hfp777:
I was thinking about using gravity to pour the water and use a small water hose on a gliding rail to move from plant to plant in a linear fashion with a valve to control the water flow.
Why so complicated? Just run a small hose between each plant and seal up the far end. Punch a small hole in the hose near each plant (more than one for thirsty or larger plants). Use your arduino to control a pump (duration and/or pressure) dependent on the moisture content of the soil (has it rained today?), average ambient temperature (on hotter days, plants require more water.) and light level (you shouldn't be watering plants in direct sunlight. Dusk is the best time.) You could also use the arduino to keep a check on the level of water left in your rain butt so that it doesn't attempt to pump when there's no water left.