Giant onion

Giant onion: how do you recognise it?

Habit

Bare, perfectly straight stems up to 1.4 m, above a rosette of leaves about 30 cm wide.

Leaves

Broad, strap-shaped and grey-green, at the base. They appear early in spring and wither while the plant is still in flower.

Flower

Dense, spherical heads of about 10–15 cm, made of countless starry flowers, in June–July.

Colour

Lilac to purple; the seed heads later turn straw-coloured.

Scent

Bruised leaves and broken stems smell of onion.

Flowering: June–July · Plants: September–November

Why grow it?

• Spectacular purple globes at 1.4 m: a real eye-catcher in June–July.
• Bees, bumblebees and butterflies flock to the flowers.
• A long-lasting cut flower, and dried, a decoration for months.
• The seed heads stay decorative in the border after flowering.
• It takes up little ground space: you can tuck it in between other perennials.

Giant onion: where does it grow best?

• Full sun: in shade the stems become weak and the flowers smaller.
• Well-drained sandy, loamy or clay soil, neutral to chalky; lighten heavy clay with coarse sand.
• Dry to moist; in wet soil, especially in winter, the bulb rots.
• Sheltered from strong wind. Plant 25–30 cm apart, in groups of three to five among lower perennials.

Light

Sun

Soil type

Sand, loam or clay

Acidity

Neutral or chalky

Moisture

Dry or normal

Size

1.4 m tall, 0.3 m wide

Hardiness

Hardy (to -20 °C)

Goes well with

• Catmint: A low blue haze that hides the withering leaves.
• Cranesbill: A dense pink or blue ground cover around the bare stems.
• Lady’s mantle: Soft green leaves and lime-green flowers at the base.
• Salvia: Purple spikes at knee height beneath the purple globes.
• Ornamental grasses: Airy stems with the globes rising above them.

Giant onion: when and how to plant it?

Plant in September–November.

1. Plant in September–November, so the bulb makes roots before winter.
2. Dig a hole 20 cm deep, about three times the height of the bulb.
3. In heavy soil, put a layer of coarse sand or gravel in the bottom so water does not collect around the bulb.
4. Set the bulb pointed end up, 25–30 cm apart, and cover with soil.
5. Plant lower perennials in front to hide the withering leaves later on.

Care

Water

Rarely needed. Only in long dry spells in spring, while the leaves and stem are growing.

Feeding

Scatter a handful of compost or organic fertiliser around the plant as the shoots appear.

Support

In a windy spot, a thin cane next to the stem, or plant it among sturdy perennials.

Leaves

Let the leaves wither naturally: the bulb uses them to build up strength. Hide them behind lower plants.

Deadheading

Leave the seed heads as decoration, or cut them for drying.

Tip: Plant 20 cm deep in full sun and put lower perennials in front: the leaves wither while it is still in flower.

Giant onion: when to prune it?

When: No pruning is needed. Only remove the leaves once they have completely withered.

How:

• Pull or cut off the leaves when they are completely yellow and dry.
• Leave the stems with seed heads as long as they look good, or cut them for drying.
• If you do not want seedlings, cut off the seed heads before the seed falls.
• Cut the stems off at the base when they flop in autumn.

Caution: Do not cut the leaves while they are green, even if they look a little untidy: the bulb needs them.

Pruning calendar and basic rules

Propagation

Offsets

Older bulbs form offsets. Lift them in July–August when the leaves have withered and replant them in autumn. They take a few years to flower.

Seed

Sow ripe seed in autumn in a pot outdoors; the cold helps germination. It takes years to get a flowering plant.

Giant onion: 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 more dangerous than frost: in heavy clay the bulbs rot. A layer of gravel under the bulb helps.
• The leaves appear early and can get brown tips from a late frost; this is mostly cosmetic.
• In a pot: choose a deep pot with good drainage and move it against a wall or wrap it in hard frost.

Diseases and pests

• Bulb rot: The bulb turns soft and the plant no longer comes up, mostly in wet, heavy soil. Plant in free-draining soil with gravel under the bulb.
• Slugs: They eat the young leaves in spring. Pick them off in the evening or spread a layer of gravel around the plant.
• Withering leaves: Normal: the leaves wither while the plant is still in flower. Plant lower perennials in front.
• Falling stems: In a windy spot or in soil that is too rich and wet. Add a cane or plant in a sheltered spot.

Through the year

• March: The leaves appear. Watch for slugs.
• April: Add compost or organic fertiliser. Stake in windy spots.
• May: Flower buds appear. The leaves start to wither.
• June: Flowering. Cut flowers for the vase.
• July: Flowers and seed heads. Cut for drying.
• August: Remove withered leaves. Take off offsets.
• September: Plant bulbs.
• October: Plant bulbs.
• 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.

Butterflies

Nectar for butterflies.

Did you know?

• Allium is the Latin name for garlic. The giant onion is related to onion, leek and garlic; its bruised leaves smell of onion too.
• The dried flower heads keep their shape for months and are widely used in dried arrangements.
• When plant families were reclassified, the onions were moved into the amaryllis family: the giant onion is now a relative of the daffodil.