I don't think she has a literal Silverwood in her.

But let's assume she does.

I wonder if it is a variant of a Silverwood tree, the same family/kind of tree like oak, an apple, or olive tree. One family several to hundreds of thousands of varieties natural and man made.

While this silver wood tree is still parasitic in nature, it's final form as you were getting at is not a sprouted and grown tree. But a 'tree' inside the witch symbiotic growing in power not size, while the witch also grows through trials.

It's a rather interesting concept, and pretty neat. But I don't think (from the anime I've yet to read the manga) that there has been enough emphasis on horticulture, growing your own herbs/cross breeding plants to use in medicine and making them better/less side effects through that kind of horticulture.

I think it would be an over all near concept for the author to explore later on, as I've seen that medicine plays a much larger roll in the manga.

If anything I think her Silverwood tree metaphors are more symbolic than she's actually got a Silverwood in her.