Planting and Growing Allium Bulbs

A Comprehensive Guide 

How to grow beautiful alliums that come back year after year

By the team at DIY Gardening

Alliums are perfect for bridging the gap between spring and summer, and it’s easy to see why they’ve become a staple for garden designers in the UK.

The rounded pom-pom flower heads look majestic in borders set amongst ornamental grasses. Still, they are versatile enough to be grown in cottage gardens, pots and containers where they pack a punch and grab attention.

Incredibly long-lived and flowering for months, these showy flowering bulbs are low-maintenance, and you don’t need to dig them up each year.

Below you’ll find DIY Gardening’s ultimate guide to planting and growing alliums.

purple allium flower heads

1) The Best Time to Plant Allium Bulbs

The best time to plant allium bulbs is in October or early November. 

You can plant in late September if its a cold month or even into early December if there’s no frost.

Allium bulbs don’t like to be kept out of the soil for long periods, so plant them reasonably soon after you buy them.

If you must store them, place them in a dark, well ventilated and cool, frost-free location.

2) General Location and Soil Condition

The best place to grow any allium is in full sun so pick the brightest part of the garden, both plant and bulb are drought tolerant so don’t be afraid to pick the driest part of the garden.

The bulbs dislike being in wet soil for long periods so choose a well-drained location. Common places to see alliums thrive are in elevated borders, large pots and other raised areas of the garden. 

Avoid shaded, damp areas where rainwater may collect as alliums won’t thrive here.

The soil should be well aerated and not too heavy or compacted.

For heavy, clay soils in the garden, add around 5mm of grit under the bulbs to help with drainage and aeration.

In pots, add some grit or broken terracotta to the base which will aid drainage. If you intend to keep the bulbs in the containers or pots for more than one season, add grit or perlite to the compost mix to help drainage and aeration.

Alliums only require only a yearly mulch of compost or well-rotted manure; you don’t need to fertilise the soil or compost throughout the growing season as you might with bedding plants.

3) Optional Products

The following products are entirely optional, but the bulb feed, in particular, is a worthwhile investment:

 Bulb Feed

Use this bulb feed at the time of planting as it contains mycorrhizal fungi which help to develop strong roots. This is an RHS endorsed product.

Bulb mycorrhizal fungi

 Potash For Poor Soil

If your soil is very poor, mulch with compost in spring and add a sprinkle of potash feed to boost the nutrient levels. Potash can be used with any type of bulb.

Potash feed for alliums

 Bulb Planting Set

Got hundreds of bulbs to plant in the garden? This bulb planter and large dibber makes planting bulbs easy, even in grass and heavily compacted soil.

Allium bulb planter

4) How to Plant Allium Bulbs

Bulb Depth

The bulb planting depth will depend on the size of the bulb but as a general rule of thumb; the absolute minimum depth is twice the length of the bulb. Ideally, the bulbs should be planted between three and four times their height.

Last autumn we purchased “Purple Sensation” allium bulbs which measured 3cm in length, and we planted them at 12cm depth.

For alliums in pots and containers, you may struggle to fit the bulb in them due to the limited space, make sure there’s at least 4cm of soil under the bulb, ideally more. 

If you’re raising an area of the garden, perhaps a flowerbed, do consider that rainfall will compact the freshly turned soil and may reduce the amount of soil above the bulbs. You may want to allow the ground to settle before planting the bulbs.

Bulb Spacing

Allium bulb spacing will depend on both the size of the bulb and the size of the flowerhead or pom-pom as I like to call them.

You can space the smaller bulbs (5mm) around 7.5cm-10cm apart.

Medium-sized bulbs such as allium Purple Sensations (roughly 3cm) can be planted between 10cm and 15cm apart.

Large bulbs, such as Purple Rain, which has a flower head of around 15cm should be planted at 30cm apart to prevent the flower heads from damaging each other in the wind.

Huge varieties, such as the Globemaster, have giant flower heads between 15cm and 20cm in width, so the bulbs should be spaced around 35cm to 40cm apart.

In pots and containers, you can place the bulbs closer together and even layer the bulbs with other varieties (see bulb lasagne).

purple allium flower heads

  5) Where to Plant Alliums

The usual advice is for gardeners to locate shorter plants at the front of a border while taller ones should go at the back, but allium foliage turns this advice upside down.

Allium foliage will turn an unsightly yellow and wilt, on larger alliums this may even occur before the plant has finished flowering, careful attention should be given to how you can hide this foliage.

If you follow the traditional advice of small alliums at the front and taller ones at the back, you’ll have a very gappy border with lots of yellow, fading foliage.

Tall allium stems can look beautiful, or they can look leggy and sparse. Which companion plants you place around the alliums is the key to success.

Alliums don’t need staking unless they’re grown in an exposed location.

6) Allium Companion Plants

As allium foliage is needed to feed the bulb for the next season’s growth, you shouldn’t remove it until the flower has finished blooming and the foliage has completely faded and wilted.

The solution is to choose companion plants that cover the unsightly foliage while allowing the stems to rise above and show off the beautiful flowering pom-pom blooms.

Try these plants:

  • Yellow, sun-tolerant hostas
  • Ornamental grasses
  • Nepeta (catmint)
  • Sedum
  • Hardy geranium

These Google Images search results may provide you with some inspiration: 

Alliums and hostas on Google Images

Alliums and catmint on Google Images

Alliums and ornamental grass on Google Images

Alliums and hardy geraniums on Google Images

We don’t recommend growing alliums as standalone border plants due to the foliage discolouration and wilt. Alliums look best with companion plants that compliment the tall stems (i.e. grasses) and cover the low-level foliage (i.e. hostas, catmint or hardy geraniums).

Longfield Gardens has a blog post with insightful tips on how to combine alliums with perennials. You can check out their guide here.

If you choose the ground-covering plants carefully, you can be very creative with your alliums, even placing the larger varieties such as the Globemaster at the front of the border. The tall stems allow a line of sight through to the plants at the back of the border.

7) Water, Feed and Care

Alliums don’t require a ton of water, and soggy soil can rot the bulbs, but like all plants that do need moisture.

We use about half an inch of water per week on our alliums during the growing season, and they do just fine but our border is raised and in full sun, so the soil does dry out very quickly. Alliums are generally drought tolerant and in many gardens require only rainwater.

Alliums in pots should be watered more frequently, but the pot doesn’t become waterlogged.

There’s no need to fertilise alliums throughout the growing season, just mulch once a year with some well-rotted manure or compost, that’s all they generally need.

If your soil is very poor, a dose of potash or a bulb feed such as this, deployed in spring will help, as it would any spring bulb.

You don’t need to deadhead alliums but doing so won’t harm them. Many gardeners leave the dead blooms in place as they add interest to the garden.

It would be best if you waited until the foliage has fully wilted before cutting it back, as it helps the bulbs to store nutrients for the next growing cycle.

8) When to Dig Up Allium Bulbs

Alliums bulbs don’t need to be lifted and stored each year. In fact, the bulbs don’t like being disturbed and certainly don’t like being stored for long periods.

You can safely leave the bulbs in the soil for years, and it won’t affect the performance of the plant.

We recommend digging up the bulbs every four to five years so you can discard any diseased or rotten bulbs and split any large ones into separate bulbs. The bulbs should ideally be replanted straight away rather than going into storage.

9) Pests and Diseases

Squirrels, foxes and other hungry animals often dig up ornamental bulbs, but fortunately, alliums have a slight onion odour and taste, so these garden pests usually steer clear of them.

(If you are having a problem with foxes in your garden, do check out our guide to deterring garden foxes)

There are a few other pests and several diseases that affect alliums.

Allium leaf miner is a common pest and treatment is usually impossible so the crop should be lifted and burnt as soon as the problem is detected. Leaf rust is also common but can be treated or the affected leaves removed. Several bacterial infections may be of concern, but most can be dealt with by removing leaves early. You can also treat infected plants with a fungicide.

10) Which Alliums are the Best to Grow?

With over 400 species of allium to choose from, there’s plenty of choices.

We love the classic Purple Sensation, but the large Globemaster is a real eye-catcher.

The drumstick alliums are unique and add interest to the collection.

Check out this image created by Longfield Gardens:

Allium infographic

Beautiful Allium Photos:

Green and purple allium flower head
Purple alliums on stems
Red allium flower
Alliums in border
Alliums and herbaceous flowers
Bees on allium flower heads

FAQs:

When should allium bulbs be planted?

In the UK, allium bubs can be planted between September and early December. The best time is generally between mid-October and mid-November.

What happens if I plant the bulbs too deep or too shallow?

As a general rule of thumb, a little too deep is better than too shallow as frost can damage the bulb. If you want the bulbs to grow year after year, plant a little deeper – between 3 and 4 times the length of the bulb.

What happens if I plant allium bulbs too close together?

You can place a few bulbs close together for effect but in general, the roots may become tangled and compete for nutrients and moisture. Dehydrated bulbs usually fail so it’s best to have some space between the bulbs, especially if you’re planting lots of bulbs.

Do alliums spread?

The bulbs do multiply in clumps which can be lifted and split into separate bulbs every few years. If you don’t split the clumps of bulbs, the roots will compete for moisture and nutrients and the flowers will be less impressive each year until split.

What are the tallest alliums?

The tallest alliums are Allium Gigantium, His Excellency, White Giant, Ambassador and Gladiator. These alliums can reach a height of up to 120cm (4ft) and may require staking unless planted in a sheltered location.

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This guide to planting and growing alliums was created by the Hannah from DIY Gardening

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Hannah Miller