Plant ornamental allium bulbs in fall, at the same time as tulips and daffodils: mid-September through October in most northern states once soil falls below about 60°F (University of Minnesota, Rutgers, Nebraska Extension) and October in Iowa (Iowa State). The RHS says early to mid autumn. Plant them about three to four times as deep as the bulb is wide (Piedmont Master Gardeners, RHS): roughly 6 to 8 inches for big alliums and 3 to 5 inches for small ones (Iowa State, NC State).
Ornamental alliums, or ornamental onions, are cousins of the onions, garlic and chives in your vegetable beds, grown for their round flower heads in purple, pink, white, blue and yellow (NC State Extension, Clemson). Like other spring bulbs they need a period of cold to bloom, which is why they are planted in fall (Piedmont Master Gardeners). They flower later than most spring bulbs, from May into summer, so they bridge the gap between tulips and summer perennials (NC State, Piedmont Master Gardeners).
When to plant alliums by region
Alliums follow the fall bulb calendar. Here is what each extension service says for its area:
| Area | When to plant alliums |
|---|---|
| Minnesota | Mid-September to mid-October; purple allium (A. aflatunense) and giant allium are on the university's list of bulbs hardy in zones 3 and 4 (University of Minnesota) |
| Wisconsin | Once the ground cools to about 50 to 55°F or lower, until a couple of weeks before it freezes (Wisconsin Horticulture) |
| Iowa | October is ideal; as late as December if the ground isn't frozen (Iowa State) |
| Nebraska and New Jersey | Late or mid-September through October, when soil falls below 60°F (Nebraska Extension, Rutgers) |
| South Carolina | Once soil at planting depth stays below 60°F; in coastal areas most bulbs go in late December or early January (Clemson) |
Iowa State warns that planting too early, while the soil is still warm, can make some bulbs sprout in fall. If your state isn't listed, wait for soil at planting depth to drop below about 60°F, and plant several weeks before the ground freezes (GardenPal summary of the sources above).
How deep and far apart to plant alliums
| Allium | Depth | Spacing | Height and bloom |
|---|---|---|---|
| Giant allium (A. giganteum) | 8 in (NC State); 6 in (Iowa State, Nebraska Extension) | 12 in, about one bulb per square foot (NC State); 12 to 18 in (Iowa State) | 3 to 4 ft (Nebraska Extension); early summer for 2 to 3 weeks (NC State) |
| Persian onion (A. aflatunense, 'Purple Sensation') | 4 to 6 in (NC State) | 4 to 6 in (NC State) | Lavender heads in May on tall stems, for 2 to 3 weeks (NC State) |
| Drumstick allium (A. sphaerocephalon) | 4 to 5 in (NC State) | 2 to 3 in (NC State) | Small egg-shaped heads, late spring to summer (NC State); July and August in the UK (RHS) |
| Small alliums (A. moly and similar) | 3 to 5 in (Iowa State) | 6 to 8 in (Iowa State); 3 to 4 in (RHS, Piedmont Master Gardeners) | Front of the border or rock garden (RHS, Piedmont Master Gardeners) |
Piedmont Master Gardeners say planting too shallowly can mean poor flowering. For the South, Clemson names Naples onion (A. neapolitanum), drumstick allium and star of Persia (A. christophii) among the best alliums, all blooming in late spring.
Sun, soil and water
- Sun: full sun is best; they tolerate a little shade (Piedmont Master Gardeners, NC State).
- Drainage: the one thing alliums insist on. Most come from dry, sandy soils and can't tolerate soggy ground, especially in winter (Piedmont Master Gardeners), and NC State says damp conditions lead to bulb rot and other fungal problems. Heavy clay works if amended for drainage (Piedmont Master Gardeners).
- Water: keep giant alliums moist while they are growing (NC State), but the RHS says alliums in the ground rarely need watering and summer irrigation can rot the bulbs.
- Feeding: the RHS says regular feeding isn't needed and to avoid freshly manured soil.
- Wind: tall kinds do best in a sheltered spot (RHS). NC State says giant alliums may need staking; Piedmont Master Gardeners say most don't unless the site is very windy.
The fading-leaf problem
Allium leaves start dying back just as the flowers open (NC State, RHS), so the plant can look tatty at its best moment. The RHS and Piedmont Master Gardeners suggest planting alliums among perennials that hide the foliage; Piedmont Master Gardeners name peonies, irises, roses and catmint, and NC State suggests daylilies and low grasses for giant alliums. The RHS says yellowing leaves can be removed with no ill effects. After flowering, the dried seed heads stay attractive for weeks and can be cut for arrangements (RHS, Piedmont Master Gardeners).
Deer, voles and pets
Alliums are one of the most animal-resistant bulbs. Piedmont Master Gardeners say deer, rabbits, voles, squirrels and chipmunks avoid them because of the oniony taste, and NC State says drumstick allium is normally not bothered by mammals. Pets are a different matter: NC State lists ornamental alliums as low-severity poisonous if eaten, and the ASPCA lists onion, a close relative, as toxic to dogs, cats and horses. Keep bulbs where pets can't dig them up, and ask a veterinarian if a pet eats one.
Alliums and the vegetable garden
Ornamental alliums can share disease and pest problems with edible onions. The RHS says they can get onion white rot, downy mildew and onion fly, and advises against planting them where onion crops have grown. In practice, keep ornamental alliums in borders and beds outside your onion and garlic rotation (GardenPal suggestion based on the RHS). They are worth having nearby: Piedmont Master Gardeners say they attract bees and butterflies, and NC State lists them for pollinator gardens. See flowers that attract beneficial insects, and for other bulbs planted now, when to plant tulips and when to plant snowdrops.
Frequently asked questions
When should I plant allium bulbs?
In fall with tulips and daffodils: mid-September through October in most northern states once soil is below about 60°F, and October in Iowa. The RHS says early to mid autumn.
How deep do you plant allium bulbs?
About three to four times the bulb's width. Iowa State gives 6 inches for large alliums and 3 to 5 inches for small ones; NC State says 8 inches for giant alliums.
Can I plant alliums in spring?
They need fall planting for spring and summer flowers; Piedmont Master Gardeners say alliums need a cold period to bloom. Potted plants bought in spring can go in then, like other perennials (GardenPal note).
Why are my allium leaves turning yellow before they bloom?
That is normal. NC State and the RHS say the leaves die back as the flowers open, so plant alliums among perennials that hide them.
Do deer and squirrels eat alliums?
Rarely. Piedmont Master Gardeners say deer, rabbits, voles, squirrels and chipmunks avoid them because of the oniony taste.
Can I plant ornamental alliums in my vegetable garden?
Keep them out of your onion and garlic beds. The RHS says they share diseases such as onion white rot and advises against planting where onion crops have grown.
Enter your frost dates to plan fall crops and bulb planting together.
Open itSources
- Virginia Cooperative Extension, "Companion Planting in Gardening" (SPES-620P)
- University of Maryland Extension, "Vegetable Succession and Companion Planting"
- Oregon State University Extension, "Companion Planting"
- Penn State Extension, "Herbs Make Good Plant Partners and Companions"
- University of Minnesota Extension, "Planting the vegetable garden"
- USDA Plant Hardiness Zone Map
- NOAA NCEI U.S. Climate Normals (frost/freeze data)
- NC State Extension Gardener Plant Toolbox, "Allium"
- NC State Extension Gardener Plant Toolbox, "Allium giganteum" (giant ornamental onion)
- NC State Extension Gardener Plant Toolbox, "Allium aflatunense" (Persian onion)
- NC State Extension Gardener Plant Toolbox, "Allium sphaerocephalon" (drumstick allium)
- Royal Horticultural Society, "How to grow alliums"
- Piedmont Master Gardeners (Virginia Cooperative Extension), "Allium"
- Iowa State University Extension, Yard and Garden, "Selecting and Planting Spring-Blooming Bulbs"
- Nebraska Extension, "Spring Flowering Bulbs" (G1742)
- Rutgers NJAES, "Spring Flowering Bulbs" (FS1220)
- University of Minnesota Extension, "Planting bulbs, tubers and rhizomes"
- Clemson Cooperative Extension HGIC, "Spring-Flowering Bulbs" (HGIC 1155)
- University of Wisconsin-Madison Extension, Wisconsin Horticulture, "Plant Bulbs Now for Spring Color"
- ASPCA Animal Poison Control, "Onion"
Pairings are summarized in our own words from extension-service guidance. Research support for many companion effects is limited, so treat them as good habits, not guarantees.