Actaea racemosa aka Black Cohosh

Actaea racemosa, or black cohosh, is a woodland perennial. It belongs to the buttercup family, Ranunculaceae. Its native range covers eastern North America, including eastern Canada. It grows in woods, ravines, and along streams. Garden plants form upright clumps and lose their leaves each year. Roots spread through rhizomes beneath the soil. Growth is moderate, though new plants can take time to settle.

Size estimates differ between sources. RHS gives a height near 5 feet [1.5 metres]. NC State lists 4–7 feet [1.2–2.1 metres]. Its spread ranges from about 1.6–4 feet [0.5–1.2 metres]. Large green leaves form a broad base beneath the flowers. Basal leaves can reach 3 feet [0.9 metres] across. They divide into several tiers of toothed, lobed leaflets. Smooth flower stems range from green to purple.

Small white flowers rise well above the leaves in branched racemes. These flower clusters reach 1–2 feet [0.3–0.6 metres] long. Each flower lacks petals and carries many stamens. Together, the stamens give the clusters a soft, fluffy look. Bloom dates vary, with records spanning May through August. Missouri Botanical Garden lists June to July. The flowers have a scent that some find unpleasant. Dry brown follicles follow, with seed pods present from July to September. Gardeners use the seed heads in flower arrangements.

Grow black cohosh in partial shade with cool, moist soil. It also tolerates deeper shade, but too little light limits bloom. Soil should contain plenty of organic matter. Water during dry spells to keep the soil moist. Dry soil can brown leaf edges and slow growth. Choose a site sheltered from strong winds. Tall flower stems may need support and bend toward light. The plant suits USDA zones 3–8 and has RHS hardiness H7. Rust and leaf spot occur, but serious pest problems are rare.

Black cohosh adds height to shaded borders and woodland gardens. Groups work well, while mature clumps can stand alone. Dark backdrops help the pale flowers show. Butterflies visit, and azure butterfly larvae feed on the flowers. Deer seldom cause much damage. Native Americans used the roots and tubers as medicine. Evidence does not establish it as a treatment for specific conditions. The plant has also served as an insect repellent. PFAF records cooked leaves as food but warns about toxicity. RHS warns that eating the plant can cause harm. Large doses can cause illness; medicinal use is unsafe during pregnancy.