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.
Baneful Herbs P18 Hemlock(Conium maculatum) Hemlock is definitely another famous Witch’s herb. It is also known as Warlock’s Weed, Poison Hemlock and Winter Fern. Hemlock was once the go to plant for death by poisoning in Ancient Greece and famously it is the classical philosopher Socrates who was poisoned by this plant.
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.