The CBC has a great story about four students at New York University's interactive telecommunications program, who have created a system of moisture sensors that are linked to a decision tree, excuse the pun, and pre-recorded phone messages. The system allows plants to let the students know via telephone when they require watering, and "There are set thresholds, unique to each plant, for minimum and maximum soil moisture." There are currently 5 messages possible, from I need water to You've over watered me. The students say they are working on a system for home users and one that will allow plants within a room to talk to each other. My mother always claimed that talking to plants was good for them, I just didn't think they'd ever talk back!