Growing guide
How to grow alliums
These ornamental onions are bold and architectural, with large rounded heads of usually purple flowers, followed by attractive seedheads. Weave them through sunny borders or combine them with feathery grasses for best effect.
Quick facts
- Easy to grow
- Flowers in spring and early summer
- Best planted in well-drained soil
- Thrive in full sun and drought tolerant
- Avoid damp sites on heavy soil
- Propagate from offsets, aerial bulbils or seed
- Good cut flowers
- Attractive to pollinators
All you need to know
Choosing alliums
To choose the right allium for you consider the following:
- Flower size and colour Alliums are available in shades of blue, purple, mauve, pink, yellow and white and with a range of flower head sizes from just a few centimetres to 18-20cm (7-8in) in diameter
- Height Tall alliums such as Allium stipitatum ‘Mount Everest’, which can be over 90cm (3ft) in height, add an architectural quality to your garden. You can plant them in the middle of a border where they can stand above shorter neighbours. You can plant smaller-flowered alliums, for example Allium cristophii, in clumps nearer the front of the border or in the rock garden
- Flowering time Most flower in May and June. For later flowers, try Allium sphaerocephalon for colour in July and August
- Consider how you would like your alliums to combine or contrast with other early-flowering
Perennials are any plant living for at least three years. The term is also commonly used for herbaceous perennials which grow for many years (To compare: annual = one year, biennial = two years).
- Specialist alliums, such as Allium insumbicum, are best grown in containers or the rock garden so you can give them the care they need more easily
- If you have cooler more moist conditions you could grow one of the smaller alliums more suited to woodland conditions e.g. Allium moly ‘Jeannine’.
Discover award-winning alliums
Buying Alliums
These are fleshy, rounded, underground storage organs, usually sold and planted while dormant. Examples include daffodils, tulips, hyacinths, lilies, onions and garlic. The term is often used to cover other underground storage organs, including corms, tubers and rhizomes.
To track down specific cultivars, you can use the RHS Find a Plant or visit a specialist nursery.
When to plant
Plant allium bulbs in early to mid autumn.
Where to plant
- For best results border alliums need a sheltered site (to avoid the flower spikes getting blown over) and a free-draining soil with plenty of sunshine
- Allium can also be grown in containers
- Avoid planting in cold exposed or waterlogged conditions as bulbs can rot
- It's best not to plant the bulbs in areas of the garden that are regularly cultivated as it is easy to damage the bulbs when digging
- The leaves die down at flowering time, so it’s best to plant alliums where this foliage is masked by that of other plants. As the leaves start to die back they can be removed with no ill effects
How to plant
Prepare your soil
Dig over the soil and remove any weeds before planting. Avoid planting into freshly manured soils which may be too nutrient rich.
In borders
Plant bulbs in early autumn at a depth of about four times the diameter of the bulbs. Plant smaller growing alliums 7.5-10cm (3-4in) apart, and taller species need at least 20cm (8in) between the bulbs.
An introduction to planting bulbs
In containers
Most alliums will do well in deep pots. Either use a good peat-free general purpose potting compost or mix equal parts of John Innes No. 3, peat-free multi-purpose compost and horticultural grit.
Alliums in pots can be placed behind other containers to mask the fading foliage.
How to plant bulbs in a pot
Watering
Alliums are drought tolerant and watering of plants grown in the ground is not usually necessary. Plants dislike summer irrigation because this may cause the bulbs to rot. Alliums grown in containers will need regular watering, but make sure the compost does not become waterlogged.
Feeding
Regular feeding is not necessary but on poor soils, apply balanced fertiliser such as Growmore in spring.
Deadheading
You can cut off the spent flower heads at the base but this is not essential as the dried flower heads look attractive in the border.
Overwintering
Plants in the ground are hardy and don’t need any special care over winter
Plants in containers can be more susceptible to winter cold. To help them survive the winter
- Move containers to a sheltered spot
- Give them some protection from winter rain by standing them in the lee of a wall or in a cold frame or greenhouse
Overwintering plants
Caring for older plants
If clumps of older plants are becoming overcrowded you can lift, divide and replant them after the foliage and flowers have died down.
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.
You can propagate alliums by seed, however hybrids will not ‘come true’ (i.e. they may vary in colour and shape from the parents) by this method. It is best to sow ripe seeds as soon as possible. Sow into trays of gritty compost and and cover the seeds with 5mm of grit. Place the containers outside in a shady spot. Alternatively store seeds 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.
RHS Guide to propagating bulbs
Discover alliums
Everything you need to know about choosing the right allium for you.
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