Hemlock, henbane, poison ivy, khat, giant hogweed, poppy, stinging nettle, yew, rhododendron, and many other toxic plants can be found in a poison garden.
Why? In old times, to make medicine, probably. The difference between poison and medicine is often dosage/timing. These days it's mainly for educational purposes. Gets folk into botany, helps prevent poisonings, teaches respect for nature.
As one would imagine, they require special care to maintain.
The sculptures reminded me of Bene Gesserit, so naturally my next thought went to poison needles and dangerous caretakers.