You would also need some method of sensing the level of the water, possibly, if you were to use a water level float sensor in a pipe that is inserted in the dirt, this may work. Liquid Level Sensors - Float | Newark element14 Canada
Lastly, you will need the some sort of micro-controller that can communicate with the sensor and valve.
If you wanted to do this for many plants, then you would probably want to have individual valves and sensors.
This might not be the best way to do it, but it might work.
The best thing to do is to use drip system which will help you water the plants whether you're or aren't at home. way better then watering plants manually.
