It is certainly worthy of a try. My neighbour has one, and it works well. I don't think he had it running for an hour, but we are in a much more moderate growing zone, here in southern Ontario.

To avoid the mildew problem, be sure to pinch off the lower 4 inches of leaves ( when the plant gets mature enough), so there is an air flow. Other than that, watch it carefully, and if you see mold developing, change the situations somehow, by either reducing the water flow, or pinching the leaves higher.

To be honest, I have seriously thought about doing something similar.

Keep us posted.

Dear Jroot - THANK YOU! I had almost despaired of anyone answering my question. I have already planted the tub with white, lavender-pink, and carmine Cosmos in the middle and dark blue Crystal Palace lobelia around the edges ALONG WITH: several adorable plants selling at the nursery under the name "Dahliaettes". The miniature or dwarf dahlias are beautiful, with flowers in a wonderful shade of warm pink. I went to the nursery in a neighboring town and fell in love with these little plants. I've been tinkering with the amount of water that comes out of the sprinklers and have fine-tuned it so that the plants don't get battered. Your advice about the lower leaves is EXACTLY what I needed to know - the dahlias are so compact and sort of dense that I am concerned about mold, etc. We generally have a dry climate here throughout the summer, but I'm sure the mold could still happen - I just didn't know what to do about it. I will post an update on the situation - whether the sprinklers work out or not. I really hope they do - aside from the practical aspects, they are wonderful to watch. (Of course, I am seldom up at four a.m., but I see them when I'm adjusting them in the daytime). Thank you again! Laura