The idea is to use some kind of electro-valve connected to the water source, and a gardening hose with small holes around all the plants. Maybe set a timer for 15 minutes very 8 hours or something like that.
Consider not attaching it to a water source but rather use a reservoir of some sort - bucket, drum, whatever; it will limit the damage you can do if there's a bug in your code. If you elevate the reservoir, you may be able to get away with gravity feed and just need to be able to control the valves.