WOLFSBANE: a toxic also known as “Aconitum”, flowering plant that can be used to repel Werewolves if great care is taken in its usage; an excessive amount actually increases the chances of savage transformations.
Monkshood: Aconitum nagarum, dark purple form. I can't pin down a variety or a source for seeds, sadly. You will have to do a bit of sleuthing if you want one. Excellent for a poison garden; not so good if you have kids around.