Tuck: I need the bark of an elm tree
and some nettle to grind. Grinded nettle?

Benjamin: A poultice? That'll help. It draws out the evils (infections?)
while keeping the wound clean.
Madhulkand some nettle to grind. Grinded nettle?

Past participle of "grind" is "ground", so we would say "ground nettle".

He is not actually asking for ground nettle. He's just asking for nettle, and then he is going to grind it (mash it up).
MadhulkIt draws out the evils (infections?)
Yes, more or less, but at that time there was no modern notion of "infections", so these may have been viewed as visitations of evil. I'd guess that the word "evil" is consciously used with this in mind, though it's not impossible that it would be used figuratively in this sense in modern times too.
Thanks, Mr Wordy! For both the grammar and historical refences.