How to Grow Leeks from Seed (Allium porrum)

A leek occupies its patch of ground for the best part of a year and gives you nothing at all until the rest of the vegetable garden has been cleared. That is exactly the point. From late September until April, when almost nothing else is standing outside, a row of Allium porrum is a living store cupboard: no clamp, no freezer, no shelf — you lift what you need on the day you need it. Leeks are also, quietly, easy to get wrong, because most of the skill sits in one move made in June or July: how deep you plant the seedling. Get that right and you have long, sweet, blanched shafts. Get it wrong and you have thin green onions. Here is how to grow leeks from seed, which varieties to choose, and how to keep a bed cropping through the coldest months of the year.

• Sow Jan–Feb under cover · Feb–Apr in modules or a seedbed · to early May for late types
• Soil Deep, rich and firm, never freshly dug · full sun · pH 6.5–7.5
• Spacing Dib 15 cm deep · plants 15 cm apart · rows 30–40 cm
• Harvest Sept–Dec from autumn types · Dec–April from winter types · lift as needed

An onion that never makes a bulb

The leek is a cultivated form of Allium ampeloprasum, grown around the Mediterranean since antiquity and carried north by the Romans, where it found a climate that suited it far better than the one it came from. Botanically it belongs with the onions, but it never swells a bulb. What you eat is a pseudostem: a tight roll of leaf bases wrapped around one another like a rolled newspaper. That has one consequence worth understanding before you sow anything, because it explains every technique below. The shaft is white only where light cannot reach it. Bury 15 cm of a young leek and it makes 15 cm of white; leave it sitting on the surface and the whole plant turns green, tough and oniony. Length is not a variety trait so much as a planting depth.

The second thing to know is that a leek is a biennial. It builds its shaft through one summer, stands through the winter, and in its second spring throws a hollow flower stem topped with a drumstick of purple florets — beautiful, and the end of the plant as a vegetable. That is why leeks are sown in late winter and lifted before May, and why there is no such thing as an autumn sowing that works. If you want an allium to sow at this time of year, it is the overwintering onion, not the leek.

Choosing a variety: autumn leeks, winter leeks and the giants

Leeks split neatly into two groups, and the mistake is to treat them as interchangeable. Autumn and early-winter types grow fast and make long, pale, slender shafts, but they are only moderately hardy and should be lifted by about Christmas: Herfstreuzen 3 (the Dutch autumn giant, sold here as Elephant), Blue Green Autumn, the fast Bulgarian Giant and the old French De Carentan 2, which we also stock as organic seed. Winter types are shorter and thicker with darker, bluer, more upright leaves, and they shrug off hard frost: Winterreuzen 3, Bluegreen Winter and its selection Hiverbleu, the broad Zwitserse Reuzen (Swiss Giant), Farinto — also available pelleted, which makes tiny leek seed far easier to space — and Tornado, which can be sown as late as June for a spring crop. For an early start under glass, organic Hannibal is tagged for January sowing. Browse the full range in the leek seed collection, by botanical name under Allium porrum, or in organic leek. There is also organic leek sprouting seed for a windowsill tray — a mild, oniony microgreen that gives you leek flavour in about three weeks rather than eleven months, alongside the rest of our sprouting seeds.

How to grow leeks from seed, step by step

1. Sow in January–April, not later Leeks need a long season, so early is better: January or February in a heated propagator, March or April in modules or a spare corner of the garden. Sow 1 cm deep, thinly, at around 12–15 °C — see starting seed indoors. Leek seed is short-lived; if the packet is more than two years old, run a germination test first.
2. Grow them on to pencil thickness This is the target for a transplant: roughly 20 cm tall and 8 mm across the base, which takes eight to twelve weeks. Seedlings look like blades of grass for the first month — that is normal. Trimming the leaf tips back to about 15 cm makes handling easier and stops them flopping; it does not speed anything up.
3. Prepare rich, firm ground Leeks are hungry and want a bed manured for a previous crop, not one dug over last week: loose, fluffy soil collapses into the planting hole. Dig in compost in autumn or spring, then tread and rake before planting — our guide to preparing your soil covers it. They follow peas, beans or early potatoes nicely in a crop rotation.
4. Dib and puddle in June or July The move that makes the crop. With a thick dibber or a broom handle, make holes 15 cm deep and about 4 cm wide, 15 cm apart in rows 30–40 cm apart. Drop one seedling into each hole so the leaves sit just above the rim — and do not fill it in. Instead, fill the hole with water. The soil washes gently in around the roots over the following weeks, the shaft grows up inside a light-proof cylinder, and it stays white and free of grit.
5. Water and feed through the summer Leeks have shallow, sparse roots and swell in response to steady moisture: a deep soak once a week in dry weather beats a daily splash, as in watering your garden. A nitrogen-rich feed in July and August thickens the shafts; stop by early September so growth hardens off before the frosts.
6. Keep the row clean, and earth up if you want more white Weeds beat leeks easily in the early weeks — hoe carefully or mulch, per mulching and weed control. From September you can draw dry soil up around the shafts in two or three stages, adding another 5 cm of blanched stem. Never earth up wet soil or into the leaf joint: that is how grit gets into the heart.
7. Start lifting, and leave the rest standing Autumn types are ready from late September; winter types are better after a frost or two, which sweetens them. Lift with a fork rather than pulling — a leek snaps at the base surprisingly easily — and take only what you need that week.

Care from midsummer to the first frosts

Once they are in the ground leeks ask for very little, and the few problems they do get are worth recognising. Leek rust (Puccinia allii) shows as bright orange pustules on the leaves in wet, mild summers; it looks alarming but is largely cosmetic, and affected plants are perfectly good to eat once the worst leaves are stripped off. Crowded rows, still air and too much nitrogen all make it worse, so space generously and rotate — the same logic as for most plant diseases. Leek moth and allium leaf miner are the two pests that matter in a bad year, both tunnelling into the shaft and leaving a slimy, unusable heart; the only reliable defence is fine insect mesh over the row from planting until autumn, which is far easier than any of the remedies in our guide to common garden pests.

Bolting is the other thing to watch for, and it is almost always a sowing-date problem. A leek that is checked by cold when it is already a good size — typically an overwintered plant, or one sown in autumn — reads the cold spell as a winter, and runs to flower the following spring instead of thickening. Sow between January and May, plant out at pencil thickness, and you will not see it.

Autumn leeks vs winter leeks

Grow a short row of each and the harvest runs from September to April without a gap. Leeks also make good company in a winter bed: kale and hardy Swiss chard stand alongside them, carrots can be lifted from the same ground, and chicory or winter salad leaves fill the spaces between. If you are short of ground, leeks will grow in a deep container — see growing from seed in pots — though they rarely reach the size they manage in open soil.

Harvesting: the bed as a store cupboard

Leeks do not store the way onions do. There is no drying, no plaiting and no dry shelf: the plant simply waits in the ground until you want it, which is why a row is worth more in January than the same weight of anything in a freezer. Push a fork in well clear of the plant, lever the soil up and lift the whole leek; trim the roots and the coarse upper leaves, then split the shaft lengthways under a tap if any grit has worked its way in. Once lifted, a leek keeps a week or two in the fridge and no longer, so lift on the day. The exceptions are worth planning for: if a long, hard freeze or a flooded bed is forecast, lift a batch early and heel them into a box of damp sand or a sheltered trench, where they will hold for a month. Sliced leek also freezes well without blanching, though it comes back softer — the usual trade-offs are in harvesting and storing your produce. Clear the last of them by early May, before the flower stems come, and the ground is free for a summer crop.

Common leek mistakes

• Sowing too late. A leek sown in June is a spring onion in November. January to April is the window; only Tornado stretches it.
• Planting shallow. No depth, no white shaft. Dib 15 cm and drop the seedling right in.
• Backfilling the hole with soil. Fill it with water instead. Soil pushed in around a seedling packs against the stem and traps grit between the leaves.
• Planting into freshly dug, fluffy ground. The holes collapse. Firm the bed first, and manure it the season before.
• Crowding the row. Leeks at 8 cm stay the thickness of a pencil. Fifteen centimetres each way is the minimum for a decent shaft.
• Feeding into October. Late nitrogen gives soft growth that frost damages and rust loves. Stop in early September.
• Treating them like onions at the end. No curing, no netted sack — leave them in the soil and lift on demand.

Frequently asked questions

Can I sow leeks in September?
No — and it is the most common autumn question about them. A leek sown now would go into winter as a seedling, be checked by cold, and bolt in spring rather than thicken. Leeks are sown from January to April for harvest the following autumn and winter. If you want an allium to sow this month, it is the overwintering onion: see growing onions from seed, and what to sow in September for everything else that still has a window.

Why are my leeks so thin?
Four usual causes, in order of likelihood: sown too late, planted out smaller than pencil thickness, spaced too closely, or short of water and feed in July and August. Depth affects how much of the shaft is white; thickness comes from time, spacing and food. Next year, sow in February and give each plant 15 cm.

Do I have to earth up leeks?
Not if you dib deep — that is the whole point of the 15 cm hole. Earthing up in autumn adds perhaps another 5 cm of blanched stem and is worth doing on a light soil if you want show-length leeks. Do it in stages, with dry soil, and keep it out of the leaf joint.

What is the orange dust on my leek leaves?
Leek rust — a fungus that arrives in mild, damp weather and is very common by late summer. It rarely kills a plant and does not spoil the shaft: strip the worst leaves, bin rather than compost them, avoid high-nitrogen feeds late in the season, and do not grow leeks or onions in the same bed two years running.

Leeks are a long game with an unusually reliable pay-off: eleven months of quiet occupation, then a vegetable standing in the ground exactly when the garden has nothing else to offer. Order the seed now while you are thinking about it, sow in the new year, and remember the dibber in June. In the meantime the other jobs of the moment are indoors — drying this year's chillies and herbs for the winter store. Fill the rest of the bed from our vegetable seeds or the organic range, start with our beginner's guide to growing vegetables from seed, and browse the rest of the growing guides for whatever the next bed needs.