How to Grow Allium (Ornamental Onion)

alliums

The Spruce / Letícia Almeida  

Most of us don’t think of onions as beautiful plants, but onions have some very close cousins that definitely deserve a place in your flower garden. Fast-growing ornamental alliums grow tall and have round flower heads composed of dozens of star-shaped flowers. While these plants are not edible, their leaves do have a slight onion-like scent when crushed.

Ornamental alliums won’t spice up your cooking, but their cheerful spherical flowers will enliven your garden. These are extremely tough plants that are both drought-resistant and cold tolerant. They’re not even bothered by deer or rodents, and there are plenty to choose from for any garden. Allium bulbs should be planted in the fall.

Botanical Name   Allium
Common Name  Ornamental allium
Plant Type  Bulb or rhizome
Mature Size   1-4 feet tall, 3-10 inches wide
Sun Exposure  Full sun
Soil Type   All; must be well-draining
Soil pH   5.5-6.5
Bloom Time  Spring, but there are fall bloomers
Flower Color   Pink, purple, yellow, white, and green
Hardiness Zones   4-10 (USDA)
Native Area   Middle East
Toxicity  Safe for humans; toxic to dogs and cats

Allium Care

The majority of alliums are bulb-forming; however, there is a handful that grows from rhizomes, the way common chives (Allium schoenoprasum) do. These may never form any kind of bulb. Allium leaves tend to be long and strappy. Some—like the cork-screw allium—remain attractive all season, with a blue-green color that complements the flowers. Most early blooming alliums have foliage that tends to die-back early, as the plants go dormant for the summer.

The flowers form in clusters and are best known in the round pom-pom form, but they can be star-shaped, cup-shaped, semi-circular, or pendulous. There's a good amount of variety in allium plants. Drumstick alliums only grow about one foot tall with 1-inch flower heads, while giant 'Globemaster' can top 4 feet in height and sport huge globes of 8- to 10-inch flower heads.

Most allium bulbs grow quickly, and they bloom in the spring or early summer after the earliest spring bulbs have faded. However, there are a few varieties that bloom later in the season, even well into fall.

The plump, round shape of the flowers looks charming poking through other plants, whether low-growing mats such as hardy geraniums or shrubby roses. The purple color is a great asset that complements most other late spring flowers, from peonies to iris to catmint. The shape also works well with other medium height plants, like foxglove or monarda.

The big drawback to early-blooming alliums is how their leaves can start to go downhill, even before the plants have flowered. If at all possible, try to hide the foliage behind a denser plant; daylilies work well for this.

Light

For the best flowering and healthiest plants, place your alliums in a site that gets a full day of sun. They will grow in partial shade, but since so many of them have short seasons, give them as much sun as you can.

Soil

Alliums prefer a soil pH that is slightly acidic, at around 5.5 to 6.5. However, how well the soil drains is far more important than soil pH. Do not let the bulbs sit in damp soil, especially during their dormant season. If they remain wet for too long, they will rot. Adding a good amount of organic matter to the soil before planting will improve draining while allowing enough water to reach the bulbs. 

Water

Alliums need infrequent watering, and if it rains often that should do the trick. Otherwise, watering every three to five days is fine.

Temperature and Humidity

Hardiness depends on the variety being grown and the growing conditions, but most alliums will do well in USDA hardiness zones 4-10.

Fertilizer

If you regularly amend your soil, you may not need to feed them at all. However, if your soil is less than ideal, a little balanced fertilizer as they start to set flowers will help them replenish all the energy they use blooming.

Is Allium Toxic?

Allium is safe for humans but the entire plant is very poisonous for cats and dogs if ingested because it contains organic sulfur compounds that convert to highly reactive oxidants in the intestinal tract.

Symptoms of Poisoning

Symptoms of toxicity include elevated heart and respiratory rates, loss of consciousness, reduced appetite, lethargy, weakness, diarrhea, abdominal discomfort, and jaundice. Call your veterinarian or veterinary ER immediately.

Allium Varieties

  • 'Drumstick allium': (Allium Sphaerocephalon) Their 1-inch flower clusters bloom in early summer, and start off greenish and eventually start to resemble red clover. They look best when allowed to waft their way throughout the garden so that they can surprise you by peeking out through other flowers
  • 'Corkscrew allium': (Allium senescens ssp. montanum var. glaucum) It’s the blue-green leaves that twist and turn and give this allium its common name. You may find this one in the perennial section of garden centers since it grows from rhizomes rather than a bulb. The 2-inch lavender flower heads are flattened balls that bloom in mid- to late summer
  • 'Globemaster': Globemaster flower stalks are super-sized, can reach 3 to 4 feet tall, and topped with flowers that form a ball that’s 8 to 10 inches in diameter. Mount Everest is another imposing, tall allium. It’s not quite the size of Globemaster and it blooms in a creamy white
  • 'Japanese onion': (Allium thunbergii) Offering small, 1-inch pink flower heads that bloom in early fall, they form more of a mop than a round globe. Some cultivars to look for include Ozawa with larger purple flower heads and Alba which has white cup-shaped florets. This allium grows from rhizomes, rather than forming bulbs, and may be available in containers
  • 'Nodding onion': (Allium cernuum) This variety produces flopping mop heads of flowers in pretty shades of pink and purple. The flower stems average 2 to 3 feet. This is one of the most widely adaptable alliums and can even be grown well in partial shade
  • 'Purple Sensation': The flower stalks reach about 2 feet tall and are topped with a 2- to 4-inch globe of bright purple flowers. Purple Sensation tends to be a long-lived bulb, but its leaves tend to get yellow or brown quite early, which can detract from the fabulous flowers
  • 'Schubert allium': (Allium schubertii) The flower heads of Schubertii alliums look like a fireworks display. Even as they fade, they retain their explosive look. Another common name for Schubertii is the tumbleweed onion.

Pruning

Alliums do not repeat bloom. Trim the flower stalks down after flowering in order to send the plant's focus back into storing energy in the bulb. However, the dried flower heads are as attractive as the live flowers and many gardeners like to keep them standing.

Propagating Allium

The bulb forming alliums will need to be planted in the fall. The planting depth should be two to three times the diameter of the bulbs. (So if you have a 2-inch bulb, you would plant it 4 to 6 inches deep.) Water them well after planting, then cross your fingers and wait for spring.

Bulb-forming alliums are very slow to multiply; however, they will eventually start forming small offsets on the original bulbs or perhaps even on the flower head. Once the plants have finished flowering, you can lift the bulbs and remove the offsets. These can be replanted immediately, but it may take a couple of years before they flower.

The rhizome forming alliums can be planted anytime. You may not find the fall-blooming varieties in the garden center until late summer. Rhizome-forming alliums can be lifted and divided any time the clump starts looking crowded. Don't wait until the center of the plant dies out, before dividing.

Common Pests/Diseases

Ornamental onions, like their culinary cousins, don't attract too many pests. Deer and rodents avoid them.

They can get a few fungal diseases, like downy mildew and rot, but these are not as much of a problem in a flower border as they would be in a vegetable garden. To fix, avoid overhead watering and remove infected bulbs.

As far as insect pests, watch out for snails and slugs, as well as the allium leaf miner. However, since the foliage does not last very long, cosmetic damage to the leaves is not something to worry about.

closeup of allium
The Spruce / Letícia Almeida 
allium as an edging plant
The Spruce / Letícia Almeida  
closeup of allium group
The Spruce / Letícia Almeida  
allium in a field
The Spruce / Letícia Almeida  
garden with alliums
Clive Nichols / Getty Images
Allium in the garden
Jason Moskowitz Photography/Getty Images