If you are interested in buffering for a short time frame (ex. a holiday) this is the right solution for you, otherwise go to the project BP005 for a self-controlled watering system that operates 365 days per year.
Some years ago I had the opportunity to spend a whole month holiday in Portugal. I didn’t want to ask my friends, or neighbours, to water my plants everyday for me. I thought therefore to build a system for that purpose using the most common microcontroller PIC16F84A.
I thought and tested lots of different solutions in order to find the best one. Reliability was the most important aspect to ensure: a fault could have killed my plants or, worse, flooded my neighbour’s house. Once the system was built, only a few days remained before my departure. So, I set the irrigation timing as best as I could, I asked a friend of mine to go and check once if everything was fine and I left the house crossing my fingers.