If something's made of wood, or looks like it is, it's ligneous. A tree is a ligneous plant. A stiff politician is a ligneous person.
Ligneous is a technical term, the kind that scientists like, so if you want to describe something as wood-like in a casual conversation or an informal piece of writing, it's probably best to go with, well, wood-like, or woody. Use ligneous if you're deliberately trying to sound like someone who spends a lot of time looking into a microscope.