Allium
I knew I had hit a home run – landscape wise – when I discovered some visiting friends peering intently at some plantings in the far corner of my ornamental garden. As I drew near to join in their conversation, I found that their attention had been drawn to a swath of deep purple irises backed by tall, silvery purple alliums. The golden foliage of nearby shrubs provided a pleasing and harmonious contrast to the composition’s various purple hues. Although quite lovely in their own right, it was neither the irises nor the shrubs that garnered so many comments from my friends. It was the alliums that had captured their fancy!
It wasn’t all that long ago that alliums, also known as ornamental onions, were a rarity in the landscape. However, they’re really catching on with gardeners, who appreciate their beauty and recognize their merits as an architectural contrast to the mounding or spreading forms of other ornamental plants. It’s exciting to see these charming bulbs appearing in landscapes everywhere.
Ornamental onions belong to a large botanical genus that includes chives, shallots, leeks, onions and garlic. But, unlike their edible cousins, the ornamental varieties are grown strictly for their showy spherical flower heads. Ranging from 1 to 10 inches wide, the flower heads are formed from clusters of tiny, star-shaped flowers. The flower heads on some of the larger varieties may contain 100 or more individual flowers. Depending on the selection, the flowers range in color from silvery lavender to reddish purple as well as white, yellow and blue.
Allium aflatunense ‘Purple Sensation’
The flower heads on most ornamental onion varieties rise well above a clump of strap-like leaves. The foliage of the early bloomers emerges very early in spring and generally dies back just as the plants come into bloom. Late-blooming alliums maintain their attractive green foliage much longer into the season. It’s wise to position the early bloomers in the landscape so that surrounding plantings hide the deteriorating leaves.
In addition to providing beauty and architectural interest to the ornamental garden, alliums attract bees and butterflies. If that’s not a good enough reason to try alliums, here’s more good news. Deer, rabbits, voles, squirrels, chipmunks and other critters avoid them because of the faint oniony taste of the foliage and bulbs.
A SAMPLING OF ALLIUM SPECIES
The ornamental allium family consists of more than 300 species. The bulbs vary considerably in size, generally in proportion to the plant’s overall height. Some of the shorter allium selections grow to just 6 inches while some of the taller selections top out at around 4 feet. Most varieties grow to 18 inches or more in height and are good choices for the middle or back of the border.
A sampling of the more popular allium species and hybrids grown in American gardens include:
- A. aflatunense ‘Purple Sensation’ – While I like all ornamental onions, this selection is one of my favorites. I love the 4- to 5-inch wide, violet-purple flower heads that rise on 2- to 3-ft. tall stems in mid spring. The saturated color adds depth and drama to the landscape.
- A. Caeruleum – The smallish 1- to 2-in. flower heads on 2-ft. tall stems are a beautiful shade of sky blue.
- A. christophii – Also known as Star of Persia, this species has one of the largest flower heads of the allium family. A typical flower head consists of hundreds of star-shaped florets arranged in a loose, airy 10- to 12-in. wide configuration.
- A. Giganteum—Topping out at 4 feet or more, this is one of the tallest members of the Allium genus. The long-lasting flower heads typically measure about 4 in. across.
- A. ‘Globemaster’ — Silvery purple softball-size flower heads bloom in May on 3-ft. tall stems. This is the variety that enchanted visitors to my garden.
- A. moly ‘Jeannine’ — This shorter species blooms on 12-in. stems and produces bright yellow flowers in mid-summer.
- A. schubertii — This is clearly one of the more extraordinary members of the allium family. The flower heads are quite large and flamboyant. Some sources describe it as a lavender fireworks display.
- A. sphaerocephalon – Commonly known as drumstick allium,
Allium sphaerocephalon (Drumstick Allum)
this later-blooming variety sports 1- to 2-in. wide egg-shaped, reddish-purple blossoms around June. This selection looks best planted in a large grouping. It can take a little more moisture than its siblings, particularly in the summer.
- A. stipitatum ‘Mount Everest’ — One of many white-blooming species, this selection produces 6- to 8-in. snow-white blossoms with dark green eyes atop 3-ft. tall stems.
USES FOR ALLIUMS IN THE LANDSCAPE
- Plant the smaller, low-growing varieties at the front of the mixed border or in a rock garden. Use medium-height varieties in the middle of the border to add texture and height. The taller species and hybrids provide attention-getting vertical elements in mixed borders. While they may grow 3 to 4 feet tall, they do not require staking unless the site is very windy.
- Use as a bridge between early spring-blooming bulbs and summer-blooming bulbs.
- Plant them to attract bees, butterflies and hummingbirds.
- Combine with plants that will help hide allium foliage as it dies back. Peonies, irises, roses, and catmint are a few suggested companions.
- Combine with other colors ranging from the palest lavender to the deepest purples. For example, plant reddish-purple selection A. aflatunense ‘Purple Sensation’ near Salvia nemorosa ‘Caradonna’, which echoes the same basic shade of purple. Plant ‘Giganteum’ with a deeper shade of purple Iris such as ‘Swingtown.’ For a more saturated color combination, plant burgundy-colored drumstick allium near the contrasting hues of golden Coreopsis.
- Combine the purple varieties with plants that have silver foliage, such as Artemisia or Stachys, or gold foliage, such as Agastache foeniculum ‘Golden Jubilee’, for a classic color combination.
- Try a white variety with pale pink roses or plants with variegated green and white foliage for a sophisticated and elegant combination.
- Stagger blooming times to keep the show going. For example, ‘Purple Sensation’ is one of the earliest to bloom in May, followed by A. giganteum. Drumstick allium picks up the baton in early summer and pink-blooming A. thunbergii ‘Ozawa’ blooms in late summer to early fall. A. stellatum (American prairie onion), a native selection, blooms from late summer into fall.
- After the flowers fade, leave the seed heads in place to provide interest in the garden for the rest of the summer.
- Use the dried flower heads in flower arrangements or even Christmas decorations.
- Allow alliums to slowly colonize to increase the display. Certain varieties are better colonizers than others, but the species commonly available for sale are not considered invasive.
HOW TO GROW ALLIUMS
Ornamental alliums are very easy to grow but they do have some specific requirements.
- Planting Time — Plant the bulbs in autumn. Like most spring-flowering bulbs, alliums require a period of cold weather in order to bloom.
- Soil – Most alliums are native to sandy, dry soils but will grow in clay provided it is amended to improve drainage. Good drainage is particularly important in the winter. Most alliums prefer soil on the dry side and cannot tolerate soggy soil.
- Light – Alliums will tolerate a little shade but they perform best in full sun. Make sure their foliage is not shaded by other plants.
- Plant Depth — Plant alliums at a depth of 3 to 4 times the diameter of the bulb. Poor flowering may result if bulbs are planted too shallowly.
- Plant Spacing — Space smaller species of alliums about 3 to 4 in. apart. Space large species 8 to 12 in. apart, depending on the selection.
- Foliage – As the strap-like foliage emerges in spring, it may suffer frost damage. That won’t keep the bulb from blooming but the damaged foliage may detract from the plant’s appearance. The foliage of spring-blooming species dies back in late spring or early summer.
ALLIUM PROPAGATION
If they are happy in a sunny, well drained spot, alliums may form colonies or self-seed. Overcrowding may hinder flower production. That’s the bad news. The good news is that you now have the opportunity to grow additional alliums for your garden or to share with friends. Alliums may be propagated in several ways:
- Division – Many allium bulbs develop offsets. When dividing overcrowded alliums, lift them from the soil after the foliage has died and the flowers have faded. Gently detach the offsets from the “mother” bulb. Replant both the “mother” bulb and the offsets.
- Bulbils — A few alliums, such as A. roseum and A. sphaerocephalon produce aerial bulbils in the flower head. Those may be carefully separated from the flower head and planted in freely draining compost. Cover them with a thin layer of compost about ¼ inch deep.
- Seeds — Alliums may also be started from the seeds but this method will produce plants that won’t reach flowering size for several years. To plant seeds, either plant them as soon as they are ripe or store them in the refrigerator until spring and plant when soil warms to at least 55 degrees. The seeds should sprout within about 12 weeks. Note that seeds from hybrids will not come true. A word of caution: The seeds of some species may be poisonous. Please don’t let pets or children ingest them.
ALLIUM PESTS AND PLANT DISEASES
Ornamental onions are normally trouble free, but they are subject to the same pests and diseases as their culinary cousins. They may suffer from onion white rot, downy mildew, and onion flies. Pests include slugs, snails and allium leaf miner.
SOURCES
Armitage’s Garden Perennials (Armitage, Allan M., 2d. ed. 2011).
A-Z Encyclopedia of Garden Plants, The Definitive Horticultural Reference for the 21st Century,(The American Horticultural Society, rev. ed. 2004).
Bulbs (Bryan, John E., rev. ed. 2002).
The Perennial Care Manual, (Ondra, Nancy J., 2009).