Poisonous plant and classic Witch garden flowers. All of these plant while lovely are indeed toxic. For more information read my book "Garden Witches Herbal". There you will find a chapter devoted to Gothic Witch Garden Plants. -Ellen Dugan
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.
In some November, some year, she took in Datura. And now November holds her while awake, in all the rest, she appears. #Datura didn't mean to cast the hex; if managed by a practiced witch, Datura's toxicity can be muted or prevented while its benefits can be felt. To find her, try the Gryffindor linen closet at Hogwarts.