This plant has square stems up to 5 feet tall covered with fine white hairs (A,E). Opposite, stalked oval-shaped leaves have pointed tips and coarsely toothed margins (B). Branched stems bear spikes of purple, blue or whitish flowers, all not opening at the same time (C). Individual tubular flowers have 2 lips and 4 long protruding stamens (D). The base of the flower is enclosed in a toothed, greenish, sometimes rosy tinged cup-like calyx with whitish margins (D,E).
Hairs on the stem, green calyces with whitish margins and green leaf undersides separate this plant from its very similar cousin, Lavender Giant Hyssop (Agastache foeniculum) which has calyces that are entirely or partially lavender and leaf undersides covered with fine white hairs. When in flower, similar Yellow Giant Hyssop is easily separated by its flower color. Since both the Lavender and Purple species are extremely rare, any non-flowering hyssop you see will most likely be Yellow Giant Hyssop.
Found in moist woodlands or edges flowering from July to September. Rare at Neale Woods, where a single flowering plant was observed just beyond the upper parking lot entrance in the summer of 2024. Although native to eastern Nebraska, it seems likely this plant is a survivor from the previous garden planting at this site.