Hardy and reliable, Agastache 'Black Adder' (Giant Hyssop) is a charming herbaceous perennial boasting bottle-brush, terminal spikes, 6-8 in. long (15-20 cm), packed with fragrant, tubular, two-lipped red-violet flowers emerging from dark buds. Blooming from mid-summer to fall, they provide a remarkably long floral display, and add color and a lovely spiky texture to the garden.