How to Grow Three-cornered Leeks UK | Regional Planting Calendar

Regional Planting Calendar

Quick Facts

At a Glance

Growth Stages

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

Named for its stem

The distinctive triangular (three-cornered) cross-section of the flower stalk is what gives this plant its name -- run a finger down it and the three flat edges are easy to feel, unlike the round stems of most other alliums.

A green stripe on white

RHS: drooping, bell-shaped white flowers with a central green vein on each petal, spring to early summer -- the detail that separates it from ramsons at a glance, whose flowers are pure white and star-shaped rather than bell-shaped.

A cluster in bloom

Flowers open in a loose, drooping cluster above the narrow leaves, spring to early summer.

Why the legal restriction exists

A single clump like this represents years of unchecked spread by both seed and bulbils. This is exactly the growth habit that put the species on Schedule 9 -- it is straightforward to establish and very difficult to remove once it has this kind of hold.

What "colonising large areas" looks like

Reader accounts describe exactly this outcome even from careful, contained initial planting -- a whole patch of ground given over to one species. It is why RHS is explicit that it is an offence to plant or cause this to grow in the wild in Great Britain, and why vigilant deadheading matters if it is grown at all.

References

1. Plants For A Future (PFAF) - Allium triquetrum

   Tier 2 -- a compiled plant-use reference, not 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.

2. Royal Horticultural Society - Allium leaf miner: identification and control

   country=UK

3. Royal Horticultural Society - Allium triquetrum (three-cornered garlic, three-cornered leek)

   Tier 1. 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.

4. Royal Horticultural Society - Onion white rot

   country=UK

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.