Allium: The Ornamental Onion
Height 30-150cm (1 -5ft)
Planting distance 8-30cm (3-12in)
Flowers in summer
Sunny open position
Bulbs available in autumn
Ornamental onions are becoming increasingly popular because of their easy-going nature, tough constitution and long-lasting. The flowers appear in summer and, according to variety, range in colour from white, yellow and blue to deep lilac and rose. Some alliums are ideal for mixed or herbaceous borders, others for rock gardens, sink gardens and even indoor displays. Their onionlike smell is only a problem when leaves and stems are bruised.
Allium albopilosum (syn. Christophii) is best grown among herbaceous plants that hide its untidy grey-blue leaves without giving too much shade. Plant the bulbs 10-12cm (4-5in) apart. In early summer the large heads of striking star-shaped lilac-pink flowers appear on stems 60cm (2ft) tall. The seed heads that follow in autumn and winter are excellent for indoor flower arrangements.
Allium caeruleum (syn. Azureum) has densely packed balls of star-like flax-blue flowers among long thin mid-green leaves in early and mid summer. Set the bulbs 15cm (6in) apart. As the flowers are carried on 60cm (2ft) high stems, they are suitable as cut flowers.
Allium giganteum stands well above most other herbaceous plants, often reaching an impressive height of 1.5m (5ft). It is grown for its large decorative flower heads: balls of mauve star-shaped florets that appear in early summer. Like many of the ornamental onions, it has long thin grey-blue leaves. Plant 23-30cm (9-12in) apart.
Allium moly forms a vivid display in early and mid summer with its bright yellow star-shaped flowers among grey lance-shaped leaves. It reaches only 30cm (12in) high, but is particularly invasive, so avoid planting it among small delicate plants. Space the bulbs 10cm (4in) apart.
Allium oreophilum var. ostrow-skianum, a popular member of the family, reaches only 30cm (12in) high. Planted 8cm (3in) apart it can spread rapidly. The small rose-coloured flowers and drooping grey-blue leaves are excellent for livening up rockeries in early summer.
Allium roseum has elegant rosy-pink star-shaped flowers that come out in early summer. It reaches 30cm (1ft) high and has long, broad, mid-green leaves. Plant the bulbs 10cm (4in) apart, in a sunny and sheltered site.
Alliums grow best in ordinary, well-drained soil in open sunny sites. Plant the bulbs in early to mid autumn, setting them in clumps of six or more so the flowers have impact. Cover each bulb with soil to twice its depth — if the dormant bulb is 4cm (1/2in) high plant it 8cm (3in) deep.
Every spring apply a dressing of general fertilizer or bone-meal. When the flowers are over, cut off the heads (some make good dried arrangements). In autumn, remove the dead leaves and stems. Tall alliums growing in a windy position may need support with canes.
Detach bulblets from the base of mature bulbs in autumn or in spring and replant immediately in moist soil. Or increase from ripe, home-saved seed.
Pests and diseases
Young shoots, leaves and stems are often eaten by slugs. White rot can cause the leaves to turn yellow and die back – the roots also rot and the bulbs become covered with a fluffy white fungus growth.