Tree Onion (Allium x proliferum)
4 sources cited · Verified
The short answer
- Plant
- Early September to late November
- Site
- 6+ hours of sun, soil pH 6.3–6.8
- Biggest risk
- Allium white rot
In the UK, tree onions go in the ground early September to late November.
Tree onions (Allium x proliferum) — herbaceous perennial.
Timing is the decision that matters most in the UK: there are 4 separate regional sowing windows, ranging from early September in Southern England to early September in Scotland.
Core growing advice on this page is checked against named horticultural and university-extension sources — see References.
Regional Planting Calendar
| Region | Sow |
|---|---|
| Southern England | early September to late November |
| Midlands | early September to late November |
| Northern England | early September to late November |
| Scotland | early September to late November |
Southern England - detail & sources
Penn State: fall is the optimal planting season for bulbils, though they can go in "spring, summer, fall, or even winter as long as the ground is not frozen." A hardy zone-3 perennial with no frost-driven timing rule from either source, so one UK-wide autumn-planting window is used across all four sub-regions rather than an invented regional gradient.
Midlands - detail & sources
Penn State: fall is the optimal planting season for bulbils, though they can go in "spring, summer, fall, or even winter as long as the ground is not frozen." A hardy zone-3 perennial with no frost-driven timing rule from either source, so one UK-wide autumn-planting window is used across all four sub-regions rather than an invented regional gradient.
Northern England - detail & sources
Penn State: fall is the optimal planting season for bulbils, though they can go in "spring, summer, fall, or even winter as long as the ground is not frozen." A hardy zone-3 perennial with no frost-driven timing rule from either source, so one UK-wide autumn-planting window is used across all four sub-regions rather than an invented regional gradient.
Scotland - detail & sources
Penn State: fall is the optimal planting season for bulbils, though they can go in "spring, summer, fall, or even winter as long as the ground is not frozen." A hardy zone-3 perennial with no frost-driven timing rule from either source, so one UK-wide autumn-planting window is used across all four sub-regions rather than an invented regional gradient.
Quick Facts
| Factor | Value |
|---|---|
| Soil pH | 6.3 - 6.8 |
| Sunlight | 6-6 hours direct sun |
| Soil texture | Loamy sand or sandy loam (MSU) |
| Drainage | Free-draining |
| Lifecycle | herbaceous perennial |
| Edible parts | leaves, bulbils, bulbs |
At a Glance
Soil & Growing Conditions
Water: About 1in of water a week; MSU notes it does not thrive in wet or extremely heavy soils.
Soil pH range
Span reflects genuine variation between the cited authorities, not a single agreed figure.
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. |
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
References
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Tier 1. 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.
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Tier 1. Heirloom perennial onion producing a cluster of bulbils (topsets) at the top of the stalk, green at first, turning reddish or purple as they mature. When the cluster gets too heavy the stalk falls over and takes root where it lands -- the plant "walks" roughly 2-3ft across the garden per year. Plant bulbils 2in deep in full sun; fall is the optimal planting season, though bulbils can go in any time the ground isn't frozen. Leaves used like green onions; topsets fried, cooked in soups, or eaten raw in salads; underground bulbs are edible but very hot, and harvesting them prevents regrowth from that point.
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4 independent sources are cited for this crop. Where they disagree - soil pH is the clearest case - every figure is preserved rather than averaged into false precision.
Frequently Asked Questions
When should I sow tree onions in the UK?
In Southern England, from early September. In Scotland, wait until early September. Treat these as a guide rather than a fixed date — the governing rule is to sow only once frost risk has passed and temperatures are reliably warm enough.
When do I sow tree onions?
Sow early September to late November.
What soil pH do tree onions need?
A pH of about 6.3–6.8. That span reflects genuine variation between the authorities we cite rather than a single agreed figure — the References section lists each source's exact range.
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
Most of the advice on this page comes down to timing and spacing — sowing at the right moment, rotating beds, and giving plants enough airflow. The UK Garden Planner lays your plot out month by month so those decisions are already made before the season starts.
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Last verified 2026-08-28
Where To Go From Here
Take this crop from reading into a written plan for your own beds.
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.