Giant alliums bring statuesque blooms in rich colors to the late spring and early summer display. Their impressive flower clusters are perfect globes that stand tall among other plantings. These architectural beauties are easy-to-grow perennial bulbs. An onion relative, gardeners prize these giants for their ornamental value. Plant the bulbs in the fall to enjoy their sculptural flowers as temperatures warm.
Here, I’ll share what you need to grow showstopping giant alliums in your garden!
Overview
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What are Giant Alliums?
Feature dramatic softball-sized flower clusters.
Giant alliums feature dramatic softball-sized flower clusters in spring and early summer. They bring high form blended with whimsy. The gorgeous globes in purple, pink, mauve, lavender, and white top tall, sturdy stems. Long and strappy blue-green leaves emerge in a bundle.
Giant ornamental onions average three to five feet tall, depending on the variety. Their size makes them ideal for the back of the perennial border or among shrubs like hydrangeas and roses. With their highly decorative flower heads, they’re exceptional in groups. Plant at least five to seven bulbs or more.
Their architectural form complements formal and contemporary garden styles. Plant bulbs in a row to create linear interest. Site them behind a low-growing boxwood hedge for a structure that delights and surprises.
The towering rounds also embellish naturalized or cottage garden schemes among looser plantings. Contrasting foliage from plants like yarrow, nepeta, and artemisia adds softness. Daylilies and grasses add sway and movement. Colorful geraniums anchor the base of plants.
The perennial bulbs are low-maintenance and easy-care, growing best in full sun. The plants are durable and have few landscape problems. Their unique forms enliven the planting arrangement with pom-poms of rich color. Butterflies and other pollinators flock to the large rounds.
Characteristics
Globes measure five to six inches in diameter.
The distinguishing characteristic of this tall perennial is, of course, its enormous blooms. The large ball flowers are dense umbels (clusters) of petite, star-shaped, six-petaled florets. Globes measure five to six inches in diameter and appear in late spring and early summer.
Flowers rise on strong, single three to four-foot tall scapes (stems). Blooming lasts for two to three weeks. Flowers are lovely on the stem and in fresh or dried floral arrangements. Blooms left on the stem dry in place for lasting interest beyond the bloom time. Small, capsule fruits develop after flowering.
Giant alliums have straplike blueish-green leaves that reach up to 18 inches long. The foliage forms a basal rosette and emerges in a bunch. When flowering, it lays down at the plant’s base. Leaves begin to yellow, wither, and fade as the plant uses energy to bloom. To conceal fading foliage, use other plants as a foil.
As a member of the onion family, bulbs and leaves have a garlicky onion scent, most noticeable when leaves are bruised or clipped. The flowers don’t emit a strong onion fragrance. A. giganteum isn’t used for culinary purposes like other onion bulbs. While the bulbs are edible, some plant parts are toxic, depending on the age and stage of the plant.
Ornamental onions have few pests and diseases and are rabbit and deer-resistant. They’re drought-tolerant and long-lived garden performers in optimal growing conditions.
Native Area
The bulbous plants are native to Western and Central Asia, including the Himalayas.
The bulbous plants are native to Western and Central Asia, including the Himalayas. They grow in woodland, naturalized, and disturbed areas. Soils tend to be sandy, dry, and extremely well-draining.
There are over 700 species of Allium. Their family, Amaryllidaceae, includes agapanthus, amaryllis, daffodils, garlic, onions, leeks, and many others.
Planting
A strong gust can topple tall bloomscapes.
Fall is the best time to plant A. giganteum. Tuck them in as you plant daffodils and tulips and await the spring display.
Dig a hole two to three times the bulb’s diameter, roughly four to eight inches deep. Bulbs are large and measure two to three inches across. Space plants 9-12 inches apart. One bulb per square foot is a good rule of thumb for giant alliums. At maturity, plants grow one to two feet wide.
Site giant onions in an area protected from high winds. A strong gust can topple tall bloomscapes. If strong winds are likely, stake stems as a precaution.
The bloomers grow well as container features as long as pots are large enough to house them. Provide consistent water, a quality potting mixture, and appropriate light conditions. Potting mix rather than garden soil is essential, as alliums need loose, lightweight, well-draining soils to thrive in containers. Topdress with compost for added nutrition.
To hide fading foliage during peak bloom, clip browning ends or disguise them among other perennials, annuals, and shrubs.
Transplanting
It allows roots to develop before freezing temperatures.
Early fall is optimal for moving bulbs. It allows roots to develop before freezing temperatures and soils. Water bulbs after planting or dividing, but avoid regular irrigation. Overly saturated soils lead to bulb rot. Natural fall and winter seasonal moisture is sufficient for dormant bulbs.
How to Grow
Alliums grow best in full sun.
A. giganteum is carefree and unfussy, with low maintenance requirements. Certain cultural conditions promote health and vigor for perennialization.
Light
Alliums grow best in full sun.
Alliums grow best in full sun, with at least six hours of sunlight daily. Morning and late afternoon sun with midday shade protection is ideal in most planting areas.
Direct afternoon sun in hot climates can shorten bloom time. Provide a dappled midday light situation to protect plants from intense afternoon rays. Plants growing in more sun require increased water to sustain blooming.
Water
Watering at the plant’s base avoids splashing the leaves.
Once established, giant ornamental onions are rugged and drought-tolerant. Still, they do best with regular moisture during the growing season. Average rainfall is usually enough to support the bulbs. Supplement with irrigation during dry spells and prolonged heat or drought. Aim for evenly moist soils. Avoid irrigating in winter when bulbs are dormant.
Even moisture ensures the plant’s overall health and vigor, but too much water and saturated soils lead to a decline in flowering and disease issues. Watering deeply, rather than frequently and shallowly, is best for root growth and moisture uptake.
Watering at the plant’s base avoids splashing the leaves and is best for preventing the spread of fungal and bacterial diseases. Drip irrigation, soaker hoses, or direct hand watering accomplish this. If using spray heads or overhead irrigation, water early in the day so plants have a chance to dry out.
If you’re growing them in pots, ensure the containers are well-draining and check the moisture level regularly. Reduce watering sessions in the winter, watering overwintered containers only when the soil feels dry.
Soil
Moist soil during the growing season best supports plants.
These early summer beauties prefer well-draining soils with medium moisture. They tolerate various soil types, from loamy to sand to clay, as long as they have good drainage. Ideal soils are slightly acidic, with a pH near 5.5 to 6.5, but they’ll grow in acidic, neutral, or alkaline soils. Good drainage is the key factor.
Alliums tolerate periods of drought, but moist soil during the growing season best supports plants. Avoid over-irrigating and wet soils.
For poor soils like clay and sand, generously add composted organic material to the native soil to improve aeration, moisture retention, drainage, and nutrition. Spread a generous layer over the entire area at planting and in the fall to provide insulation and soil improvement as bulbs enter dormancy.
Temperature and Humidity
Provide a sheltered location for overwintering container-grown plantings.
Giant alliums tolerate a range of summer and winter temperatures in zones 5-8. They’ll benefit from good air circulation to prevent humid conditions. The bulbs don’t thrive in hot, humid growing areas. Damp conditions lead to fungal issues and weaker plants.
Provide a sheltered location for overwintering container-grown plantings. For best success, overwinter pots in an unheated garage or basement.
Fertilizing
Nutrient-rich soils don’t require additional fertilizers.
The bulbous perennials benefit from compost amendments at planting and each fall. A small amount of bone or blood meal at planting helps promote strong roots. Nutrient-rich soils don’t require additional fertilizers.
In poor soils, consider a balanced granular fertilizer with compost at planting. Stop fertilizing in the fall as plants stop actively growing and prepare for winter dormancy.
Maintenance
Giant ornamental onions need little ongoing maintenance.
Giant ornamental onions need little ongoing maintenance. Allow leaves to turn yellow and fade naturally before plucking or clipping them. They’ll continue to supply energy to the bulbs for storage.
Post-flowering, tidy up plants as they decline. Remove browned plant parts as plants enter winter dormancy. Enjoy fresh or dried flower umbels in floral arrangements by cutting stems back to the ground. Allow flowers to remain on the stem for continued seasonal interest.
Propagation
These alliums propagate easily through division.
Alliums propagate easily through division. Divide mature bulbs in the fall to expand the colony.
Division
Lift the bulbs from the soil in the fall and peel off any offsets.
Here are simple steps for best practices in dividing bulbs:
- Lift the bulbs from the soil in the fall and peel off any offsets (bulblets) from the main bulb.
- Replant the mother bulb.
- Choose a spot with full sun and good drainage for the new alliums. Dig a hole roughly four to five inches deep for each bulblet.
- Water deeply for evenly moist soil.
- Enjoy the expanded collection as blooms appear in spring.
Popular Varieties
Many beautiful varieties are available. They look particularly stunning when interplanted in the same flower bed.
They bring a burst of color in uniform lollipop clusters.
A. giganteum selections “wow” garden visitors with impressive sizes and rich colors. They bring a burst of color in uniform lollipop clusters.
‘Globemaster’
Deep lavender florets have a silvery sheen and pack the enormous spheres.
‘Globemaster’ is a hallmark of giant ornamental onions and a recipient of the Royal Horticultural Society Award of Garden Merit. The hybrid has showy, impressive orbs that measure 10 inches across on stout two to four-foot stems.
Deep lavender florets have a silvery sheen and pack the enormous spheres. The umbels are long-lasting, with new florets replacing the originals as they fade.
‘Mount Everest’
The tightly formed spheres beautifully contrast with violet varieties.
This variety stands tall with creamy white flowers with lime green eyes. The tightly formed spheres beautifully contrast with violet varieties.
‘Mount Everest’s’ snow-capped blooms top three-foot tall stems. Plants are hardy in zones 4 through 8.
‘Gladiator’
Flowers have a sweet scent that draws pollinators.
‘Gladiator’ boasts large, lavender-blue florets that reach six inches across. One of the tallest, along with ‘Globemaster,’ scapes rise three to four feet tall.
Flowers have a sweet scent that draws pollinators. Group bulbs in a mass for a majestic show of color. ‘Gladiator’ is a Royal Horticultural Society Award of Garden Merit recipient.
Common Problems
The best prevention is ensuring proper cultural conditions.
These carefree growers are free of most pests and diseases. Allium leaf miners may be occasional visitors. Root rot and powdery mildew are sometimes problems. The best prevention is ensuring proper cultural conditions through even watering, air circulation, and healthy soils.
Pests
A simple horticultural soap or neem oil treats infestations.
Early detection is the best way to control insects. Allium leaf miners are insects that feed on tissues and sap in leaves and stems. They lead to plant weakness and decline and invite fungal infection, like rot.
The pest leaves trails of little white dots in a line along leaf surfaces as they feed. Adult females are small flies that lay eggs beneath the leaf surface. The larvae feed on tissue and sap as they mine leaves.
If you notice signs of the insect, remove leaves after they die back. Fortunately, the foliage of giant alliums fades after flowering, limiting the habitat for leaf miners. A simple horticultural soap or neem oil treats infestations.
Diseases
The best disease control is prevention through cultural conditions.
As with pests, the best disease control is prevention through cultural conditions. Common fungal diseases may be an issue with water fluctuations or warm, humid conditions.
Bulb rot occurs in overly wet soils and periods of prolonged saturation. The bacteria cause the bulbs and plant material to become mushy and rot. While there’s no reversing bulb rot once it sets in, improved drainage and cultural conditions can help other bulbs. Remove the impacted bulbs and plants from the bed or planter.
Powdery mildew is another common fungal disease that impacts bigleaf varieties. A gray-white powdery substance appears on leaves, stems, and buds, which may distort and drop.
Remove affected plant parts and any dropped leaves. Horticultural oils like neem can prevent early fungal infections. These treatments impact beneficial insects, so be sure to follow application requirements.
Frequently Asked Questions
If your bulb isn’t blooming, it may be in too much shade, overly wet or dry soils, or not planted deeply enough. Bulbs in too much shade won’t have enough energy to produce blooms. Overly wet soils may mean fungal conditions like rot, while dry soils can stress roots. Planting at the wrong time of year, too, causes a lack of root development and less vigorous plants. Removing foliage too early in the season reduces energy storage for overwintering bulbs. Leave the foliage intact as until it yellows and browns
The bulbous perennials, hardy in USDA zones 5-8, bloom for a few weeks in late spring to early summer. The enormous clusters dry naturally on the stem after bloom, creating a striking texture.
If your garden doesn’t have space for the enormous bloomers or your climate exceeds their growing range, consider other ornamental Allium selections. The smaller hybrid allium ‘Millenium’ grows 15 to 20 inches tall and produces stems full of violet globes. Hardy in zones 4-8, plants are slightly more heat and cold-tolerant in borderline growing areas.
