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Yellow giant hyssop plants are 4-7' tall; the stems are square and conspicuously winged. Flower spikes are up to 8" long. The calyx of the flowers is tubular and green in color; the corolla is ~3/8" long, nominally yellow, with an upper and lower lip between which protrude 4 stamens with yellow anthers and a style with a cleft tip. When in bloom, yellow giant hyssop is easily distinguished from other giant hyssops by the yellow/cream flowers.

Agastache

Agastache nepetoides

yellow giant hyssop [Blooms: Jul-?]

Purple giant hyssop plants are up to 6' tall; the stems are square. Flower spikes are short (<6"). The calyx of flowers is tubular and green; the corolla is ~1/4" long, lavender to pink and occasionally white (the common form in Jackson Park), with an upper and lower lip between which protrude 4 stamens with pink anthers and a style with a cleft tip. Similar to blue giant hyssop (Agastache foeniculum) but the latter smells of anise, has purple rather than green calyxes on the flowers, and the underside of the leaves is white, not green.

Agastache

Agastache scrophulariifolia

purple giant hyssop [Blooms: Jul-Aug]