Hi!
I have made a automatic watering system with 10 solenoid valves and a small 12v pump (2A 7,5 l/min).
For the pump I use a relay and for the valves I have 2*ULN2803A transistor arrays.
I also use 2*74HC595 shift registers to control the ULN2803A (for saving output pins).
The valves is opened one at a time to keep the water pressure high. The system works like a charm, I can control watering by different schedules (days, hours, which plants that should get water).
Here is a picture of the valve I use (common on ebay).
/Olof