How to Grow Tree Onions UK
On this page
In the UK, tree onions go in the ground early September to late November.
Timing is the decision that matters most in the UK: sowing early September, and that same window applies across all 4 regions covered here.
The short answer
- Plant
- Early September to late November
- Biggest risk
- Allium white rot
Core growing advice on this page is checked against named horticultural sources — see References.
Tree Onion (Allium x proliferum)
5 sources cited · Verified
When to Plant Tree Onions in the UK
When to plant tree onions in the UK: in Southern England, sow early September to late November. Find your area in the table below.
| 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 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.
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
Plan your Tree Onions planting
When to sow and plant Tree Onions depends on your own frost dates. Check the UK vegetable planting calendar. 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.
References
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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.
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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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Allium x proliferum is a cross between the cultivated onion (A. cepa) and the Welsh onion (A. fistulosum), hardy in USDA zones 3-10. Instead of flowers and seed it makes a cluster of BULBILS (topsets) at the top of the stalk; if they are not picked, the stalk flops over under their weight and the bulbils root where they land -- which is how the plant "walks" across the bed, moving only 2-3ft a year. Propagate by breaking off bulbils and planting them no more than an inch deep and 4-5in apart. Plant in FULL SUN somewhere you are content to keep them long term, since they are perennial. Eat the tender young shoots and stems as spring onions, the small underdeveloped basal onions (VERY HOT), and any bulbils not replanted, which keep a few months in store. Pests: thrips can be a problem in hot dry summers but the damage is mainly cosmetic. Not susceptible to juglone.
5 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 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.
Get the UK Garden Planner →Instant PDF download · Written for UK growing conditions
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 RHS and Met Office guidance. 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
Plan Tree Onions by month
The months this guide gives for sowing or planting, each with the full list of jobs for that month in the UK:
More from the same group (the onion family):
Growing advice for your part of the UK
Timing and conditions change a lot inside one country. These guides cover frost dates and growing seasons region by region.
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.