BONSAI!! Sorry, I couldn't help myself. Day 18 reveals 5 bonsai stamps. The stamps show the most common trees used for bonsai. These include: Sierra Juniper, Trident Maple, Black Pine, Azalea, and Banyan trees.
Bonsai is basically the Japanese art of carefully pruning trees to stunt their growth. The trees are usually planted in trays or some other small containers. They can be started from seed (which is amazing to me) or use a small sapling. And they not only prune the limbs, but they also prune the roots of the tree. From everything I have read...it is definitely an artform.