Caulophyllum thalictroides aka Blue Cohosh

Caulophyllum thalictroides, or blue cohosh, belongs to the barberry family. This herbaceous perennial comes from eastern and central North America. Its range reaches from New Brunswick south to South Carolina. It extends west to Arkansas, North Dakota, and Manitoba. It grows in rich woods, on wooded slopes, and in ravines. In gardens, it suits shaded beds and woodland plantings.

Plants usually stand 1–2 feet [0.3–0.61 metres] tall. They spread about 0.5–1 foot [0.15–0.3 metres] wide. RHS gives larger limits of 3.3 feet [1 metre] tall and 1.6 feet [0.5 metres] wide. Upright stems grow without branches and have smooth surfaces. Their colour ranges from pale green to pale purple. Young growth has a pale wax coating. Leaves emerge blue, then turn blue-green as they age. The compound leaves divide into groups of three. Their broad leaflets have smooth edges and no hairs. The plant sheds its leaves each year.

Small flowers open in spring before the leaves fully expand. Missouri Botanical Garden lists April as the bloom month. Each flower has six coloured sepals and tiny petals. The sepals range from green-brown to yellow-green. Rounded flower clusters rise above the leaves on upright stems. Bees and other insects visit for pollen and nectar. Green seeds follow the flowers and turn bright blue in summer. Their fleshy coats make them look like small berries. These blue clusters can remain showy into autumn. Woodland birds spread the seeds, and mice feed on them.

Blue cohosh needs partial to full shade and rich soil. Keep the soil moist, but allow excess water to drain. Acid to neutral soil suits it, with plenty of humus. Sources list USDA zones 3–8 and UK hardiness H5. Growth is slow, and rhizomes gradually form small colonies. Seed-grown plants may take three or four years to flower. Established plants can live a long time with little disturbance. Gardeners can divide them, though settled clumps are best left alone. They need no pruning. Sources report few serious pest or disease problems. Deer and rabbits seldom eat the leaves.

The seeds and roots pose a poisoning risk to people. Eating them can cause vomiting, diarrhoea, and raised blood pressure. Contact may also irritate the skin. PFAF records roasted seeds as a past coffee substitute. That report does not remove the warnings about poisonous seeds. The roots have a history of use in childbirth remedies. PFAF warns against use during pregnancy before labour begins. It also notes heart and blood-pressure risks from excessive doses.