Answer:
I am flora not fauna. I am foliage not trees. I am shrubbery not grass.
I have a mouth on my head and eat everything.
What is easy to get into, but hard to get out of?
Two in a whole and four in a pair. Six in a trio you see. Eight's a quartet but what you must get is the name that fits just one of me.
I move without wings between silken strings. I leave as you find my substance behind
Though not a plant, has leaves. Though not a beast, has spine. Though many wouldn't need this thing, It's more valuable than wine.