Allium

Alliums, also known as ornamental onions, are grown for their showy flower heads, which come in wide range of sizes and shades of blue, purple, white and yellow. Even when the plants die back, the dried flower heads look attractive in the garden, or can be cut for indoor display.

Alliums. Image: RHS

Quick facts

Common name Ornamental onion
Latin name Allium
Group Bulb
Flowering time Spring to mid-summer
Planting time Autumn
Height and spread Up to 1.8m (6ft) and 30cm (1ft)
Aspect Sun
Hardiness Hardy to borderline hardy
Difficulty Easy to Moderate

Cultivation notes

Alliums thrive in a sheltered, sunny, open, well-drained site. They do not like cold, exposed or waterlogged conditions. Also avoid planting in freshly manured soils. A number of smaller growing species do well in cooler, moister conditions such as Allium cyathophorum var. farreri and A. moly.

Depending on height, alliums can be grown in the front or back of the border.

Planting

Bulbs should be planted in early autumn at a depth of about four times the diameter of the bulb. Plant smaller growing alliums 8-10cm (3-4in) apart, taller species need at least 20cm (8in) between the bulbs.

On poorer soils, apply a balanced fertiliser in spring such as Growmore.

Containers

Most alliums will do well in deep pots. Although any good multipupose potting media will do, a mix of equal parts of John Innes No 3, multipurpose compost and horticultural grit is sometime preferred. Mask the fading foliage by placing the pots behind other containers or just plunge them in the border. Re-pot annually in the autumn.

Aftercare

The often straggly leaves begin to die down at flowering and for this reason alliums are best planted so they are masked behind the foliage of other plants.

Lift, divide and replant overcrowded clumps after the foliage and flowers have died down.

Propagation

Many allium species produce offsets. Once flowering is over and leaves have died down, lift the bulbs and detach the offsets. Plant them in the final position or grow on in gritty compost.

Some alliums (Allium roseum, A. sphaerocephalon and A. vineale) produce aerial bulbils in flower head. Carefully remove and separate the bulbils and plant them in moist free draining compost about 2.5cm (1in) apart and cover with 1cm (3/8in) layer of compost.

It is possible to propagate alliums by seed, however hybrids will not come true from this method. It is best to sow ripe seeds as soon as possible. Alternatively store in a fridge and sow in spring at about 13°C (55°F). Most should germinate within 12 weeks. It will take several years to reach the flowering size.

Cultivar Selection

Allium ‘Globemaster’ AGM: This reliable allium has deep violet heads, 15-20cm (6-8in) across, in summer, and reaches a height of about 80cm (32in).
Allium hollandicum ‘Purple Sensation’ AGM: This allium has smaller heads of deep violet, 8cm (3in) across, and reaches a height of about 1m (3ft).
Allium karataviense AGM: This low-growing allium can be planted at the front of a border as it only reaches 10-25cm (4-10in) in height; it has small heads of pale pink star-shaped flowers and greyish purple broad basal leaves.

Links

RHS Plant Finder
RHS Plant Selector
AGM plants

Problems

Ornamental alliums suffer from the same pest and diseases as culinary onions such as onion white rot, downy mildew and onion fly, but they tend to be trouble free in the border. However, it is best to avoid planting where onion crops have been grown. Pests to be on the watch for include slugs, snails and allium leaf miner.

Poor flowering is usually due to growing conditions. Shallow planting, growing on very light or wet soils is likely to result in decline of the vigour. Plant alliums in full sun and make sure that the foliage is not shaded by other plants.

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