How To Grow Allium Bulbs: Guide For Ornamental Onions

How To Grow Allium Bulbs: Guide For Ornamental Onions

Alliums are often referred to as ornamental onions because they are in the same family as onions, leeks and garlic. Plus, if you crush their leaves, you get that oniony smell.

Like edible onion plants, the flowers are quite spectacular as they appear in a round ball on top of tall stems. The round flower heads are actually a tightly bound structure with many tiny flowers.

Alliums come in a range of different floral colours including white, yellow, pink and blue. These delightful plants make a great addition to cottage gardens and are ideal for planting at the back of a border. They also make excellent cut flowers.

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What’s The Best Time To Plant Allium Bulbs?

Being part of the onion family, you would expect alliums to grow from bulbs and you would be right. This makes them fairly easy to plant in late autumn or fall. This gives the bulbs time to become established over winter so that they can put on their floral display in late spring to early summer.

Where Should You Plant Your Allium Bulbs?

Large allium flower heads

Alliums can be planted in full sun in your garden but many will also handle partial shade, especially in the afternoon.

These plants need well-drained soil that is fairly rich in organic material. Alliums don’t like heavy soil and should not be planted in soggy soil either.

When the plants start to flower, the green strappy leaves will turn yellow and die back. This is why they are perfect for planting at the back of your borders.

How To Plant Your Allium Bulbs

White allium flower heads

The easiest way to plant bulbs of any kind is to use a tool called, you guessed it, a bulb planter. These tools are specifically designed to create a planting hole to the correct depth and will excavate the soil at the same time.

These are even available with a long handle for gardeners who have trouble kneeling down in the garden. However, if you don’t have one of these special tools, you can just use a hand trowel instead. 

When planting your allium bulbs, you want to plant them quite deep. In fact, the hole should be four times deeper than the height or size of the bulb.

Make sure that you plant your bulbs with the root nodes at the base and the pointy end up, so that they can grow correctly. Then, fill the planting hole again and firm the soil down gently. 

The most important thing to remember is to ensure that the soil is free-draining and does not become waterlogged. If the bulbs are wet for an extended time, they might rot.

How To Care For Your Ornamental Alliums

bright pink allium flower heads

Alliums are fairly hardy plants and don’t require a lot of expert care. When the new foliage emerges in early spring, watch out for slugs and snails. You can deter these by using either snail baits or pet and wildlife-friendly snail pellets.

Some gardeners even suggest placing crushed eggshells around your plants as snails and slugs don’t like crawling over them. I’ve had some success keeping snails out of pots by using copper tape to encircle the outside of the pot.

Once your alliums are flowering, feed them with a slow-release fertiliser. This will help the bulbs to develop further and store enough food to produce flowers in the following year.

Leave the foliage until it has died back completely and then you can safely remove it. While the leaves are still green, they’ll be helping the bulbs to store food.

Alliums also don’t need to be deadheaded after flowering. This is because even the dried allium flowers are quite attractive and will add an extra dimension to your garden borders until at least late summer. 

How To Propagate Allium Plants

hands holding a bunch of allium bulbs

Like most plants that grow from bulbs, alliums will produce small flower bulbs from the mother plant. These are commonly referred to as offsets. Each of these small bulbs will eventually turn into a full-grown flowering plant. However, it usually takes a few years before they will start to produce flowers.

You can use this to your advantage to plant additional bulbs in other parts of your garden or even in some pots. 

Once your plants have finished flowering, you can gently dig up the bulbs using a small garden fork. Take care not to damage the bulbs as you’re doing this.

Remove the soil from the bulbs and look for tiny offsets growing from the main bulb. You can easily remove these and plant them elsewhere in your garden.

Pests And Diseases To Look Out For

While alliums are fairly hardy, there are a few pests and diseases to be aware of. Here are the major ones.

White Rot

This is a fungus disease which is common in plants in the onion family. It is mainly a problem if the soil is not well-drained. The disease causes the bulbs to rot and the foliage to turn yellow and wilt.

Unfortunately, there’s no effective control against this disease. If your plants do succumb to this, make sure that you don’t plant any bulbs in the same spot in your garden. This is because the fungal spores can live in the soil for many years.

Downy Mildew

This is another fungal disease but it mainly affects the foliage. The leaves end up with spots and will eventually die. 

To avoid your plants succumbing to this disease water only in the morning so that the leaves are not wet overnight. If you see any affected leaves, remove them immediately and throw them in the garbage. This will stop the disease from spreading. 

Allium Leaf Miner

The larvae of these insect pests will feed on the bulbs while the adults will suck the sap out of the foliage. This is another pest that is difficult to control because the larvae live in the soil. 

Allium giganteum

There are quite a few different varieties of alliums that you can grow. Here are just some of the more popular ones:

  • ‘Purple Sensation’ – tall alliums with gorgeous dark purple flowers
  • ‘Mount Everest’ – even taller alliums with lovely white flowers
  • ‘Silver Spring’ – with small white flowers that have purple centres
  • Allium giganteum – a six-foot-tall giant allium with purple flowers
  • Allium sphaerocephalon – with maroon flower heads on tall stems

If you want more flower-growing tips, check out:

5 Edible Flowers That Can Add Colour To Your Veggie Patch

Best Soil For Dahlias

Frequently Asked Questions

Can alliums be grown in pots?

Yes, alliums will do quite well in pots as long as the pot is deep enough and the plants are kept well watered. It’s also essential for the pots to have good drainage.

How many allium bulbs should I plant together?

When planting allium bulbs, it’s best to just plant one bulb per planting hole and to spread the bulbs around 3 to 6 inches (7 to 10 cm) apart. This ensures enough airflow around the plants and helps to prevent fungal diseases.

Can you leave allium bulbs in the ground all year?

Yes, these bulbs will do fine in the ground, even over winter. This means that you can enjoy their flowers year after year.

Final Thoughts

Alliums or ornamental onions make a delightful addition to your flower garden, Their flowers add a spot of colour and form and they’re relatively easy to grow. 

And, once you plant these in your garden, you can enjoy them for years to come.

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