Growbotica

OpenSprinkler Pi is “an extension board for the Raspberry Pi” that “makes use of the Raspi’s GPIO pins to directly interface with sprinkler valves. ”  Read more on this OpenSprinkler Pi page.

Here’s a good tutorial on building a moisture sensor project for Raspberry Pi.

Here’s an excellent (and well-commented) Instructables how-to on making a watering system that monitors soil moister and The only drawback I see is the need to remember to fill the resevoir — but I can see this extended to send SMS alerts or to use plumbing or a large-capacity water supply waters as needed.

The article contains a good parts list for everything except the Ardiono Uno. All the items are available at Radio Shack except perhaps the TOM Aquarium lifter pump — available from Amazon.