How do I grow alliums, and why do the leaves die before the flowers?

Ornamental alliums are among the easiest bulbs in a British garden and among the most misunderstood. The leaves of the big drumstick types start dying back just as the flower opens, which looks like a disease and is simply how the plant works. Everything else about growing them is straightforward.

What you'll need

• Allium bulbs, planted in autumn
• Spade or bulb planter
• Horticultural grit for heavy soil (optional)
• Low perennials to plant through (optional)

Step by step

1. Plant in autumn, September to November.They need the winter cold to perform. Bulbs planted in spring rarely flower well in their first year.
2. Plant deep — three to four times the bulb's own height.For a large Allium 'Globemaster' or 'Purple Sensation' bulb that means 150 to 200mm of soil above it. Deep planting anchors the tall stem and produces better flowers.
3. Give them full sun and sharp drainage.Alliums are dry-hillside plants. They rot in wet soil, and heavy clay is the main reason a planting dwindles. On clay, work grit into the planting area or grow them in raised beds or containers.
4. Expect the foliage to look terrible at flowering time.The strappy leaves of the large ornamental alliums yellow and collapse as the flower buds open. This is normal and is not a disease, a deficiency or a watering problem. It is why they are almost never planted as a stand-alone group.
5. Plant them through low perennials.This is the practical answer. Hardy geraniums, nepeta, alchemilla, low grasses and emerging herbaceous growth all cover the dying allium leaves at exactly the right moment while the flower heads stand above them.
6. Leave the leaves to die back naturally.Untidy as they are, they are feeding the bulb. Do not cut them off while they are still green.
7. Leave the seedheads standing.The dried spherical seedheads are one of the best features of the plant and hold their shape into autumn and often winter. Cut them for indoor use if you want, but do not tidy them away as a matter of course.
8. Decide whether you want them to seed.Some alliums, particularly Allium hollandicum 'Purple Sensation' and Allium cristophii, self-seed enthusiastically. In a relaxed border that is a bonus. If you do not want dozens of seedlings, remove the heads before the seed ripens and falls.
9. Feed and leave them alone.A general fertiliser in spring, and no further attention. Alliums do not need dividing often and resent disturbance.
10. Lift and divide only when flowering declines.After several years a clump may become congested. Lift it once the foliage has died down in summer, separate the bulbs, discard the smallest, and replant.

Tips

• Small alliums are a different proposition. Allium moly, Allium sphaerocephalon and Allium cernuum are shorter, later, and keep better foliage.
• Allium seedheads sprayed or left plain make excellent winter structure. They stand up to weather remarkably well.
• Alliums are relatives of the onion and are largely ignored by slugs, deer and rabbits, which makes them useful in gardens with browsing problems.
• Do not plant ornamental alliums in ground that has grown onions or leeks recently, or vice versa — they share pests and diseases such as white rot.

Common mistakes

• Cutting the yellowing leaves off before flowering — They are still feeding the bulb, and removing them reduces next year's flower. Hide them with other planting instead.
• Planting them in heavy wet soil without improving it — The bulbs rot over winter and the planting fades away within a couple of years.
• Planting them too shallow — Tall allium stems need anchoring, and shallow bulbs produce smaller flowers on stems that keel over.
• Tidying the seedheads away in July — They are one of the most valuable things the plant does. Leaving them extends the interest by months.

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