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
The leek is a member of the onion family that never bothers with a bulb. What you eat is the shank, a long cylinder of tightly wrapped leaf bases that you blanch white by planting deep and mounding soil up the stem as it grows. The flavor is milder and sweeter than an onion, and it stays that way through cooking.
This is a crop for the patient gardener. Leeks need 120 to 150 days to mature, so most growers start seed indoors in late winter and set out pencil-thick seedlings around the last frost. The payoff is a vegetable that holds in the ground for weeks, and with a hardy variety under mulch, right through winter.
Give a leek deep, rich soil, steady water, and plenty of food, and it grows slow and strong. Crowd it, starve it, or let the bed dry out and you get thin, tough shanks instead of the fat white stems you were after.
Care
The short version: full sun, deep and fertile soil, and steady moisture all season. Plant 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 to size up properly, though they'll put up with a little afternoon shade. Skimp on light and the shanks stay thin and the plants grow slack.
Water
Keep the bed evenly moist and never let it dry out fully, or the shanks turn thin and tough. Aim 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 the long roots room and the heavy feeder its fuel. A pH between 6.0 and 7.0 suits them well.
Temperature
Leeks are tough, taking anything from 14°F to 86°F (-10°C to 30°C), and a touch of frost actually sweetens the shank. Heat is the bigger problem, since hot, dry spells stress the plants and invite thrips.
Humidity
Ordinary outdoor humidity is fine, so there's no number to chase. What matters is airflow, because crowded, damp leaves are exactly how leek rust gets started.
Feeding
Leeks are heavy feeders that want food all season long. Side-dress with a nitrogen-rich or balanced fertilizer about once a month from spring through late summer while the shanks are building.
Pruning
Trim seedling tops back to about 4 to 5 inches (10 to 13 cm) once or twice before transplanting, which builds stockier plants. If one starts to bolt, snap the flower stalk out fast, 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 reach 12 to 35 inches (30 to 90 cm) tall and stay narrow, just 4 to 6 inches (10 to 15 cm) across. Give each one room so air moves through and the shanks can fatten without crowding.
When to plant
Leeks run on a long clock and need an early start. 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 never fatten. Catch the signs early and you can usually turn it around before harvest. Match what you see below, then act while it still helps.
A flower stalk shoots up and the shank turns woody
When a leek throws up a hard, round flower stalk in its first year, it has bolted, usually after a long cold snap chilled large transplants into thinking a winter had passed. Drought and transplant shock can tip it over the edge too. Once a stalk forms the quality only slides, so harvest those plants right away 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 powdery streaks running along the leaves
This is leek rust, a fungus that takes hold in humid weather and tight, crowded plantings. Strip off the worst leaves, open the rows up so air can move through, and ease back on the nitrogen that pushes soft, susceptible growth. Rotate your alliums on a 3 to 4 year cycle so the spores have nowhere to overwinter.
Fix:
- Remove and destroy heavily infected leaves
- Thin or widen spacing to improve airflow
- Avoid overhead watering late in the day
Silvery flecks and a grayish cast on the leaves
Those silvery scratches are the work of onion thrips, tiny insects that rasp the leaf surface and run riot in hot, dry weather. Drought makes the damage worse, so keep the bed watered and mulched, and blast the plants with strong jets of water to knock the thrips back. Reach for insecticidal soap in the inner leaf folds only when pressure is heavy.
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 stay thin instead of fattening up
Thin, pencil-like shanks usually mean the plants are crowded, went in late, or never got the fertility they need. Leeks take 120 to 150 days to mature and feed hard the whole time, so thin them out, weed the bed clean, and side-dress with nitrogen while watering deeply and evenly. There's no rush to pull them, since leeks hold for weeks in cool soil and undersized plants keep right on bulking up.
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 and a real hazard for animals. Like every allium, they carry organosulfur compounds that destroy red blood cells and cause hemolytic anemia in pets and livestock, raw or cooked alike. Keep growing plants, kitchen trimmings, and compost scraps well away from cats, dogs, horses, and grazing animals, since cats are especially sensitive and the signs can take days to appear.
Toxic to
Cats · Dogs · Horses · Rabbits · Livestock · Birds
Safe for
People
Cats Severe
Cats are the most sensitive of all, and even a small nibble can be dangerous. Watch for vomiting, drooling, weakness, and pale gums, then rapid breathing and red or brown urine as the red blood cells break down into Heinz-body hemolytic anemia.
Dogs Moderate
A dog that chews any part of a leek can come down with vomiting, diarrhea, belly pain, and growing weakness. Larger or repeated helpings push it into the same hemolytic anemia, with pale gums, a racing heart, and dark urine.
Horses Moderate
Horses that graze a real quantity of leek turn dull and weak, with pale or yellow-tinged gums and dark urine as their blood cells give way.
Rabbits Toxic
Leeks and every other allium are off the menu for rabbits. The same organosulfur compounds attack their red blood cells, so keep all parts well away from the hutch.
Livestock Toxic
Cattle, sheep, and goats are all vulnerable, and trouble can build up from grazing alliums over time. Expect weakness, pale gums, and dark urine as anemia sets in.
Birds Toxic
Birds, backyard hens included, should never be offered leek scraps. The toxin damages their blood cells just as it does in mammals.
People Safe
Leeks are a kitchen staple and perfectly safe for people to grow, handle, and eat. The catch is the cleanup: keep trimmings and compost scraps where pets can't reach 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 split mainly into early types for fall and hardy late types bred to stand through winter. These are the ones worth knowing by name.
King Richard
An early type with long, slender shanks that's superb for a fall harvest, though it's one of the least winter-hardy leeks you can grow.
American Flag (Broad London)
A classic open-pollinated heirloom prized for its thick shanks and the cold tolerance that lets it stand through to spring.
Bandit
A very hardy late leek with dark blue-green leaves, bred to hold in the ground all winter for fresh harvest in the cold.
Lancelot
A uniform, upright mid-season hybrid whose long white shanks are easy to clean.
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
Leeks are grown one way, from seed, and the 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
Leeks are grown from seed, and it takes patience. Sow indoors 8 to 12 weeks before your last frost, and the seed sprouts in a week or two at 65°F to 75°F (18°C to 24°C) before slowly reaching transplant size. Set the pencil-thick seedlings 6 inches (15 cm) deep in dibble holes or a trench, water them in without firming the soil, and keep hilling soil up the stem all season to blanch a long, sweet white shank.
Myths
Leeks form a bulb like onions and are ready when the tops fall over.
A leek never makes a true bulb. The white shank you eat is a cylinder of tightly bundled leaf bases, blanched white by deep planting and hilling soil up the stem. You harvest by shank size, not by waiting for the tops to flop.
Cooked leeks are safe to share with cats and dogs.
Cooking, drying, or grinding into powder does nothing to break down the organosulfur compounds in an allium. A cooked leek is every bit as dangerous as a raw one, and cats can suffer Heinz-body hemolytic anemia from surprisingly little.