Vegetable · Amaryllidaceae
Leek
Allium ampeloprasum
Plant them deep, feed them hard, and hill the soil up, and a leek hands you a long sweet shank that stands through the cold.
Full sun
Every 3–7 days
120–150 days
USDA 5–10
Overview
A leek is part of the onion family, but it never bothers making a bulb. What you eat is the shank, a long cylinder of tightly wrapped leaf bases that goes white when you plant it deep and mound soil up the stem as it grows. And the flavor? Milder and sweeter than an onion, and it stays that way once it's cooked.
This one's for the patient gardener. Leeks need 120 to 150 days to mature, so most people start the seed indoors in late winter and set out pencil-thick seedlings around the last frost. What you get back is a vegetable that holds in the ground for weeks, and with a hardy variety under mulch, right through the winter.
Give a leek deep, rich soil, steady water, and plenty to eat, and it grows slow and strong. But crowd it, starve it, or let the bed dry out, and you're looking at thin, tough shanks instead of those fat white stems you had in mind.
Care
The short version: full sun, deep and fertile soil, and steady moisture all season. Set the seedlings deep, hill soil up the shank to blanch it white, and feed regularly, because leeks are hungry plants.
Light
Leeks want full sun if they're going to size up right, though they'll put up with a bit of afternoon shade. Give them too little and the shanks stay thin while the plants go slack.
Water
Keep the bed evenly moist and never let it dry out all the way, or the shanks turn thin and tough. Shoot for about 1 inch (2.5 cm) of water a week, since the roots sit shallow, and lay down mulch to hold that moisture between waterings.
Soil
Plant in deep, loose, fertile loam worked over with compost or aged manure, which gives those long roots room and feeds a plant that eats a lot. A pH between 6.0 and 7.0 is right where they like it.
Temperature
Leeks are tough as nails, taking anything from 14°F to 86°F (-10°C to 30°C), and honestly a touch of frost just sweetens the shank. Heat's the bigger worry, since hot, dry spells stress the plants and pull in thrips.
Humidity
Regular outdoor humidity is fine, so there's no number to chase here. What actually matters is airflow, because crowded, damp leaves are exactly how leek rust gets going.
Feeding
Leeks are heavy feeders that want food the whole season. Side-dress with a nitrogen-rich or balanced fertilizer about once a month from spring through late summer while the shanks are filling out.
Pruning
Trim the seedling tops back to about 4 to 5 inches (10 to 13 cm) once or twice before transplanting, and you'll get stockier plants for it. If one starts to bolt, snap that flower stalk out quick, because a bolted shank turns woody.
Repotting
not applicable; grown in open ground (or a deep container) for a single long season and harvested
Size & spacing
Plants get 12 to 35 inches (30 to 90 cm) tall and stay skinny, only 4 to 6 inches (10 to 15 cm) across. Leave each one a little room so air moves through and the shanks can fatten up without crowding.
When to plant
Leeks run on a long clock, so an early start really pays off. Sow indoors in late winter, set the seedlings out around your last frost once they're pencil-thick, then harvest from late summer through fall and, with a hardy variety under mulch, deep into winter.
Northern hemisphere
Southern hemisphere
Typical months for a temperate climate: start earlier in warm zones, later where spring frosts linger.
- Sow: 8-12 weeks before the last spring frost
- Plant: around the last frost date, once seedlings are pencil-thick; set them deep in dibble holes or a trench and hill soil up as they grow to blanch the shank
- Harvest: harvest from late summer through fall and, for hardy cultivars under mulch, into winter; flowers only in the second year if overwintered
Hardiness
USDA hardiness zones: 5–10
Temperature: down to -29°C
Troubleshooting
Most leek trouble shows up on the leaves or in skinny shanks that won't fatten. Catch it early, though, and you can usually turn things around before harvest. Find what matches below, then act while it still does some good.
A flower stalk pops up and the shank goes woody
If a leek throws up a hard, round flower stalk in its first year, it's bolted. Usually a long cold snap tricked a big transplant into thinking a whole winter came and went, and drought or a rough transplant can nudge it that way too. Once the stalk shows up the shank only gets tougher, so just pull those plants and eat them first.
Fix:
- Harvest bolted plants immediately and use them; quality only declines
- Snap out very young stalks and use those plants first
Orange powder streaking down the leaves
That's leek rust, a fungus that loves muggy weather and plants packed in too tight. Pull off the worst leaves, open up the rows so a breeze can get through, and go easy on the nitrogen, since all that soft growth is exactly what the rust is after. And here's the long game: rotate your alliums on a 3 to 4 year cycle so the spores can't just wait around for next season.
Fix:
- Remove and destroy heavily infected leaves
- Thin or widen spacing to improve airflow
- Avoid overhead watering late in the day
Silver flecks and a gray look on the leaves
Those silvery scratches? That's onion thrips, little bugs that scrape at the leaf and go wild when it's hot and dry. Dry plants have it worst, so keeping the bed watered and mulched already helps a lot, and a good blast of water knocks plenty of them off. If the numbers really pile up, work some insecticidal soap into the inner leaf folds where they like to hide.
Fix:
- Knock thrips back with strong jets of water
- Keep plants well watered and mulched
- Use insecticidal soap on the inner leaf folds if pressure is heavy
Shanks staying skinny instead of filling in
Thin, pencil-like shanks usually come down to one of three things: too crowded, planted too late, or just not enough food in the bed. Leeks want the full 120 to 150 days and they eat the whole time, so thin them out, get the weeds gone, side-dress with nitrogen, and water deep and even. And don't rush to pull them, right? Leeks sit happily for weeks in cool soil, and those little ones keep fattening while they wait.
Fix:
- Thin to proper spacing and weed thoroughly
- Side-dress with nitrogen and water deeply and evenly
- Leave undersized plants in the ground longer; leeks hold well in cool soil
Toxicity
Leeks are a treat for people but a real hazard for animals, so it's worth being clear on that. Like every allium, they carry organosulfur compounds that wreck red blood cells and cause hemolytic anemia in pets and livestock, whether they're raw or cooked. Keep the growing plants, kitchen trimmings, and compost scraps well away from cats, dogs, horses, and grazing animals, and remember cats are especially sensitive and the signs can take days to turn up.
Toxic to
Cats · Dogs · Horses · Rabbits · Livestock · Birds
Safe for
People
Cats Severe
Cats are the ones to watch most, and even a tiny nibble can cause real trouble. It usually starts with vomiting, drooling, weakness, and pale gums, then moves to fast breathing and reddish or brown urine as the red blood cells break down into Heinz-body hemolytic anemia.
Dogs Moderate
A dog that chews on any part of a leek can end up with vomiting, diarrhea, a sore belly, and slowly building weakness. And if it's a bigger or repeated helping, that tips into the same hemolytic anemia, with pale gums, a racing heart, and dark urine.
Horses Moderate
A horse that grazes a real amount of leek goes dull and weak, with pale or yellow-tinged gums and dark urine as the blood cells start giving way.
Rabbits Toxic
Leeks and every other allium are simply a no for rabbits. The same organosulfur compounds go after their red blood cells, so keep all the parts well away from the hutch.
Livestock Toxic
Cattle, sheep, and goats are all at risk too, and the harm can build up as they graze alliums over time. You'd see weakness, pale gums, and dark urine as the anemia sets in.
Birds Toxic
Birds, backyard hens included, should never get leek scraps. The toxin hits their blood cells the same way it does in mammals.
People Safe
For people, leeks are a kitchen favorite and totally safe to grow, handle, and eat. The only thing to stay on top of is the cleanup, so keep trimmings and compost scraps somewhere the pets can't get to them.
Safe for people to grow and handle; keep growing plants, kitchen trimmings, and compost scraps out of reach of pets. Onset of anemia signs in pets can be delayed by several days after ingestion.
Helpline numbers
United States
ASPCA Animal Poison Control: (888) 426-4435 (fee applies)
United Kingdom
Animal PoisonLine: 01202 509000 (fee applies)
Australia
Animal Poisons Helpline: 1300 869 738
Varieties
Leeks mostly split into early types for fall and hardy late types bred to stand through winter. These are the ones worth knowing by name.
King Richard
It's an early leek with long, slender shanks that's great for a fall harvest, though it's one of the least winter-hardy ones out there.
American Flag (Broad London)
It's a well-loved open-pollinated heirloom with thick shanks and enough cold tolerance to keep standing right through to spring.
Bandit
It's a really hardy late leek with dark blue-green leaves, and it's happy to sit in the ground all winter for fresh harvests in the cold.
Lancelot
It's an even, upright mid-season hybrid whose long white shanks clean up without much fuss.
Companions
Leeks pair well with crops that swap pest protection and share their need for rich, moist soil, and badly with the legumes that alliums tend to hold back.
Plant near
carrot: Classic pairing: leeks are reputed to deter carrot rust fly while carrots help deter leek moth, and the crops share bed conditions without competing strongly
celery: Similar needs for rich soil and steady moisture make celery a compatible bed partner that uses the space between slow-growing leeks
Keep apart
beans: Alliums are traditionally reported to inhibit the growth of beans and other legumes when interplanted
peas: Like beans, peas are reported to grow poorly alongside Alliums and should be kept in a separate bed
How to propagate
There's just the one way to grow a leek, from seed, and the whole trick is starting early enough and planting deep enough to blanch a long white shank.
SeedModerate
Sow indoors 8-12 weeks before the last spring frost; transplant outdoors around the last frost date · Germinates in about 1-2 weeks at 18-24°C (65-75°F); reaches transplant size in 8-12 weeks
There's really just one way to grow a leek, and that's from seed, so a bit of patience is part of the deal. Start them indoors 8 to 12 weeks before your last frost, where the seed pops up in a week or two at 65°F to 75°F (18°C to 24°C) and then takes its sweet time getting to transplant size. When they're about pencil-thick, tuck them 6 inches (15 cm) deep into dibble holes or a trench, water them in without firming the soil, and keep hilling soil up the stem all season so you end up with a long, sweet white shank.
Myths
Leeks form a bulb like onions and are ready when the tops fall over.
Here's the thing: a leek never actually makes a bulb. That white shank you cook with is just a cylinder of tightly bundled leaf bases, turned white because you planted it deep and hilled soil up the stem. Go by the size of the shank, not by waiting for the tops to flop over.
Cooked leeks are safe to share with cats and dogs.
Cooking, drying, or grinding a leek into powder doesn't touch the organosulfur compounds inside an allium, so this one's a trap. A cooked leek is every bit as dangerous as a raw one, and cats can get Heinz-body hemolytic anemia from surprisingly little.