Allium nigrum

Allium nigrum: how do you recognise it?

Habit

One or a few sturdy, straight stems 60–70 cm tall above a low rosette of leaves; about 20 cm wide.

Leaves

A few broad, strap-shaped, grey-green leaves at the base, which already turn yellow around flowering time.

Flower

A flat to slightly domed, dense head about 8–10 cm across with dozens of star-shaped flowers, in May and June.

Colour

White to pale pink, with a dark, greenish-black ovary at the heart of each flower.

Fruit

Dry seed capsules that give the head a greenish, later straw-coloured structure after flowering.

Bulb

Fairly large, round bulb; plant it about 15 cm deep.

Flowering: May–June · Plants: September–November · Pruning: July

Why grow it?

• Large, flat white heads in May and June, when the border is only half full.
• The dark centre of each flower gives the head a surprising effect close up.
• A sturdy, long-lasting cut flower, fresh or dried.
• Bees love to visit the flowers.
• The greenish seed heads stay decorative in the border well into summer.

Allium nigrum: where does it grow best?

• Full sun: the stems stay sturdy and it flowers best.
• Sandy, loamy or clay soil, free-draining, neutral to chalky.
• Dry to moderately moist; in soil that stays wet in winter the bulb rots.
• Plant in groups of 5–7 bulbs, 15–20 cm apart, among perennials that hide the dying leaves.

Light

Sun

Soil type

Sand, loam or clay

Acidity

Neutral or chalky

Moisture

Dry or normal

Size

0.7 m tall, 0.2 m wide

Hardiness

Hardy (to -20 °C)

Goes well with

• Hardy geranium: Grows over the yellowing leaves in May and keeps flowering in summer.
• Lady’s mantle: Soft, rounded leaves and yellowish flowers around the white heads.
• Woodland sage: Blue to purple spikes in June, a fresh combination with white.
• Peony: Flowers at the same time; its broad leaves hide the base of the allium.

Allium nigrum: when and how to plant it?

Plant in September–November.

1. Plant the bulbs in September, October or November.
2. Loosen the soil well and, on heavy clay, add some grit to the bottom of the hole.
3. Plant the bulbs about 15 cm deep, tip pointing up.
4. Space them 15–20 cm apart and plant in groups so the white heads read as one.
5. Cover with soil and firm gently; watering is only needed in a dry autumn.
6. Mark the spot so you do not damage the bulbs when you work in the border in spring.

Care

Watering

Rarely needed. Only in a very dry spring, while the flower stem is growing.

Feeding

A thin layer of compost around the bulbs in March is enough.

Foliage

Turns yellow around flowering time. Leave it until it withers all the same: the bulb needs it.

Cut flowers

Cut the stems when about half the flowers are open. Dried seed heads look good in dried arrangements.

Deadheading

Leave the seed heads for decoration, or cut them off in July.

Dividing

Every 4–5 years, when the bulbs are crowded and flower less.

Tip: Place it among perennials that unfold their leaves in May, such as hardy geranium or lady’s mantle: they hide the leaves that already yellow during flowering, while the white heads rise above them.

Allium nigrum: when to prune it?

Pruning months: July.

When: No pruning is needed. Let the leaves die back and cut off the faded stems in July, or leave the seed heads standing for decoration.

How:

• Cut the stem with its seed head off just above the ground once it no longer looks good.
• Leave the leaves until they are yellow and dry.
• Pull away the withered leaves when they come away easily.

Caution: Do not cut the leaves off while they are green, even though they start to yellow during flowering: the bulb needs them to flower next year.

Pruning: when and how · Pruning calendar and basic rules

Propagation

Offsets

In July–August, while the plant is dormant, lift the bulbs, separate the offsets and replant them in September. They flower after 1–3 years.

Seed

Sow ripe seed in autumn in a pot left outdoors. Seedlings take 3–4 years to flower.

Allium nigrum: how does it get through the winter?

Hardy to −20 °C.

• Hardy to about −20 °C: no protection is needed in Belgium.
• Wet soil in winter is the biggest danger; make sure drainage is good.
• In a pot: stand it against a wall, wrap the pot in hard frost and do not let the compost become waterlogged.

Diseases and pests

• Yellow leaves at flowering: Normal for this species, not a disease. Hide the leaves behind lower perennials.
• Bulb rot: The bulb goes soft and fails to come up in soil that is too wet. Replant in a better-drained spot.
• Slugs: They eat holes in the young, broad leaves in spring. Pick them off in the evening.
• Rust: Orange pustules on the leaves in a damp spring. Cut off affected leaves and put them in the household waste.

Through the year

• March: Leaves appear. Compost around the bulbs.
• April: Stems grow. Watch out for slugs.
• May: Flowering begins. Cut flowers.
• June: Full flower. Bees on the flowers.
• July: Cut off faded stems or leave the seed heads. Clear withered leaves.
• August: Summer rest. Lift and divide crowded clumps.
• September: Plant bulbs, 15 cm deep.
• October: Plant bulbs. Sow ripe seed in a pot.
• November: Plant the last bulbs.

Edible and useful

Not edible. Ornamental onions are true alliums, related to onion, garlic, leek and chives, and like them they are poisonous to dogs and cats: all parts, especially the bulbs, contain sulphur compounds that damage the red blood cells and can cause anaemia. Cats are more sensitive than dogs, and the signs (lethargy, pale gums, red or brown urine) sometimes appear only after a few days. Horses, sheep and other livestock can fall ill from them too. Even though the leaves and bulbs smell of onion, they are not grown as vegetables: do not eat them, and never store the bulbs with the kitchen onions, as they are not meant for eating and may have been treated with pesticides. Do not plant them where your dog digs, keep loose bulbs out of reach of pets and children, and do not put a vase of alliums where your cat chews the stems. If your dog or cat has eaten a bulb or leaves, call your vet, even if it shows no signs yet. If any is swallowed, call a poison centre (in Belgium 070 245 245) or your doctor at once.

Good for wildlife

Bees

Nectar and pollen for honeybees and wild bees.

Did you know?

• Despite the name nigrum (‘black’), the flowers are white: the name refers to the dark ovary at the heart of each flower.
• Dried seed heads keep their shape for a long time in dried arrangements.
• Leaves yellowing around flowering time is common in many ornamental onions and is not a sign of disease.