Aconitum 'Barkers Variety' is a tall (up to 5-feet) perennial for the back of the flower or mixed shrub border, featuring long, densely packed spikes of amethyst-blue flowers from August well into October, when so many other garden plants are looking tired.

Aconitums are commonly known as monkshood, because the upper
sepal of each flower drapes like the hoods of medieval monks. They
attract a variety of butterflies and moths, and are the primary food
source for Old World bees. Another common name, wolfbane, refers to the
poison made
from this plant's roots, which was used to tip hunting arrows.
Medicinal (and poisonous) uses of this plant date back to Roman times.
What's the difference between a medicine and a poison? Dosage.