Schubert's allium (Allium schubertii) is one of several allium species grown as ornamental landscape plants, known collectively as ornamental onions. Although closely related to chives and edible table onions, these alliums are not edible. Schubert's allium is a bulb that produces a low cluster of strap-like foliage in early spring, which then sends up a tall flower scape supporting a flower cluster. While it has the same globe-shaped flowers found on other ornamental onions, the blooms on this species have a very striking resemblance to exploding fireworks. The flower heads are large globe-shaped clusters of 100 or more tiny star-shaped flowers—the entire globe can be as much as 18 inches across. Despite the very dramatic appearance, Schubert's allium is remarkably easy to grow, making it a favorite among beginning gardeners seeking a spectacular plant.
Schubert's allium bulbs, like other ornamental onions, are normally planted in the fall in order to give them the cold-chilling period they need to bloom. Potted alliums may also be available at garden centers for spring planting. Unlike many bulb species, flowering onions are long-lived, multiplying freely over time.
All parts of this plant contain sulfides that are mildly toxic to humans and somewhat more seriously toxic to pets. Consuming these plants can cause digestive upset in humans. Blood cell damage is possible in dogs and cats.
|Common Name||Schubert's allium, ornamental onion, tumbleweed onion|
|Botanical Name||Allium schubertii|
|Plant Type||Perennial bulb|
|Mature Size||12–24 in. tall, 12–18 in. wide|
|Soil Type||Dry to average moisture, well-drained loam|
|Soil pH||5.5 to 7.0 (acidic to neutral)|
|Bloom Time||Late spring|
|Flower Color||Rosy purple|
|Hardiness Zones||5–8 (USDA)|
|Native Area||Southern Eurasia|
|Toxicity||Toxic to dogs and cats, mildly toxic to humans|
Schubert's Allium Care
Schubert's allium, like most ornamental onions, is an easy perennial bulb to grow. Just about any sunny location with average well-draining soil will work for these plants. Shubert's allium is native to the dry regions of southern Eurasia and will do best in conditions that mimic those regions.
In mid-to-late fall, plant the bulbs about 4 to 8 inches deep, spaced 12 to 18 inches apart. Larger bulbs should be planted deeper, small bulbs at a shallower depth. Bulb orientation in the hole is important. Look for either a point on the bulb or tiny roots. A point indicates the top of the plant, with roots at the bottom. Bury the bulb in the soil so that the top points up.
Blend bulb fertilizer into the soil around the buried bulbs and water it in. The strap-like leaves will appear first in early spring, followed by tall flower stalks will suddenly shoot up as spring draws to a close. As the globe-like clusters of star-shaped flowers open, the leaves will begin to wither; this is entirely normal.
If planting in the spring from container-grown nursery specimens, plant at the same depth as the allium was growing in the container. Here, too, a handful of bulb fertilizer blended into the fill soil is a good idea at planting time.
Ornamental onion species all require full sun in order to bloom robustly. They can survive some shade, though the leaves and flower stalks may be overly long and floppy in shady conditions.
These plants prefer dryish or medium-moisture loamy soils. Ordinary garden soil amended with peat moss to improve drainage is often ideal. Avoid dense, wet soils for this plant.
Like many perennial bulbs, alliums don't require a lot of water; in fact, too much moisture will invite bulb rot. It's a good idea to water regularly in spring, but after blooming is complete, withhold water altogether. When feeding, the fertilizer should be thoroughly watered in.
Temperature and Humidity
Schubert's allium will do well in the temperature and humidity conditions through its hardiness range, zones 5 to 8. They often will survive in zone 4 if the bulbs are well insulated with mulch for the winter. These plants do not like extremely humid conditions, which can cause leaf spots and other fungal issues, though these problems are rarely life-threatening.
Like most alliums, Schubert's does not normally require feeding if the soil has a good blend of organic material. With barren soils, an application of granular time-release bulb fertilizer each spring can be helpful.
Types of Ornamental Onion
Though few species have flowers that rival the spectacular nature of Schubert's allium, there are several other ornamental onions that have largely the same cultural needs:
- Allium sphaerocephalon is also known as drumstick allium. It has 1-inch flower clusters that bloom in early summer,
- 'Globemaster' is a hybrid cross between A. christophii and A. macleanii. It has very tall, 3- to 4-foot flower stalks that support large 8- to 10-inch diameter flower globes.
- 'Mount Everest' is another tall hybrid, though not quite a tall as 'Globemaster'. It blooms with creamy white flowers.
- 'Purple Sensation' is another hybrid allium. Its flower stalks reach about 2 feet tall and support 2- to 4-inch globes of bright purple flowers.
It's typical for the strappy leaves on this plant to begin to wither and turn yellow at just about the same time as the flower stalks are shooting up in late spring and early summer. As the foliage dies back, you can pull it off and discard it, but leave it in place until then, as photosynthesis of the leaves will replenish the bulb's energy. The flower heads can be left in place after they fade, however, as the dried heads have their own ornamental appeal and provide food for birds.
Propagating Schubert's Allium
Like most ornamental onions, Schubert's allium is normally propagated by splitting off the offset bulblets that form around the parent bulbs or around the flower stalks. These bulblets are produced rather slowly, so don't plan to divide them up every year, but rather every three to four years. Here's how to do it:
- In fall, use a trowel or shovel to dig up the plant clump, flower stalks and all.
- Shake loose the dirt, then separate the larger parent bulbs and smaller bulblets by hand. Discard the flower stems.
- Replant the parent bulbs and smaller bulblets at a depth about four times the diameter of the bulbs. Be patient, as it may take several years for the newly planted bulblets to develop the size necessary to produce their own flowers.
- Fertilize new bulbs each spring to hasten their growth.
How to Grow Schubert's Allium From Seed
Schubert's allium often self-seeds from seeds dropped from the dried flower heads. The tiny volunteers left to grow in the garden will eventually develop their own bulbs and begin to flower, though this can take a number of years. The volunteers can also be transplanted to other locations.
Potting and Repotting Schubert's Allium
Container culture is not common for Schubert's allium, but it is possible. Alliums are best planted in mixed containers with colorful drought-tolerant annuals that will fill in the spaces left when the allium fades. Plant the bulbs in a large, wide well-draining pot filled with standard potting mix. Bulbs should be planted about 4 inches deep and spaced 6 to 8 inches apart. Alliums in containers don't respond well to excessive water, so they are best planted with other species that tolerate relatively dry conditions. Choose colors that harmonize with the rosy purple hues of the allium blossoms.
At the end of the season, the container should be moved to a sheltered location to overwinter with the allium bulbs left in place. The bulbs require some winter cold but may perish if the pots are left outdoors in an exposed location. An unheated garage, porch, or cold frame is a good spot to move potted alliums. Don't bring the pots indoors, as the bulbs need a moderate amount of winter cold to reset themselves for the following season. In the spring, the container can be replanted with new annuals and set into a sunny location to grow and flower.
In northern zones, you may want to apply a thick mulch over the pruned-back flower stalks to protect the bulbs over winter. This treatment sometimes allows Schubert's allium to survive as far north as zone 4. The mulch should be promptly removed in the spring to prevent rot.
The seed heads can be left on the plant as the flowers fade. The dried tumble-weed-like flowerheads can be quite attractive and sometimes will fall off the stems to blow around the garden. Self-seeding is common, though it may take several years for volunteer plants to develop the sizeable bulbs that produce their own flowers.
Common Pests & Plant Diseases
Schubert's allium is not affected by any truly serious insect or disease problems, but bulb rot can be a problem in very moist soils. Humid conditions can also foster mildew, rust, and leaf spots.
Onion leafminer flies and thrips can sometimes be a problem. Thrip damage reveals itself through stippled or wrinkled leaves, sometimes with black fecal spots on the leaves. Remove affected plant parts and spray the rest of the plant with horticultural oil. Onion leafminer flies cause tunneling through the leaves of allium plants when they lay eggs that grow into larvae. The best solution to leafminer infestation is to remove and destroy affected plant parts, though spraying with horticultural oil may also slow the spread of the insect.
Insect damage often corrects itself as parasitic insects arrive to prey on thrips and leafminers. A patient approach is often best, in which you tolerate small insect infestations and allow the garden to find its own balance before reaching for chemical pesticides.
How to Get Schubert's Allium to Bloom
Mature allium bulbs planted in a sunny location and relatively dry soil will normally send up flowers stalks in late spring and early summer each year. But it's common for immature bulbs, such as the bulblets split off from the parent plant, to take two or three years to gain enough size to support good blossoms.
- A patch of alliums that experiences reduced blooming may be in need of lifting and dividing the bulbs to give them more space in the garden.
- Bulbs that consistently fail to bloom may respond to a dose of bulb fertilizer applied in the spring, as extremely poor soils sometimes cause alliums to withhold flowers.
- Bulbs that are planted too shallow may experience winter frost that causes them to fail. Allium bulbs should be planted at a depth that's roughly four times the bulb diameter.
- It's also possible that an unusually cold winter or unusually warm winter may cause flower failure. Unusually low temps can destroy bulbs, and a very warm winter may not provide the chill period the bulbs need to reset.
- Bulbs planted at the wrong time of year may fail to bloom. Proper planting time is in the fall, which will provide the winter chill necessary for the bulbs to reset for bloom the following year.
Common Problems With Schubert's Allium
Aside from issues with flowering (see above) there are a couple of fairly common complaints with ornamental onions such as Schubert's allium:
Flower Stalks Collapse at Ground Level
When leaves and flower stalks turn black and mushy at ground level, it's usually because excessively moist soil is causing some type of bulb rot. Affected plants will need to be dug up, and soft bulbs discarded. To avoid this problem, improve drainage of the soil and avoid overwatering.
Flower Stalks Won't Stay Upright
If planted in full sun, the flower stalks on alliums are generally quite sturdy and don't require staking. But in shady conditions—or if the bulbs are planted too shallow—it may be necessary to stake the flower stalks to help keep them upright.
How should I use this plant in the landscape?
Ornamental onions such as Schubert's make good plants for mixed border gardens, cottage gardens, meadow plantings, and rock gardens. They are also commonly planted for use in both fresh-cut flower and dried flower arrangements.
Give Schubert's allium plenty of space, as you do not want the foliage of other plants obscuring your view during its peak display time. Avoid planting it next to large plants that will swallow it up. At the same time, it's best to position them behind lower foreground plants that can hide the bare spots that are left when the foliage fades.
I live in a cold region; is there a similar Allium that will work for me?
Gardeners in zones 3 and 4 can try one of these similar-looking types:
- Alium 'Purple Sensation is a hybrid that is hardy to zone 4
- Allium caeruleum, (blue allium) is a smaller species, hardy in zones 3 and 4.
- Allium sphaerocephalon (drumstick allium) is a 3-foot-tall species with red-purple egg-shaped flowers. It is hardy to zone 3.
- Allium aflatunense (Persian flowering onion) has 4-inch ball-like flowers. It is reliably hardy to zone 4, and zone 3 gardeners may also succeed with it.
Where does the name "Schubert's" come from?
The botantical name of this plant is in honor of Gotthilf Heinrich von Schubert (1780-1860), a German physician and plant collector.
If I live in a cold region, can I grow Schubert's allium as an annual?
Some tender perennial plants, such as gladiolus corms, are sometimes planted in the spring, then discarded at the end of the growing season if you live in an area that's too cold for the bulbs or corms to survive over the winter. But this doesn't work very well for Schubert alliums, which require a summer dormant period followed by a winter chill period—but not one that is bitterly cold—in order to bloom the following year. Thus, Schubert's allium is always planted in fall. If you live in a region outside its hardiness range, it's best to opt for a different allium species.