How To Grow And Care For Allium Ornamental Onions

Pinterest Hidden Image

There’s no doubt you’ve enjoyed onions, garlic, chives, or leeks in your food at some point. These are the most popular edible alliums out there, but you might not have heard the term allium or even about the ornamental plants that make up most of the genus.

Yes, this genus – which may include anywhere from 260 to 979 species (the exact number varies from one classification system to another) – is far more wonderous than the superfoods it’s known for.

Caring for AlliumsPin

In fact, the ornamental onions, as the flashier species and cultivars are commonly called, are some of the most fantastic plants you’ll ever see outside of Dr. Seuss.

Of course, seeing them may make one wonder if Seuss based his fanciful world on these plants.

Some are smaller, such as the drumstick allium (Allium sphaerocephalon), or several feet tall with massive globe umbels like the gigantic onion (Allium giganteum).

But no matter which ornamental alliums you choose to grow, they require almost no effort and add beauty and functionality to any ornamental or crop garden space.

How To Grow And Care For Allium Ornamental Onions?

Ornamental alliums practically grow themselves, and many will self-seed or come back year after year with little or no effort on your part.

Best of all, they repel a wide range of pests (even deer and rabbits!) while attracting pollinators when used as companion plants.

Do You Need Special Soil Or Fertilizer For Allium Plants?

The remarkable answer is no and no!

Alliums can grow in almost any soil type as long as it’s well-drained and can exist without ever being fertilized.

You may wish to mix coarse sand or perlite into your garden plot if the ground is heavy clay, just to ensure drainage.

Vibrant purple allium flowers in a garden.Pin
Photo Credit: Instagram @english_cottage_garden

Likewise, many growers like to mix in a bit of organic compost while loosening the soil for spring plantings to boost their ornamental alliums.

They love an acidic soil pH of 5.5 to 6.5 but can tolerate up to a neutral 7.0, with a few varieties able to handle up to 8.0.

Water, Temperature, And Sunlight Needs

Water these beauties when the soil feels dry 1″ to 2″ inches down using the soak and dry method, regardless of whether they’re potted or in the ground.

This method simply requires you to pour the water slowly and evenly around the base of the plant without getting the foliage wet, stopping when the ground has trouble keeping up or when you see moisture seeping from the drainage holes.

You can grow most of these ornamentals in USDA hardiness zones 4 to 8, although some varieties can handle slightly cooler or warmer zones.

They can suffer damage in temperatures below 40° degrees Fahrenheit, so you may need to overwinter the bulbs indoors in colder climates.

Also, temperatures exceeding around 80° degrees Fahrenheit in the spring can weaken the flower stalks, which won’t be able to hold the umbel’s weight.

As for sunlight, the full sun is the way to go!

In climates where the afternoon sun is too harsh, you can give them a full morning or evening sun with partial shade in the afternoon.

A minimum of 6 hours of full sun per day is generally best if you want the most out of its blooms.

White allium flowers in a garden setting.Pin
Photo Credit: Instagram @mariapaalmbacken

How To Plant Allium Bulbs In Gardens Or Pots?

Whether you want to use allium plants in borders, as a deterrent intermingled with other plants, or in a container, the rules are pretty much the same.

It’s best to plant your bulbs in the fall, although especially cold regions will need to plant them in a pot to transplant in spring.

For smaller varieties, you’ll need to make the hole 4″ inches deep, and for the big ones, go 8″ inches deep.

Loosen your soil a little deeper to help with root growth, and mix in some organic compost if you have poor soil.

You may wish to plant smaller alliums in groups of up to 15 and larger ones in spaced-out groups of 3 (exact spacing can vary from one species to another).

However, if you plan to intersperse them with other plants, try to space them around their companions, so they’re not too far apart.

If your area is prone to some crisp winters but not deep freezes, you can protect garden plantings with a layer of insulating bark or volcanic rock or set up a temporary cold frame over the spot.

In spring, you may lay a layer of mulch around the sprouts, although this isn’t necessary.

If you’re overwintering these plants in pots, transplant them in the spring to the same depth as a fall planting.

Peat pots are perfect as you won’t have to risk damaging their delicate roots by removing them from a solid pot.

Alternatively, you can keep your allium in the container to grow on a patio, deck, or windowsill.

Bloom Time For Allium Flowers

The foliage appears pretty early, creating a clump of usually blue-green, strap-like leaves that make for a nice ground cover.

Most alliums will soon send up their tall stalks, which can be up to 10 times the height of the plant.

Giant alliums tend only to have single stalks, while smaller alliums may have several.

Suddenly, a round or egg-shaped umbel bursts into dozens or hundreds of tiny flowers, often star-shaped.

Taller varieties will have umbels that can measure 4″ to 5″ inches wide, while the smaller ones will usually only be closer to 2″ inches wide.

Vibrant purple allium flowers in bloom.Pin
Photo Credit: Instagram @entreplantasviajes

The blooms will actually only live for a few days, but they dry out in full color, leaving behind a display that can last well into late summer.

If fertilized, the umbel will turn into seed heads that can be harvested for propagating, although many choose to cut the umbels for use in flower arrangements before this happens.

Please note that the foliage of early bloomers will usually die back in summer, and the plant goes dormant, and it’s very rare for alliums to bloom again.

There are, of course, a few exceptions to this rule that will bloom a second time in the fall.

Additionally, some ornamental alliums won’t bloom until late summer, so mix and match if you want a display that continues well into the fall.

If you’re overwintering the plant in the ground, cut it back to ground level and add a protective layer of bark or similar to shield it from deep frosts.

However, if you’re overwintering the bulbs indoors, you can excavate the root ball and clip the foliage after.

Just be sure to let the foliage completely die back so the bulb can gather as much energy as possible for the winter.


Finally, let’s take a brief moment to talk about propagating ornamental onions.

Most alliums have bulbs, but a few (like the drumstick mentioned above allium) have rhizomes.

Division is a popular propagation method in both cases, especially for cultivars.

Alternatively, you can harvest the seed heads and grow your alliums from scratch.

Just be warned, if you have a cultivar, the seeds will almost always produce the parent plant, and a few cultivars are actually sterile.

JOIN Our FREE Plant Care Newsletter 

By entering your email address you agree to receive a daily email newsletter from Plant Care Today. We'll respect your privacy and unsubscribe at any time.