Round-headed leek (Allium sphaerocephalon) is a perennial found throughout Europe northern Africa and western Asia. The large round “head” that blooms in summer is actually made up of hundreds of small purple flowers. Its popularity among gardeners has resulted in many subspecies and a range of local names.
I. Appearance and Characteristics
Allium sphaerocephalon is a plant species in the Amaryllis family known as round-headed leek, round-headed garlic, ball-head onion, and other variations on these names. Drumstick allium is another common name applied to this species. Some publications use the alternate spelling Allium sphaerocephalum. It is a bulbous herbaceous perennial plant.
Allium sphaerocephalon is found in the wild across all parts of Europe except in the northern and western countries (Scotland, Ireland, Netherlands, Scandinavia, and the Baltic States). Its native range extends to northern Africa and to western Asia as far east as Iran. It is also reportedly naturalised in New York State in the United States. In England it grows wild only in the Avon Gorge so is known locally as the Bristol onion.
The specific epithet sphaerocephalon derives from ancient Greek, meaning “spherical head”.
This plant has gained the Royal Horticultural Society’s Award of Garden Merit.
Allium sphaerocephalon produces egg-shaped bulbs. Small bulblets are present under the outer layer of the stem (making the plant potentially invasive when grown in gardens). Flowers are borne on a scape up to 50 cm in height, in a spherical to egg-shaped umbel, 1–6 cm in diameter, tightly packed with many flowers crowded together. The relatively small size of the umbel relative to the height of the stem makes this one of those described as “drumstick alliums”. Individual flowers are reddish-purple and are occasionally replaced by bulbils (again making this a potentially invasive species).
II. How to Grow and Care
Sunlight and Soil
Round-headed leek is easily grown in rich, sandy to gritty, dry to medium moisture, well-drained soils in full sun. Add grit when grown in clay soils to improve drainage. This Allium is drought tolerant.
Watering
Thriving in its native habitats with ample moisture, round-headed leek has adapted to require consistent hydration. It exhibits a preference for regular watering, aligning with its origins in environments that maintain steady water availability. To sustain its health and vigor, round-headed leek should be watered every week. In indoor settings, round-headed leek benefits from higher humidity levels to mimic its natural conditions. As a bulbous plant, round-headed leek’s water needs are particularly crucial during its active growing season to ensure proper development and blooming.
Fertilizing
For optimal flourishing, round-headed leek benefits greatly from balanced nutrition fertilizers, with an emphasis on high-phosphorus versions during the flowering stage to boost bloom production. Apply fertilizer every six weeks during the growing season, with quantities aligned to manufacturer’s instructions, ensuring not to overload and risk root burn. As round-headed leek enters dormancy in colder months, reduce feeding to prevent wasteful accumulation of nutrients. Practical tip: dilute the fertilizer to half strength for young plants to prevent damage. Always water round-headed leek thoroughly after fertilization to help nutrient absorption and mitigate the risk of root burn.
Propagation
- From Seed
Like many of their edible allium cousins, ornamental alliums self-seed readily.
Without any work required on your part, they’ll send seeds out into the surrounding area, and you’ll find new plants popping up all the time.
Don’t worry, though. They aren’t invasive, and it’s easy to pull unwelcome starts.
You can snip the flower heads in the fall when they turn dry, during September and October.
Deadheading helps to prevent them from spreading, with the added bonus that you can rub the seed heads in your hands to separate out the chaff and retain the rest for planting next season.
Store saved seeds in a sealed container in a cool, dark place and they can last for up to two years.
These are plants that do best in well-draining, slightly sandy soil. If you have heavy clay or dense soil, you’ll want to work in some sand to improve drainage before putting seeds in the ground.
To plant seeds, whether purchased or saved from existing plants, loosen the soil in the garden where you’re planting after the last frost has passed in the spring.
Sprinkle seeds liberally over the prepared area and gently cover with about 1/4 inch of soil. Water in well at planting time, and don’t let the seeds dry out while they’re germinating.
Once the seedlings emerge, thin them to six inches apart and reduce watering. The top inch of soil should thoroughly dry out in between at this point.
Keep in mind that you won’t get flowers for the first year or two when you plant by seed.
- By Division
In the fall after the foliage dies back, you can dig up and divide mature plants. Cut away any dead or dying foliage after you pull up the plant.
Dividing is a good idea even if you don’t plan to use the bulbs, because the plants are healthier when they are well-spaced.
Alliums don’t have particularly deep roots. Using a spade, gently dig a six-inch margin around the plant, going down about nine inches.
Lift the clump of soil out and gently tease out up to half of the bulbs.
Replace some of the bulbs in the soil in the original planting area with four to six inches between each at the same depth they were growing before you dug them up, and fill in with dirt.
The plants will return next spring.
Take the rest of the bulbs and plant them individually in a new location. Prepare the soil at the new planting location by adding sand if it’s particularly heavy or does not drain well.
- From Transplants or Bulbs
You can purchase bulbs, or get some bulbs or starts from a friend or neighbor for planting in the spring or fall.
Loosen the soil in the planting area and work in a little bulb fertilizer according to package directions. Place bulbs into holes about three times as deep as the bulbs are long, with the pointy ends facing up.
Place bulbs four to six inches apart. Gently cover over with soil and water deeply.
If you can’t plant your bulbs right away, put them in a cool, dark area until you can get them in the ground.
Maintenance
Deadhead the plants in the fall if you want to prevent them from spreading or naturalizing in your garden.
If you want to tidy up the garden, cut plants back to the ground, removing the seedheads if you haven’t already, as well as the dead foliage.
After the foliage has died back, you can dig up and separate bulbs for planting elsewhere. This helps to maintain good spacing, which is helpful for avoiding fungal issues.
Pests and Diseases
Drumstick allium is wonderfully untroubled by herbivores. Deer and squirrels just walk (or bounce) on by.
Most insects aren’t terribly interested in alliums either. In fact, the main problem you might encounter is fungal disease.
That said, even though there are a few pests and diseases out there to keep an eye on, ornamental alliums are generally hardy against these kinds of problems – unlike their edible cousins.
Still, there are a few things you should be aware of that we’ll cover here.
Common Pests
There are only a few insects to watch out for with these ornamental bloomers!
- Allium Leafminers
The allium leafminer (Phytomyza gymnostoma) can be devastating to onion and garlic crops, but for ornamental alliums, they’re more of an inconvenience.
This little pest is common in Europe and is just making its way into the US.
Sightings were confirmed in Pennsylvania in 2015, and since then it has spread into parts of New England and Maryland
The flies have clear wings and yellow heads. They lay their eggs in the spring and fall, and after a few days, the white or light yellow larvae hatch and start chewing their way through allium leaves.
While this can stunt plant growth or cause the foliage to become deformed, damaged allium foliage will typically die back just before blossoming, so this pest usually doesn’t usually have time to cause a major problem.
They can, however, create conditions that invite fungi to attack when they chew holes in your plant.
If you want to deter them, put down silver reflective mulch in the spring. Place yellow sticky traps within a foot or two of your plants at about six inches above the ground to attract and capture the adults at the same time.
You can also make a trap to capture adults by placing soapy water in a yellow bowl at the base of plants.
You only need a few drops of dish soap for a cup of water, and the water only needs to be an inch deep for the trap to work. The flies will visit the water and drown.
If you’re still having trouble, use a spinosad-based insecticide following the manufacturer’s directions.
- Onion Thrips
Thrips (Thrips tabaci) are tiny insects – about 1.5 millimeters long – with two sets of wings that are covered in long hair.
The adults have pale yellow or light brown bodies. Immature thrips are slightly smaller and wingless.
These pests love dry, dusty conditions.
The adults and nymphs damage plants by nibbling on the bulbs and leaves.
If they do enough damage to the leaves, this can cause a reduction of photosynthesis, which can lead to lower rates of flowering. It may even cause flowering to stop altogether.
Predator mites, pirate bugs, and lacewings all love to make a meal out of thrips, so do what you can to encourage these beneficial insects to hang out in your garden.
This naturally occurring fungus reacts to UV light, so products containing it should be applied in the evening or when the weather is cloudy, according to the manufacturer’s directions.
Common Disease
Ornamental alliums face potential issues caused by a few different types of fungi that may destroy your pretty plants.
- Downy Mildew
Downy mildew is caused by the fungus Peronospora destructor, which targets plants in the allium family. At first, you’ll see plants exhibit stunted growth, and the leaves will turn light green or yellow.
Eventually, plants will turn brown and die.
You can sometimes spot fine white furry growth on the leaves, and you may see violet lesions as well.
This type of fungi thrives in cool temperatures and high humidity. Spores move through the air and attach to plants that have moisture on the leaves.
Once air temperatures reach 75°F and higher, the fungus can no longer thrive.
Since fungi love moisture, it’s vital to water at the soil level early in the day rather than sprinkling water on leaves of your plants.
You should also trim any other plants that are growing near your alliums to make sure there’s plenty of space for adequate air circulation.
Keep your gardens weed-free to prevent the spread of disease, and buy certified disease-free seed if it is available.
- White Rot
White rot is caused by the fungus Stromatina cepivorum. It attacks all types of plants in the allium family.
The disease is soil-borne and can live on the earth for decades. It thrives when air temperatures are between 50 and 75°F.
The first sign of this disease that you’ll see is outer leaves of the plant turning yellow and dying off. Underground, the roots and bulbs will start rotting.
Once your plants have white rot, there isn’t anything you can do. Pull the plants and dispose of them in the garbage rather than on the compost pile. Don’t plant alliums in the same location again for at least 15 years, unless you sterilize the soil first.
You can sterilize the soil using solarization, which involves placing transparent plastic over the soil in the summer and allowing it to sit for 4-6 weeks.
This heats the soil up and will kill most pathogens.
III. Uses and Benefits
Round-headed leek is a very showy ornamental species often planted at garden borders and in cottage and coastal gardens. It is known for its showy purple blossoms. Round-headed leek blossoms in late summer and are considered perfect for late season gardens. It is thought to have the most ornamental impact visually when planted in groups of twenty bulbs or more.
Round-headed leek is excellent as cut flowers for both fresh and dried arrangements! Beloved by florists for its long vase life!