General characteristics:
* Hard
* Rigid
* Strong
* Durable
* Sturdy
* Tough
* Resistant
* Stable
Appearance:
* Rough
* Barky
* Fibrous
* Grainy
* Brown
* Grey
* Dark
Specific types:
* Branching (if the stem has branches)
* Trunk-like (if the stem is the main support of a tree)
* Knobby (if the stem has bumps or knots)
* Twisted (if the stem has a curved or contorted shape)
The best adjectives will depend on the specific type of woody stem and the context in which you are using them. For example, a young sapling might be described as "smooth" and "flexible," while an old oak tree trunk might be described as "gnarled" and "imposing."