Here is an open source plant watering alarm based around the ATtiny44A. It uses a humidity sensor to detect when a plant needs to be watered.
Your favorite plants need plenty of water to grow. If you keep forgetting to water your house plants you might be interested in this plant watering alarm device called Chirp. Chirp uses capacitive humidity sensing technique to measure the moisture level in the soil and alerts the user through short chirps when it is time to water the plant.