What's a skeleton's favorite plant? A bone-zai tree. But if they don't like that one, how about a s-pine tree?
Skeleton's Favorite Plants
Explanation
This is a play on words. The joke uses "bone-zai" which sounds like "bonsai", and "s-pine" which sounds like "pine", both common types of trees. The humor comes from the association of these words with bones, and the fact that it's about skeletons, which are made of bones.