With its long green stems topped by round purple or white blooms, allium, or flowering onion, is a striking addition to any garden. Interestingly enough, it's also part of the plant genus that includes garlic, onions, leeks, shallots, chives, and others.
If you're interested in growing this beautiful plant, you may wonder, "When and where should I plant allium bulbs?" Well, you're in the right place!
When and where to plant allium bulbs depends on the zone in which you live. You should plant them in autumn when the soil has cooled off after the summer.
Here's when to plant for each zone:
- Zones 3-5: late September to early October
- Zones 6-8: October to mid-November
In the rest of this article, we'll share more detailed information about when and where to plant alliums. That includes soil pH levels, sunlight needs, and more. You probably have more allium questions, too, and we'll do our best to answer those as well.
So, let's get started!
When & Where To Plant Allium Bulbs? [By Zone]
Because they need time to develop their roots over the winter before blooming in late spring, you should plant allium bulbs in autumn. And, since they need cool temperatures to help them develop correctly, you should plant them after the soil has cooled.
Of course, this timing varies depending on where you are located. In general, though, you can follow this schedule:
- Zones 3-5 [northern half of the United States]: late September to early October
- Zones 6-8 [southern half of the United States]: October to mid-November
What Kind of Soil Do Allium Bulbs Like?
The most important thing to consider when choosing soil for your allium is making sure that it's well-drained. Since alliums are bulbs, they are at high risk of rotting in damp ground.
Try working sand or grit into the planting area if your soil is dense and filled with clay. You'll also want to choose an uphill spot that doesn't collect rainwater.
Aside from drainage considerations, alliums are very tolerant of almost any type of soil. However, when given an option, they prefer soil with a neutral to slightly acidic pH of 5.5 to 6.5.
If you're unsure of your soil's pH level, you can easily check with an at-home test kit. Luster Leaf makes a kit that includes 10 test strips.
How Much Sun Do Alliums Like?
Alliums will grow happily in partial shade to full sun. However, planting them in a spot with full sunlight maximizes their fairly short growing season.
Receiving more sunlight also helps them grow stronger stems. If you want to avoid ending up with drooping flowers, choose a sunny spot for your alliums.
How To Plant Allium Bulbs
Planting allium bulbs is pretty easy, but we'll walk you through the process since there are a few details you'll need to be aware of for the best results.
What You'll Need
- Hose or watering can
- Tape measure or yardstick [optional]
Pick A Spot
First, choose a spot to plant your alliums using the light and soil requirements mentioned above. Additionally, alliums come in various heights, ranging from 5" to 4' tall, with most clocking in around 2'.
Because of this, you'll want to consider the plants growing near your alliums and ensure that the taller plants are growing behind, the shorter ones.
Prepare The Hole
Next, use your shovel to dig a hole. The size of the hole depends on the size of the allium bulb. A good rule is to plant the bulb about 2 to 3 times deeper than its height.
For example, if your bulb is 3" long, you'll need to dig a 6" to 9" deep hole. You can use a tape measure or yardstick to measure the bulb and the depth of the hole, or you can simply eyeball it.
After figuring out the hole's depth, dig it wide enough for the bulb to fit snugly. The width of the hole doesn't matter as much as the depth.
If you're planting more than one allium bulb, space the holes 4" to 8" apart. Again, this depends on the size of your chosen allium variety. You can figure out the spacing by finding out how big the flower head will be, then add 1" or 2".
Since alliums grow straight up instead of spreading, they typically look better when planted as close together as possible.
Plant The Bulbs
Plant the allium bulb by placing it in the hole with the pointy end facing up. Then, refill the hole with dirt and pack it gently around the bulb.
After you've planted all the bulbs, give them a generous soak with water from the watering can or hose. This helps the soil settle around them and gives your bulbs a great start as they begin growing!
How To Care For Alliums
Alliums are an easy-to-care-for plant, but you can do a few things to ensure they stay healthy and have a productive growing season.
Here are some supplies you might need to care for your alliums throughout their growing season:
What You'll Need
- Watering can or hose
- Stakes or long, thin branches
As they grow, you should aim to give your alliums about 1" of water weekly. This means that you'll need to water whenever the top inch of soil dries out.
However, be mindful of over-watering. Since alliums are bulbs, it's easy for them to start rotting if they get too damp.
Use Stakes If Necessary
If you have one of the very tall allium varieties that reach heights of 3' or 4', you may need to stake them. Keep an eye on the alliums as they grow, and if they start drooping or falling over, you can give them some support with a stake.
To stake an allium, find a stake that is just slightly shorter than your plant. You can purchase wooden or metal stakes or simply use a thin but strong stick from a tree in your yard.
Push the stake about 4" into the soil next to the allium, or however deep it needs to go to be secure.
Now, take a piece of string between 12" and 24" long and wrap it gently around the allium and stake.
It should be snug enough to hold the plant's stem close to the stake but not so tight that it cuts into the stem. Tie the ends of the string together to hold everything in place.
There are two times during your allium's growing season that you should apply fertilizer. One is immediately after the shoots appear, and the other is when the plants start to flower. Use 5-10-10 fertilizer on the shoots and 0-0-10 or 0-0-50 fertilizer as they bloom.
In addition to these two times during the growing season, add 5-10-10 or all-purpose flower fertilizer in the spring of every year. If you'd rather not use fertilizer, you can get similar results from adding aged manure or compost to the soil.
To learn more about fertilizer, you can check out our article "How Long Does Fertilizer Last In Soil?"
Watch Out For Diseases
Mildew usually shows up as powdery white on the tops of the leaves or sometimes on the stems. Rust, on the other hand, is reddish-brown spots that eventually turn into pustules.
Remove Faded Blooms and Foliage
Keep an eye on your allium as early summer approaches, and the blooms start to fade. When this happens, cut the flower heads off and use them in fresh flower arrangements.
They also dry extremely well, which can provide you with colorful flowers all year round!
After cutting off the flowers, allow the leaves to grow for a few more weeks. This gives the allium time to continue photosynthesizing and saving energy in the bulb for the long winter months.
Around the middle of summer, you'll notice your allium's leaves and stems start to turn yellow and shrivel. You can remove all of the foliage by gently pulling it out of the ground.
It should easily detach from the bulb with a gentle tug. If there's any resistance, leave it alone and try again in a few more days.
After the foliage has all been removed, your allium is in a dormant state and will remain that way until next spring.
Allium bulbs should be planted in the fall. You can grow them in zones 3-5 from late September to October. In zones 6-8, on the other hand, you should wait until October to mid-November.
Plant the bulbs in well-drained, neutral to slightly acidic soil, and choose a spot that gets partial shade to full sun for the best results.
If you're interested in growing other types of bulbs, you might also like our article "How To Grow Tulip Bulbs."