Juglone is found in English Walnuts, though to a lesser degree than the infamous Black Walnut. It is found in the soil, in the fallen leaves and nuts, AND in the run-off from the canopy and stems! Considering that rhododendrons are very sensitive to julgone, I'd think twice about bringing in more plants to the area.

If you decide to experiment, be sure to let us know what the results are.

I have rhododendrons that grow near black walnuts. The black walnuts killed some before I discovered the problem and killed the walnuts. I just girdled the walnut trees. It took them three years to die and after over 15 years they are just now beginning to fall over. The rhododendrons in the area are coming back strong and healthy. I think that if you do as you say and remove the leaves, nuts, etc., they will be OK. I can't imagine the dripping rain water is that bad. The live roots are what did mine in. I think it is worth a try.

Thank you both. I'm going for it.
Oh, and by the way. When I use half wine barrels, I drill the drainage holes on the SIDES instead of the bottom. That way roots are a little less likely to wander in, but mostly if the bottom gets plugged up with dirt, you don't get the drainage problems if your holes are on the sides

Very good idea. I would also put a layer of gravel or stone in the bottom that goes up above the holes to discourage roots from going down to the puddle that forms below the holes. The stone will also help aerate the roots. There are materials such as compressed moss or window screening that you can place over the stone to keep the soil mix from penetrating the stone.