Common name: Ornamental onion
Latin name: Allium
Group: Bulb
Cultivation
Alliums are grown for their showy flower heads on leafless stems. They come in wide range of sizes. The colour range is dominated by shades of blue and mauve, but white, yellow and deep purple can be found. The attractive, dried flower heads of taller species can be left in situ or collected for indoor display.
Alliums dislike cold, exposed or waterlogged conditions. Plant in early autumn, avoiding freshly manured soils (which encourage soft growth that is more prone to damage), and plant deeply for good flowering performance, about four times the diameter of the bulb. Space smaller selections 8–10cm (3-4in) apart, taller types at least 20cm (8in) apart. On poorer soils apply a balanced fertiliser in spring. Most alliums do well in deep pots (equal parts of John Innes No 3, multipurpose compost and horticultural grit is ideal). Repot each autumn.
Depending on their size, alliums can be grown in the front or back of the border, in a container and smaller species can be used in rockeries. 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. A number of smaller growing species do well in cooler moister conditions such as A. cyathophorum var. farreri and A. moly.
Most alliums will do well in deep pots. Plant in a mix of equal parts of John Innes No. 3, multipurpose compost and horticultural grit. Mask the fading foliage by placing the pots behind other containers or just plunge them in the border. Re-pot annually in the autumn.
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 (A. 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.
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 used to be grown. In most cases the decline of the flowering performance is down 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.
Cultivar selection
Allium ‘Globemaster’ AGM
Allium hollandicum ‘Purple Sensation’ AGM
Allium karataviense AGM
Click here for other recommended AGM varieities