Ornamental native onions

Credit: Richard Hebda

Flowering onions are making a comeback in the garden. From the stunning sky-blue of Allium caeruleum to the purple-violet of towering Allium giganteum, the choice is huge. And British Columbia gardeners are fortunate to be able to choose from three native onions that qualify as both attractive and surprisingly hardy garden subjects.

Nodding onion (Allium cernuum), the most extensively grown native North American onion, arises from a cluster of small narrow bulbs covered in a thin grey membrane. Under the covering, the bulbs have a distinctive pinkish lining, while several grass-like, grey-green leaves arise from the top of the bulb. Up to 20 centimetres long, these onion-scented leaves remain green throughout the year. This onion’s flowering stalks stand 30 to 50 centimetres tall and display a nodding cluster of pale to intensely rosy-pink flowers that measure about half a centimetre across and consist of six tepals (three petals and three modified sepals). These blooms appear from May to August. Nodding onion grows in open woodlands, meadows and rocky knolls from the Peace River country southwards in B.C., especially in the Interior.

Relatively uncommon in our province, Hooker’s onion (Allium acuminatum) flourishes on the east side of Vancouver Island and in dry parts of southwest B.C. It favours pockets of shallow, dry soil on rocky knolls. This native onion rises from a small grey-brown bulb covered in a netted tunic. Two to four slim grass-like leaves sprout early and wither before the flowers appear. The 10- to 30-centimetre-tall, stiff flower stalk bears a loose cluster of five to 30 pink to intense rosy-purple blooms, but unlike the flowers of nodding onion, these blooms face upward and the tips of the three (of six) outside tepals curl back in a distinctive manner. The flowers open from June to July.

The little-known slimleaf onion (Allium amplectens) performs superbly in a dry native species bed in our garden. Growing 30 to 45 centimetres tall from a small, brownish, scaly bulb, slimleaf onion has scented leaves that wither early. Its stiff flower stalk bears a nearly spherical mass of white to light-pink flowers measuring five centimetres across. Its six pointed tepals form a saucer-shaped bloom about one centimetre across. Flowers appear as early as late March, but more typically in May and June. A rare species in B.C., slimleaf onion inhabits spring-moist rocky bluffs and meadows from southeast Vancouver Island and the Gulf Islands to California.

All three native onion species are exceptionally easy to grow and drought resistant. The best site is a sunny, well-drained rock garden or dry border. During the dormant season (midsummer to early fall), plant the bulbs of Hooker’s and slimleaf onion near the front of the bed five centimetres deep to the top of bulb. Keep the ground clear of weeds for the best display. Leave them in the ground for several years before digging up and dividing the nest of little bulbs. Plant the matted masses of nodding onion bulbs five to 10 centimetres deep in well-drained humusy soil in fall or spring and water in well. Being generally more robust than the other two species, nodding onion will thrive in partial shade at the edge of shrubs and among modestly vigorous perennials. Divide it every three to four years by digging up and pulling apart the bulb masses and replanting immediately.

Our wild onions require little after-planting care, but start them off with a spoonful of bone meal in the planting hole. Most garden centres carry nodding onion and sometimes Hooker’s, however the slimleaf onion is more rare. Check for it through local native plant groups and specialists. All three species can be raised from fall-sown seed, though slimleaf onion rarely produces it.

The following plants are hardy to the zone number indicated:

• Allium acuminatum (Hooker’s onion) – zone 3
• A. amplectens (slimleaf onion) – zone 2
• A. caeruleum (sky-blue onion) – zone 3
• A. cernuum (nodding onion) – zone 2
• A. giganteum – zone 4

An expert on native plants, Richard Hebda is curator of Botany and Earth History at the Royal British Columbia Museum in Victoria.

Related links:
Allium giganteum video
Ornamental onions
How to dry alliums for winter decoration