Agastache nepetoides aka Yellow Giant Hyssop

Agastache nepetoides is also called yellow giant hyssop. It is part of the mint family. This plant lives for years but has no woody stems. It comes from eastern Canada and central and eastern America. It grows in parts of that range with a temperate climate. In Missouri, it grows in rich, moist, open woods. It also grows in thickets and at the edges of woods. It is not native to Britain or Ireland.

The plant grows fast and forms a tall, upright clump. Missouri Botanical Garden gives its height as 4–6 feet [1.2–1.83 metres]. RHS gives a wider range of 5–8.2 feet [1.5–2.5 metres]. Plants spread about 1.6–3.3 feet [0.5–1 metre]. Thick, square stems hold flowers above the green leaves. The leaves have coarse teeth and a shape like an arrowhead. They reach about 0.5 feet [0.15 metres] long. Sources do not agree on their scent. RHS says the leaves have a scent. Missouri Botanical Garden says they lack the scent of many mints. The leaves fall when the growing season ends.

Small, tube-shaped flowers form dense spikes at the stem tips. The spikes reach about 0.4 feet [0.13 metres] long. Pale greenish-yellow blooms form rings, called whorls, on each spike. Missouri Botanical Garden gives the bloom time as July through September. Blooms may last until the first frost. Bees and butterflies visit them. The pale flowers and tall stems suit mixed borders of plants that live for years. Groups suit wildflower plots and gardens with woods or wildlife. Plants can spread and grow wild where the site suits them.

Grow yellow giant hyssop in sun or part shade. It needs moist soil that drains well. It dislikes dry spells in summer. RHS lists clay, loam, sand, and chalk as suitable soils. The plant accepts acid, neutral, and alkaline soils, too. Missouri Botanical Garden says it copes with summer heat and damp air. That source lists USDA zones 2–8. RHS gives the UK rating H3. These ratings reflect how growing conditions differ from region to region. RHS says to choose a sheltered site.

The plant needs little care and rarely has serious pest problems. It can get powdery mildew. Yet RHS says pests tend to leave it alone. Deer tend not to eat it. This helps where deer limit which plants can grow. Remove the flower spikes once their blooms fade. Grow new plants from seed or split clumps in spring. Plants may also grow from their own seeds where conditions suit them.