Bonsai trees are not like other plants. There’s no single watering schedule that can be applied to a bonsai and the best way to tell if the bonsai needs water is to touch the soil. Experienced growers know when a tree needs to be watered by observing the foliage or just by the weight of the pot. If you are not used to taking care of this type of tree, Bonsai Watchdog could be the perfect project for you. It runs on Arduino and Genuino Uno and makes it really easy to monitor the moisture level in the soil.

Thomas Baum, created it and shared it some days ago on the Arduino Community on G+ :

Two pencil leads, an Arduino and a 12864 (ST7565) LCD watches out my little bonsai. The filling level shows how often the sapling need to be watered.
source and discription (in german) you can find here:
http://tiny.systems/categorie/lcdProjekt/BonsaiWatchdog.html