How to Grow Tree Onions
On this page
Tree onions (Allium x proliferum) — herbaceous perennial.
Unlike an annual crop, the work here is front-loaded into establishment: get the planting and the site right and the same plants crop for several seasons.
Core growing advice on this page is checked against named horticultural sources — see References.
Tree Onion (Allium x proliferum)
5 sources cited · Verified
Soil & Growing Conditions
Water: About 1in of water a week; MSU notes it does not thrive in wet or extremely heavy soils.
Growth Stages
| Stage | BBCH | Notes |
|---|---|---|
| Planting bulbils | 0 | Plant bulbils (topsets) 2in deep in full sun. Fall is the optimal planting season, though Penn State notes they can go in spring, summer, fall or winter as long as the ground is not frozen. Larger bulblets establish better than smaller ones (MSU). |
| Establishment & overwintering | 3 | A hardy perennial to zone 3 that stays green through winter (MSU) -- unlike most alliums it does not need to be lifted or protected for the cold months. |
| Leaf & topset development | 6 | Hollow leaves can be cut and used like chives or green onions at any point. Through summer the plant sends up a flowering stem that produces a cluster of bulbils (topsets) instead of a normal flower, green at first. |
| Topsets mature & the plant "walks" | 8 | Topsets turn reddish or purple as they mature (Penn State) and can be picked at that point. Left in place, the stem eventually dries, weakens and topples under their weight, and the bulbils root wherever they land -- moving the plant roughly 2-3ft (Penn State) to about one stem-length (MSU) across the garden each year. The underground parent bulb can also be harvested, though it ends further growth from that spot. |
Quick Facts
| Factor | Value |
|---|---|
| Soil texture | Loamy sand or sandy loam (MSU) |
| Drainage | Free-draining |
| Lifecycle | herbaceous perennial |
| Edible parts | leaves, bulbils, bulbs |
At a Glance
Harvest & Storage
When to pick: Topsets can be picked once they colour from green to reddish or purple (Penn State). Leaves can be cut like chives or green onions at any point while green. The underground parent bulb is edible but tough-skinned and pungent (MSU), and lifting it ends further growth from that plant.
Diseases
Pests
Tree Onion, Topset to Walk
Plan your Tree Onions planting
Timing for Tree Onions follows soil temperature and frost, not the calendar date. The Global vegetable planting calendar explains the method. To work out where it fits in your beds, sketch your plot in the free Garden Layout Planner.
Growing Tree Onion in another region
Sowing dates, frost timing and local advice differ by country. Use the guide for where you garden.
Sources
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Allium x proliferum, a hybrid of the Welsh onion. Stays green all winter; forms bulblets/topsets at the top of the flowering stem instead of a normal flower. As bulblets mature the stem dries, weakens and topples, moving the plant roughly the stem's own length from the parent each time -- larger bulblets establish better than smaller ones. Grows in full sun, loamy sand or sandy loam soil, pH around 6.5; hardy to zone 3; does not thrive in wet or heavy soils. Bulblets eaten raw, cooked, or pickled; young leaves used like chives; the underground parent bulb is tough-skinned and pungent.
These source is specific to this region. The full list of 5 sources behind this guide (Michigan State University Extension, Penn State Extension, Royal Horticultural Society and others) is in the references of our UK guide to tree onions; they describe the plant itself, which is the same wherever it grows.
Frequently Asked Questions
What problems affect tree onions?
Our data links 1 disease and 1 pest to this crop. See the sections above for each one and how to manage it.
Plan next season before the problems start
This page gives you the thresholds; the planner turns them into your own calendar. It works from what you can observe and measure in your garden rather than from dates written for somebody else’s latitude.
Get the Global Garden Planner →Instant PDF download · Works in either hemisphere
How this guide is put together
The growing data on this page — sowing windows, pests, diseases, soil and site requirements — comes from named horticultural sources, listed in References below, checked against international horticultural research and extension sources. An automated pipeline assembles that verified data into the page you're reading: it selects which facts to lead with and builds the surrounding prose and layout. It does not invent a date, a source or a number — where a verified value for a field isn't available, the page leaves it out rather than guessing. A person reviews and publishes each page; nothing goes live unverified.
Last verified 2026-09-02
Related guides
More from the same group (the onion family):
Where To Go From Here
Take this crop from reading into a written plan for your own beds.
Keep going
Plan the next step
Use the printable SoilCommander planner for your region to turn this guide into sowing dates, bed layouts and weekly garden tasks.