How to Grow Allium

Hey everyone! This is the third in a series of (probably) four fall bulb-planting posts I’ll be writing. I’m excited to be getting some bulbs in the ground at our new house—and today’s post is all about how to plant and grow allium.

You might have heard of this plant as something called “ornamental onion.” And that’s because “allium” is actually a genus of plants with hundreds of different species. The allium genus includes onions, garlic, leeks, chives, scallions, and shallots.

Ornamental allium has big bright purple poof-ball-like flowers. The most popular type of allium is probably allium hollandicum “purple sensation,” aka the Persian onion. (It’s native to Iran and Kyrgyzstan.) That’s what I’m planting.

Want more spring flowers? Check out my posts for How to Plant Tulips and How to Grow Hyacinth!

Timing & location

While alliums don’t bloom until late spring, the fall before your first frost is generally when you should plant them. If you live in an area with all four seasons, that likely means October, give or take. If you live somewhere warmer, it will likely be later. 

You might even need to chill them in the fridge before planting. Or buy them pre-chilled. Your local nursery will be the best source of help on planting time and if you need to chill the bulbs first.

Alliums like a lot of sun, so you’ll want to plant your bulbs in an area that gets great light. They will tolerate partial shade, but the flowers might not be as large, showy, and impressive.

Planting depth

Your package should include instructions on the required depth and spacing for your bulbs. However, a general rule of thumb is that if a bulb is 2 inches, you should plant it 5-6 inches deep (roughly 3x the size). Plant one bulb per hole.

Allium bulbs tolerate a variety of soils quite well. However, the soil must be well-draining and in an area where water doesn’t pool or sit for long periods of time after rain. That will rot the bulbs quickly.

We have very dense clay-based soil, so I used a bulb planter to dig out a spot about 6 inches deep. Then I dropped the bulb in with the pointy side up. When backfilling the dirt over the bulb, I mix the dirt with leaf compost. This added nutrients while also lightening the soil up—a critical step in preventing excess moisture retention.

Deadheading

After your alliums are done blooming, the flower heads will die off on their own. Much like hydrangea flowers, they also look kind of cool while they are dying. So you might choose to keep them on the plant for a bit longer.

However, you’ll eventually want to cut the stem and flower off—but not the leaves! When you cut the stem and flower off of the plant, it will allow the plant to focus all of its energy into the bulb, which is what you want for nice flowers next year.

Cutting back

Wait until your foliage dies off completely before cutting alliums totally back to the ground. This is an essential step in ensuring you get the best flowers possible during the next year.

The leaves are busy acting as solar panels, taking in sun and storing energy in their bulbs. This is the energy the bulbs need to recharge over the winter and eventually produce leaves and flowers the following spring.

It’s possible that your allium’s leaves will begin yellowing while the plant is still flowering. While this kind of sucks, it’s sometimes just part of the process. It happens to mine all the time (see below). You may choose to grow something else with nice foliage around your alliums to help camouflage the foliage.

If you are growing your allium in the ground, you can just keep it there. You don’t need to water the bulbs at all. You can dig them up if you want to—just make sure to store them in a cool, dry place until planting them again in the fall.

Bulb multiplication

Allium bulbs will multiply if you keep them in the ground and the care conditions are ideal. That means a well-draining soil where water doesn’t gather too often, plenty of light, and giving the foliage time to die off naturally before cutting it back.

After a few years, the allium bulbs will begin producing babies or “offsets.” You can dig up the bulbs and separate these offsets to plant them. In fact, after several years, you should probably dig up the bulbs to divide them so the growing area doesn’t get too crowded.

Growing in pots

You can grow allium in pots. If you don’t have space to put them in the ground or just don’t want to commit to growing them in the ground, this is a great option. The process is largely the same.

One thing to keep in mind when planting alliums in pots, though, is the difference in moisture retention. While allium plants are quite tolerant to dry conditions, they soil in pots can dry out much faster than it can in the ground.

So you might need to water your alliums a bit more in pots. But this really only goes for the spring when things start to warm up. They should be fine over the fall and winter.

Be mindful of temperatures where you live, though. If temperatures jump around a lot, the bulbs can rot from freezing and thawing repeatedly. Not a problem where I live…it mostly stays cold all winter!

Pest deterrence

As part of the allium family, ornamental onions do have a light garlicy/oniony scent. Grden pests like deer, groundhogs, rabbits, squirrels, etc., can smell it and don’t like it!

That means that planting allium flowers around other tasty bulbs or delicious flowers can be a good strategy. I planted them alongside my hyacinth and tulip bulbs—the latter of which is vulnerable to hungry critters.

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