Janie, whenever I plant a rosebush or other shrub, I also "plant" a 2-liter soda bottle, label removed, with four or five nailholes punched in the bottom. Leave the top on the bottle. I suppose you could put more holes on the bottle-sides if you wanted. Filling the bottle once a week with water and sometimes a pinch of fertilizer puts the nutrients and moisture at the shrub's roots. I haven't tried this in big pots yet, but it sounds like a great idea -- put one of these in the middle of the big tubs I grow pole-beans in, since the gravel driveway is the only place in my yard sunny enough for them.
Lynn T in the DC suburbs