Easily grown in average, medium moisture, well-drained soils in full sun to part shade. Best in full sun. Good soil drainage is essential. Plants will perform poorly and may not survive winter in hard clay soils that retain moisture. Plants tolerate heat and some dry soils once established. Deadhead spent flowers to promote additional bloom. Plants may not be reliably winter hardy throughout the St. Louis area where they should be planted in protected locations (e.g., southern exposures). Leave leaf and flower stems up during the winter for additional protection, cutting back the dead stems in late winter to early spring. Sandy/gravelly mulches will protect plants and help to avoid onset of rot. Self-seeding will not occur with this sterile cultivar.
'Black Adder' is an upright, clump-forming giant hyssop that grows to 2-3' tall. It is a hybrid resulting from a cross between A. rugosum and A. foeniculum. It is noted for its long summer-to-frost floral display of tubular, 2-lipped, smoky red-violet flowers that bloom in showy, bottle-brush, terminal spikes (to 6-8" long). Flowers appear in many-flowered verticillasters (false whorls) which are densely packed to form the flower spikes. Flowers emerge from nearly black buds. Aromatic (anise/licorice scented), lance-shaped to ovate leaves are blue green. Flowers are attractive to bees, hummingbirds and butterflies. Agastache comes from the Greek words for “much” (agan) and “grain stalk” (stachys) in reference to the flower spikes.