We’ve see a lot of clever irrigation devices for gardeners being made with the Pi, but PiPlanter is the most complete (and the best documented) system we’ve seen so far. It does far more than simple irrigation. PiPlanter monitors temperature, ambient light, ambient humidity and soil humidity; it outputs that data to a MySQL database, controls a pump to water the plants depending on that data, and outputs the data as graphs and text. (It also tweets that text and uploads the graphs to Flickr hourly so that Devon, the PiPlanter’s owner, can keep an eye on things.)
Devon has documented the build minutely, with circuit diagrams, a ton of code, and several videos. Here, he explains more about the sensor array he built.
You can read much more at Devon’s blog, and replicate the project yourself. Thanks Devon: more power to your green thumb!