How to Grow Three-cornered Leeks UK
On this page
In the UK, three-cornered leeks go in the ground early August to late September.
Timing is the decision that matters most in the UK: sowing early August, and that same window applies across all 4 regions covered here.
The short answer
- Plant
- Early August to late September
- Biggest risk
- Allium white rot
Core growing advice on this page is checked against named horticultural sources — see References.
Three-cornered Leek (Allium triquetrum)
7 sources cited · Verified
When to Plant Three in the UK
When to plant three in the UK: in Southern England, sow early August to late September. Find your area in the table below.
| Region | Sow |
|---|---|
| Southern England | early August to late September |
| Midlands | early August to late September |
| Northern England | early August to late September |
| Scotland | early August to late September |
Southern England - detail & sources
Neither source gives UK region-specific dates. PFAF: divide in summer after the plant dies down, replanting immediately; seed sown as soon as ripe. One UK-wide late-summer window is used across all four sub-regions rather than an invented gradient -- RHS H5 hardiness means no frost risk drives regional differentiation the way a tender crop would.
Midlands - detail & sources
Neither source gives UK region-specific dates. PFAF: divide in summer after the plant dies down, replanting immediately; seed sown as soon as ripe. One UK-wide late-summer window is used across all four sub-regions rather than an invented gradient -- RHS H5 hardiness means no frost risk drives regional differentiation the way a tender crop would.
Northern England - detail & sources
Neither source gives UK region-specific dates. PFAF: divide in summer after the plant dies down, replanting immediately; seed sown as soon as ripe. One UK-wide late-summer window is used across all four sub-regions rather than an invented gradient -- RHS H5 hardiness means no frost risk drives regional differentiation the way a tender crop would.
Scotland - detail & sources
Neither source gives UK region-specific dates. PFAF: divide in summer after the plant dies down, replanting immediately; seed sown as soon as ripe. One UK-wide late-summer window is used across all four sub-regions rather than an invented gradient -- RHS H5 hardiness means no frost risk drives regional differentiation the way a tender crop would.
Quick Facts
| Factor | Value |
|---|---|
| Soil texture | Chalk, clay, loam or sand (RHS) |
| Drainage | Moist but well-drained |
| Lifecycle | herbaceous perennial |
| Edible parts | bulbs, leaves, flowers |
At a Glance
Growth Stages
| Stage | BBCH | Notes |
|---|---|---|
| Planting or division | 0 | Bulbs planted fairly deeply (PFAF). Propagate by seed, sown as soon as ripe in a greenhouse or cold frame -- germinates quickly -- or by division in summer after the plant has died down, with divisions replanted straight into their permanent position. NOTE: illegal to plant or cause to grow in the wild in Great Britain (RHS, Schedule 9) -- confine to a contained garden setting only, and check local regulations elsewhere. |
| Leaf growth | 3 | RHS: in leaf from February to July, narrow green leaves on distinctively triangular (three-cornered) stalks. |
| Flowering | 6 | RHS: in flower from spring to early summer, drooping white bell-shaped flowers with a central green vein on each petal. |
| Harvest & spread control | 8 | Bulbs harvested early summer, store at least six months; leaves and flowers picked while present. RHS/PFAF both note vigorous spread by seed and bulbils -- vigilant deadheading is recommended to prevent unwanted colonisation, and is a legal necessity, not just tidiness, given the Schedule 9 restriction. |
Harvest & Storage
When to pick: Bulbs (up to 20mm across) are harvested in early summer and store for at least six months (PFAF, tier 2). Leaves can be picked as a leek substitute at any point while present -- younger leaves raw, mature leaves cooked. Flowers are picked fresh for use as a garnish.
Diseases
Pests
Three-cornered Leek, Flower to Colony
Plan your Three-cornered Leeks planting
When to sow and plant Three-cornered Leeks 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 Three-cornered Leek in another region
Sowing dates, frost timing and local advice differ by country. Use the guide for where you garden.
References
-
Listed under Schedule 9 of the Wildlife and Countryside Act in England and Wales, making it "an offence to plant or otherwise cause to grow this species in the wild". Identification: uniquely TRIANGULAR stem, which gives the plant its name; narrow three-angled strap-like leaves in basal clusters; small white star-shaped flowers in umbels April to June; garlic or onion scent when bruised. Spreads both by seed (wind, water, human activity) and vegetatively by bulb division, enabling rapid colonisation. DISPOSAL: because it is Schedule 9 listed, all plant material and associated soil that may contain seeds or bulbs is classed as CONTROLLED WASTE and can only be removed by a licensed contractor to an authorised waste facility.
-
A compiled plant-use reference rather than a university or government body. Used only for cultivation detail RHS's weeds-focused page does not cover. Bulbs planted fairly deeply. Seed sown as soon as ripe in a greenhouse or cold frame, germinates quickly; or divide in summer after the plant has died down, replanting divisions directly into their permanent position. Bulbs up to 20mm, mild garlic flavour, harvested early summer, store at least 6 months; leaves used raw or cooked as a leek substitute; flowers "juicy with a mild garlic flavour", used as a garnish. Reader reports describe it colonising large areas even from careful initial planting -- consistent with RHS's own Schedule 9 listing -- and recommend vigilant deadheading to prevent unwanted spread.
-
PFAF entry for Allium triquetrum. Fetched and parsed directly for the USDA hardiness rating.
-
LEGAL: "This plant is listed on Schedule 9 of the Wildlife and Countryside Act (1981) as an invasive non-native species." While not banned from sale, "it is an offence to plant or cause it to grow in the wild in Great Britain." Bulbous perennial with triangular stalks, narrow green leaves, white flowers with a central green vein, spring to early summer. Height/spread 0.1-0.5m (about 45cm). RHS hardiness H5. Full sun to partial shade; chalk, clay, loam or sand, moist but well-drained, acid/alkaline/neutral. Sheltered, east/south/west-facing. Not native to Great Britain or Ireland. Spreads by seed and bulbils.
-
Not native to GB/Ireland. Hardiness H5. Height and spread 0.1-0.5m. White flowers with a central green vein on each petal, spring to early summer. Chalk, clay, loam or sand; full sun or partial shade; east, south or west aspect; sheltered; moist but well-drained. INVASIVE STATUS: "This plant is listed on Schedule 9 of the Wildlife & Countryside Act (1981) as an invasive non-native species." Illegal to deliberately plant or allow to grow wild in Great Britain, though it remains legal to sell. Spreads by seed and bulbils, so containment matters.
7 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 three-cornered leeks in the UK?
In Southern England, from early August. In Scotland, wait until early August. 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 three-cornered leeks?
Sow early August to late September.
What problems affect three-cornered leeks?
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 Three-cornered Leeks 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.