Aconitum carmichailii 'Arendsii'
Huge, amethyst blue hood-shaped flowers on stately spikes bloom in September to October. Grows 4-6' tall on sturdy stems. Space 18-24" apart. Latest and perhaps most dramatic of the monkshoods.
Light to part shade in moist soil, amended with organic matter. May need staking when the plant begins to bloom. All plant parts are poisonous.