These ornamental onions are bold and architectural, with large rounded heads of usually purple flowers, followed by attractive seed heads. Weave them through sunny borders or combine them with feathery grasses for best effect.
Allium are best planted in autumn, just about when much else in the garden is beginning to fade. The ideal months are from September to mid November but as late as the middle of December is acceptable. They can also be planted in spring but won't flower as profusely during their first year.
The small and middle sized alliums (A.Moly, A.Sphaerocephalon etc) can be planted en-masse. Depending on the size of the bulb, plant 10-15cm deep and 10-15cm apart. A general rule of thumb is to plant bulbs 3x their width deep but alliums do need to be a little deeper.
Tip: Throw them in the air and plant them where they land for a more naturalised look
The larger alliums (A.Globemaster, Christophii, etc) can be planted in groups or individually at a depth of 15cm. They need much more space for their larger heads:
Make sure your growing site doesn't gather water after a heavy rain. Wet and boggy soil will hinder their growth and likely rot the bulbs. Work in lots of sand and grit if you have heavy clay soil.
Caring for your alliums
Once planted, alliums are virtually maintenance free. They might need a little water during a drought but they are rather self sufficient. A little liquid feed during flowering will extend the blooms of any perennial. You can cut the flowers for the vase just before they come into flower, don't worry this will not harm the plants at all. Do leave all foliage in place until it has completely died back. The leaves will continue to feed the plant right up until dormancy.
Alliums will die back after flowering and disappear back under ground until the following year. After a number of years they might divide so much under the soil that they start to come up 'blind' with just foliage. This is when you should dig them up and separate the bulbs.
Many allium species produce offsets, new young plants. Once flowering is over and leaves have died down, you can lift the bulbs and detach the offsets. Either plant them directly in their final positions or grow on outside in pots of gritty compost.
Some alliums (Allium roseum, A. Sphaerocephalon and A. vineale) produce aerial bulbils (small young bulbs produced instead of flowers) in the flower head. These bulbils can be carefully removed and separated. The bulbils can be planted in moist free-draining compost about 2.5cm (1in) apart and covered with 1cm (3/8in) layer of compost. It will take several years for them to reach flowering size.