How to Grow Black Cohosh in Australia

Black cohosh (Actaea racemosa, family Ranunculaceae) is a long-lived woodland perennial grown for its large, divided leaves and tall wands of creamy white flowers. The store sells it as Black Cohosh seeds, sometimes labelled with its older name, Cimicifuga racemosa. In a cool, moist, part-shaded bed its flower spikes stand 1.2 to 2 m tall. The commonest mistake is expecting quick seedlings: the seed needs a warm spell, then a winter, and often two springs.

Three mistakes that matter

1. Chilling the seed without warming it first. In a laboratory trial, seed kept at 4°C for 18 months without a warm spell did not germinate. Give the seed two weeks of moist warmth before the fridge, as Germinating the seed explains.
2. Tipping out the pot after one spring. Autumn-sown seed often waits for its second spring before any seedlings show. Keep the pot moist, shaded and labelled for at least two years, because bought seed often takes longer.
3. Planting it in hot sun and dry soil. Black cohosh comes from shady North American woods, and its leaves scorch and brown at the edges when the soil dries. Give it morning sun only, afternoon shade and a thick leaf mulch.

When to plant

The seed needs about two weeks of moist warmth at 21 to 25°C, then about three months of cold near 4°C, then warmth again. Sow outdoors in autumn, while the soil still holds the summer's warmth, and let the winter supply the cold. Or run the same cycle in the kitchen and the fridge, and sow the chilled seed into trays in spring. In cool districts, where frost arrives from about April, sow outdoors in March. One batch is usually enough, because each plant lives for many years.

Late in the autumn window, give the seed its warm spell indoors first, so winter does not arrive too soon.

Site, soil and setup

Think of a forest edge, or a shaded gully garden around Katoomba: morning light, cool shade from lunchtime, and soft soil under fallen leaves. Black cohosh copes with more light and more varied soil than many woodland plants, provided the soil never dries out. Too little light is the opposite trap, because in deep shade it may grow leaves and no flowers.

Container growing

A large pot can hold a mature plant on a shaded courtyard or verandah. The store sells only the species, with no compact selection. Allow for a plant up to 2 m tall with a spreading rhizome (a thick underground stem). Use a pot with drainage holes, of at least 40 L, 45 cm across and 40 cm deep, filled with premium potting mix. Potting mix can carry Legionella bacteria, so wet it first, wear gloves and a mask, and wash your hands afterwards.

• Watering: keep the mix evenly moist and check it daily in hot weather, because a dry pot scorches the leaves.
• Feeding: fish and seaweed blend at half strength, about 12 mL per 9 L, fortnightly from spring to late summer.
• Position: morning sun and afternoon shade, sheltered from wind so the spikes do not tip the pot.
• Winter: the leaves die down, so keep the pot barely moist until new shoots appear in spring.

Germinating the seed

Black cohosh seed is shed with a tiny, unfinished embryo that is also dormant. Botanists call this morphophysiological dormancy: the embryo must grow inside the seed before it can germinate. Warmth lets the embryo grow, and the long cold spell that follows lets the root and then the shoot emerge. Seed that misses either stage simply waits for another year.

A North Carolina extension leaflet puts growers' average germination at about 20 per cent. Bought seed often does worse than seed sown fresh. In a laboratory trial, two thirds to three quarters of seeds made a root within about three months. Kitchen results are usually lower. Sow at least five seeds for every plant you want. Keep the packet sealed in the fridge until you start, because seed stored for many months can sink into a deeper dormancy.

The same laboratory found that gibberellic acid, a hormone some nurseries use on hard seed, did not speed germination of this species. For seed treatments in general, see soaking, scarification and cold stratification.

When nothing comes up

• No warm spell. Seed that went straight into the fridge stays dormant. Give it two weeks of moist warmth, then start the cold stage again.
• Too short a winter. Three months of cold is the working minimum. Leave seed without roots in the fridge another month before sowing.
• Bag dried out or went mouldy. Keep the vermiculite damp, not wet, and check the bag every week. Remove any soft or mouldy seeds.
• Old or warm-stored seed. Long storage, especially warm, deepens dormancy. Sow seed as soon as it arrives and expect a second spring.
• Given up too soon. Many seeds sit out their first spring; see Three mistakes that matter.

Sowing Seeds

Start black cohosh in deep trays or a shaded seedbed. Use seed that has had its warm and cold spells, or sow in autumn and let the weather supply them.

Starting in trays

• Fill deep cells or small pots with seed raising mix, firm it lightly and water it well.
• Sow one sprouted seed per cell, about 6 mm deep, and cover it with mix.
• Stand the tray in bright shade at about 25°C, and keep it moist.
• Pot seedlings on singly once they show a second set of true leaves.
• Grow them on in pots in a shaded spot for one or two years.

Sowing in a seedbed

Choose a shaded, weed-free patch of fine, moist soil. Sow the seed about 6 mm deep and 4 to 5 cm apart. Cover the bed with about 2.5 cm of leaf mulch and keep it moist through two winters. Mark the spot, because nothing may show for 18 months.

Planting out

• Plant out in autumn or early spring, while the young plants are dormant or just stirring.
• Harden off first (stand the pots outdoors a little longer each day for a week, so they adjust to sun and wind).
• Space plants 60 to 90 cm apart, the crown at the depth it grew in the pot.
• Water in with seaweed solution at half strength, 15 mL per 9 L.
• Mulch with leaf litter or shredded bark, kept just clear of the crown.

Care by growth stage

Steady moisture and afternoon shade decide how well black cohosh grows. It is slow to settle, and a plant raised from seed usually flowers in its third or fourth year. Feed gently: rich compost and mulch do most of the work, and over-feeding gives soft, floppy growth.

If growth stalls in poor soil, a light application of all-purpose controlled release granular (NPK around 14-5-10) scratched in each spring can help.

Cutting Back

Black cohosh needs little pruning: a tidy-up after flowering and again when it dies down in autumn. Leave some spikes standing if you want seed or the dry seedheads for arrangements.

• Cut faded flower spikes back to the leaves if you do not want seed.
• Pick off spotted or yellowing leaves as they appear, and bin them rather than composting them.
• Thin crowded clumps so air moves freely between the leaves.
• Cut all stems to the ground once the leaves yellow and collapse in late autumn.
• Top up the mulch over the dormant crown for winter.
• Divide an old clump in autumn or early spring if you want more plants.

See dividing perennials for lifting and splitting a clump, which gives flowering-sized plants much faster than seed.

Companion planting

Pair black cohosh with plants of cool, moist, part-shaded ground, its spikes at the back.

Grows well with

• Aquilegia, which flowers in spring in the same dappled shade, before the cohosh spikes rise.
• Foxgloves, another tall spire for part shade and moist soil.
• Lady's mantle, a low, soft edging that covers bare soil under the tall leaves.

Keep apart

• Lavender and other sun lovers that want dry soil, the opposite of what black cohosh needs.
• Mint, whose running roots compete with young clumps for moisture.

Harvesting Flowers and Seed

Enjoy the flowers on the plant from early summer. Collect seed once the pods turn dark brown, split at the tips and rattle when shaken. The flowers have an unpleasant smell, so they suit the garden better than the vase.

• Expect each spike to flower for two to three weeks.
• For seed, cut a spike once most pods are brown and dry.
• Shake the spike over a paper bag to collect the loose seed.
• Sow fresh seed straight away outdoors, or start its warm spell indoors.
• Cut dry seedheads with their pods for dried arrangements.
• Leave the roots and rhizomes in the ground.

Store any seed you cannot sow in a paper envelope inside a sealed, labelled jar in the fridge, and sow it within a few months. The roots and rhizomes contain triterpene glycosides and are linked to rare liver injury, which Safety explains.

Traditional use and history

Black cohosh is native from south-eastern Canada to the north-central and eastern United States, where it grows in rich woods. Its common name comes from an Algonquin word meaning rough, after the look of the knotted rhizome. Bugbane and the old genus name, Cimicifuga, refer to the plant's smell, said to drive insects away.

Black snakeroot set it apart from another snakeroot, and rattleweed and rattle root come from the seeds rattling in their dry pods. Macrotys and fairy candles are two more. The herbalist Maud Grieve recorded that seed was sent to Europe every year. In England the plant thrived in moist, shady borders but did not ripen seed.

Most black cohosh sold for supplements has come from plants dug in the wild, and many large, easily reached stands are already gone. It is listed as endangered in Illinois and Massachusetts, and United Plant Savers lists it as at risk. In its home woods it also feeds the caterpillars of the Appalachian azure butterfly, which eat its flowers.

Safety

Happy Valley Seeds sells black cohosh as seed for growing, as an ornamental. Nothing here is advice to eat or take any part of the plant. Anyone considering using a plant should first seek advice from a qualified health practitioner.

Two plants are easily confused with it. Blue cohosh (Caulophyllum thalictroides) is a different plant with serious risks of its own. Baneberries, black cohosh's close relatives, carry poisonous berries, while black cohosh carries dry pods. Call triple zero (000) if someone collapses, stops breathing or has a seizure.

Common problems

No seedlings. Usually a missed warm spell, too short a winter, or too little patience; see Germinating the seed and its list, When nothing comes up.

Leaves but no flowers. Normal for a young plant, as Care by growth stage explains. In an older plant, deep shade is the usual cause, so give more morning sun.

Brown, crisp leaf edges. The soil dried out or the plant caught hot afternoon sun, the third of Three mistakes that matter.

Spikes leaning over. Heavy shade bends the spikes towards the light and wind pushes them over; see the Light and Shelter rows in Site, soil and setup.

Tall, pale seedlings. Too little light in a dim spot stretches them, so move trays to bright shade. Read more

Pests and diseases

Black cohosh has no serious pest or disease problems of its own, so most trouble comes at the seedling stage.

Common garden snail (Cornu aspersum) and grey field slug (Deroceras reticulatum). Both graze seedlings and new spring shoots at night, and a leaf-mulched shade bed suits them well. Hand-pick them after dark, or scatter an iron-based snail bait thinly as the label directs. Read more

Damping off (Pythium species and Rhizoctonia solani). Young seedlings rot at soil level and collapse, most often in wet, poorly drained mix. Use fresh seed raising mix and clean pots, and water only when the surface starts to dry. Read more

Cutworms (Agrotis species). These moth larvae, the Bogong moth (Agrotis infusa) among them, cut seedlings off at night and hide in the soil by day. Scratch away the surface soil around damaged plants to find and remove them. Read more

European rabbit (Oryctolagus cuniculus). In rural gardens rabbits graze seedlings and young shoots, leaving clean cuts at about 45 degrees. Fence the bed while the plants are young.

Prevention first

• Raise seedlings in fresh mix and clean pots, clear of hose splash.
• Space plants so air moves between the leaves, and never crowd them.
• Check seedlings and new shoots with a torch after rain.
• Clear boards and loose pots from the shade bed, where snails and cutworms hide.

Tips

1. Plant several together once you have them, because black cohosh looks best in a group.
2. Plant it in front of a dark hedge or fence, where the white spikes stand out on a summer evening.
3. Site it away from a window or seat, because the flowers smell unpleasant up close.