Digitidope:
Hmm but would still need valves to direct the water to different places as I want control over the exact amount going into each plant box (overenthusiastic gardener).
Regular pumps give more exact control than valves (the flow of which is highly dependent of how much water there happens to be in your reservoir).
If you want really tight control look into peristaltic pumps. They're slow but very accurate.
Instead of relays, you can also switch your pumps with a MOSFET transistor. Smaller and more efficient than a relay. The IRLZ44N is a popular one to use with Arduinos.