However, you may be right about them getting too much sun in the hot part of the afternoon. Azaleas confuse me since they seem to like more shade than I gave them, but I see them everywhere here stuck right out in the middle of the lawn with no shade trees around. I'm not giving up on them though because the colors are just so beautiful. I'll just pick a better spot next time.

At first I thought about asiatic jasmine, but it doesn't bloom. However it will live under oak trees and can handle sun just fine and stays green (here anyway) all year. Then I thought about ajuga? It can handle some sun and shade, plus it blooms and creeps. Would that be tall enough for your urns? Just a thought. Perhaps someone else will come along with some ideas. Trudyjean

I live in the Pacific NW but my favorite groundcovers under trees (dry shade, root competition) are:
Vinca major and minor- many variegated forms
Groundcover comfrey, Symphytum iberis- aggressive green rough leaves, blue/pink fading to white blooms in spring, and variegated form Goldsmith- bright, not as aggressive
Anemone nemarosa- blooming clumps in spring, goes dormant and disappears in late summer
Geranium macrorrhizum- blooms, very drought tolerant, spreads quickly
Lemon balm
Wintergreen- wonderful glossy evergreen leaves, little tubular flowers, red berries, takes a while to fill in

I would not plant most of these together except the Geranium, Comfrey, and Lemon balm.