What kind of pump, solenoids and valves are you gonna use? I have made something similar to water my tomato plants. It's really simple; a 10 l bucket of water, a cheap aquarium pump from Sparkfun, and a Uno set up to start the pump for about one second each 4 hours. I'm running a silicon hose from the pump to the potted plants at the moment. I've cut a hole halfway on the hose where it passes plant no 1, then the end of the hose goes to the second plant. If I had some valves I could get better control of the amount of water for each plant