Shrubbery with a Grudge

I hide among my other friends

Deep inside the woods.

When I am found in my domain

I cannot run away.

Humans come and break off my arms

And then with them they take.

They don't even call me by my full name

They shorten it to, Shrub.

So can you guess what I am?

If not I'll give you some last clues

I'm short, small or lage and leafy

And given to the Knights who say Ni!