How to Grow & Care for Allium (Ornamental Onion)

Ornamental onion (allium) is a fast-growing, deer-resistant plant that is tolerant of drought and cold. Planted in the fall, allium bulbs need full sun and slightly acidic soil to produce flower heads composed of dozens of star-shaped blooms. It can be mildly toxic depending on the plant's age, how much is ingested, and the sensitivity of the person or pet.

Allium Care

Here are the main care requirements for ornamental onion.

• Plant them where they can receive a full day of sun.
• Maintain a soil pH that is slightly acidic.
• Be careful not to overwater and make sure soil has good drainage.
• Fertilize in late summer especially if your soil is lacking any nutrients.
• Cut the plants back in the late fall after the leaves have dried up.
  

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 or during times of prolonged drought.

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. This can be in late summer or early fall.

Types of Allium

• '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
• '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
  
• '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

Bulb-forming alliums are very slow to multiply.; however, they will eventually start forming small offsets on the original bulbs or perhaps even the flower head. The best way to propagate them is through the division of the bulbs when they reach this stage in late fall or early spring. Here's how:

1. Wait until the plants have finished flowering.
2. Lift the bulbs and gently peel off the offsets.
3. Replant them immediately at a depth two to three times the diameter of the bulbs.
4. Water them well after planting.
5. Cross your fingers and wait for spring.

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.

How to get Ornamental Allium to Bloom

Ornamental alliums won’t spice up your cooking, but their cheerful spherical flowers will enliven your garden.

Bloom Months

Most allium bulbs grow quickly, and they bloom in the spring or early summer after the earliest spring bulbs have faded. Some early-blooming alliums have foliage that tends to die back early, as the plants go dormant for the summer. However, there are a few varieties that bloom later in the season, even well into fall.

What Does Ornamental Allium Flowers Look and Smell Like?

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 can be as many as a dozen flowers in a cluster. Blooms vary among varieties of allium. Some have only 1-inch flower heads while others have 8-inch flower heads. Colors range from white to pink and purple.

The odor from ornamental onion comes from the stalks and leaves more so than the flowers.

How to Encourage More Blooms

Promote plentiful blooms by deadheading any spent flower heads at the base. Additionally, you can stake taller varieties to support the blooms. A slow-release granular fertilizer in late summer will ensure nutrients stay balanced throughout the season.