Although most pumps are quite expensive, I found a Chinese retailer who was selling some Japtop JT-180 submersible pumps for about £5 each. These are tiny things that run on anything from 3V - 12V and only need a couple of hundred milliamps at the lower voltages. I use an ordinary motor shield to drive them (also Chinese and also only a few quid each).
Using a separate pump for each plant avoids having to deal with any complex valving and so on. And for the 'Mark 1' version without any soil moisture sensors you can just have a single pump supplying a fixed manifold with restrictors to control how much water each plant gets.