Ornamental Onion (Allium)

Onion, Ornamental Onion

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.

I. Appearance and Characteristics 

Allium is a genus of monocotyledonous flowering plants with hundreds of species, including the cultivated onion, garlic, scallion, shallot, leek, and chives. The generic name Allium is the Latin word for garlic, and the type species for the genus is Allium sativum which means “cultivated garlic”.

Carl Linnaeus first described the genus Allium in 1753. Some sources refer to Greek ἀλέω (aleo, to avoid) by reason of the smell of garlic. Various Allium have been cultivated from the earliest times, and about a dozen species are economically important as crops, or garden vegetables, and an increasing number of species are important as ornamental plants.

The genus Allium (alliums) is characterized by herbaceous geophyte perennials with true bulbs, some of which are borne on rhizomes, and an onion or garlic odor and flavor.

The bulbs are solitary or clustered and tunicate and the plants are perennialized by the bulbs reforming annually from the base of the old bulbs, or are produced on the ends of rhizomes or, in a few species, at the ends of stolons. A small number of species have tuberous roots. The bulbs’ outer coats are commonly brown or grey, with a smooth texture, and are fibrous, or with cellular reticulation. The inner coats of the bulbs are membranous.

Many alliums have basal leaves that commonly wither away from the tips downward before or while the plants flower, but some species have persistent foliage. Plants produce from one to 12 leaves, most species having linear, channeled or flat leaf blades. The leaf blades are straight or variously coiled, but some species have broad leaves, including A. victorialis and A. tricoccum. The leaves are sessile, and very rarely narrowed into a petiole.

The flowers, which are produced on scapes are erect or in some species pendent, having six petal-like tepals produced in two whorls. The flowers have one style and six epipetalous stamens; the anthers and pollen can vary in color depending on the species. The ovaries are superior, and three-lobed with three locules.

The fruits are capsules that open longitudinally along the capsule wall between the partitions of the locule. The seeds are black, and have a rounded shape.

The terete or flattened flowering scapes are normally persistent. The inflorescences are umbels, in which the outside flowers bloom first and flowering progresses to the inside. Some species produce bulbils within the umbels, and in some species, such as Allium paradoxum, the bulbils replace some or all the flowers. The umbels are subtended by noticeable spathe bracts, which are commonly fused and normally have around three veins.

Some bulbous alliums increase by forming little bulbs or “offsets” around the old one, as well as by seed. Several species can form many bulbils in the flowerhead; in the so-called “tree onion” or Egyptian onion (A. × proliferum) the bulbils are few, but large enough to be pickled.

Many of the species of Allium have been used as food items throughout their ranges. There are several unrelated species that are somewhat similar in appearance to Alliums but are poisonous (e.g. in North America, death camas, Toxicoscordion venenosum), but none of these has the distinctive scent of onions or garlic.

Dogs and cats are very susceptible to poisoning after the consumption of certain species. Even cattle have suffered onion toxicosis. Vegetables of the Allium genus can cause digestive disorders for human beings.

II. How to Grow and Care

Sunlight

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.

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.

Watering

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

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.

Fertilizing

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 for blooming.

Pruning

When allium has finished blooming, removing spent blooms can encourage the plants to store more energy for next year’s show, but it isn’t necessary. Leaving the old flowers on can add interest as they dry (they look like miniature fireworks) and encourage reseeding if you’re hoping to increase the number of allium plants in your garden.

Propagation

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.

Potting and Repotting 

Allium grown in a pot will need repotting as it outgrows its container. In autumn, dig out the bulbs of the plant and divide where needed. Replant in pots that are well-draining, or add divided bulbs to in-ground gardens before the first frost.

Pests and Diseases

There are very few pests or problems with alliums. Because alliums belong to the onion clan, these blooms are associated with that trademark odor. This smell works as an animal deterrent and, coupled with their taste, prevents alliums from being eaten by creatures like pesky rabbits, deer, and other browsing animals. Many gardeners take advantage of this and plant them among other plants to act as a barrier to troublesome critters.

Alliums can develop fungal diseases like rot and downy mildew, but these issues are easily avoided. To prevent rot and mildew, avoid overhead watering and remove any infected bulbs as soon as you notice them.

III. 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.

IV. Uses and Benefits 

Many Allium species and hybrids are cultivated as ornamentals.

The genus includes many economically important species. These include onions (A. cepa), French shallots (A. oschaninii), leeks (A. ampeloprasum), garlic (A. sativum), and herbs such as scallions (various Allium species) and chives (A. schoenoprasum). Some have been used as traditional medicines.

This genus also includes species that are abundantly gathered from the wild such as wild garlic (Allium ursinum) and ramps (Allium tricoccum).

Ornamental Onion (Allium) Details

Common name Onion, Ornamental Onion
Botanical name Allium
Plant type Bulb
Hardiness zone 4a, 4b, 5a, 5b, 6a, 6b, 7a, 7b, 8a, 8b, 9a, 9b
Growth rate Medium
Height 1 ft. 0 in. - 3 ft. 0 in.
Width 1 ft. 0 in. - 3 ft. 0 in.
Sunlight Full sun (6 or more hours of direct sunlight a day)
Soil condition High Organic Matter
Flower color Gold/Yellow
Leaf color Green