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 an onion relative that never swells into a bulb. What you eat is the shank, a smooth, firm white cylinder built from leaf bases wound tightly one around the next, bleached pale where deep soil kept the light off it. Above the white, the leaf blades fan out flat and strap-shaped in a cool blue-green, and the flesh tastes milder and sweeter than an onion, holding that softness through the pan.
This is a plant that builds slowly and asks for patience. A leek needs 120 to 150 days to bulk its shank, so most growers start the fine, thread-like seedlings indoors in late winter and set them out pencil-thick around the last frost. The reward is a stout column that lingers in the ground for weeks and, under mulch with a hardy variety, stands firm right through winter.
Give a leek deep, rich soil, steady water, and plenty of food, and the shank thickens into a heavy white stem you can wrap a hand around. Crowd it, starve it, or let the bed dry, and it stays a thin, tough, pencil-narrow thread instead of that fat pale column.
Care
The short version: full sun, deep and fertile soil, and steady moisture all season. Set the seedlings deep, draw soil up the shank to bleach it a clean white, and feed regularly, because leeks are hungry plants.
Light
Leeks want full sun to build a full, heavy shank, though they will take a little afternoon shade. Starved of light, the shank stays thin and the flat leaf blades flop slack and pale.
Water
Keep the bed evenly moist and never let it dry out fully, or the white shank stays thin and turns stringy and tough. Aim for about 1 inch (2.5 cm) of water a week, since the roots sit shallow, and lay mulch over the soil to hold that moisture between waterings.
Soil
Plant into deep, loose, fertile loam worked through with compost or aged manure, which gives the long roots room to run and the hungry plant its fuel. A pH between 6.0 and 7.0 suits them well.
Temperature
Leeks are tough, riding out anything from 14°F to 86°F (-10°C to 30°C), and a touch of frost even firms and sweetens the shank. Heat is the harder trial, since hot, dry spells wilt the blade tips and draw in thrips.
Humidity
Ordinary outdoor humidity is fine, so there is no number to chase. What matters is airflow between the leaf fans, because crowded, damp blades are exactly where the orange dust of leek rust takes hold.
Feeding
Leeks are heavy feeders that want food all season while the shank builds. Side-dress with a nitrogen-rich or balanced fertilizer about once a month from spring through late summer.
Pruning
Trim the fine seedling tops back to about 4 to 5 inches (10 to 13 cm) once or twice before transplanting, which thickens the plant into a stockier shoot. If one throws a flower stalk, snap that hard round stem out fast, since 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 stand 12 to 35 inches (30 to 90 cm) tall in a slim, upright fan just 4 to 6 inches (10 to 15 cm) across. Give each one room so air moves between the blades and the shank can swell without crowding.
When to plant
Leeks run on a long clock and need an early start. Sow the thread-thin seedlings indoors in late winter, set them out around your last frost once they thicken to pencil width, 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 writes itself across the leaf blades or shows in a shank that stays narrow and never fattens. Read the marks early and you can usually turn it around before harvest. Match what you see below, then act while it still helps.
A hard round stalk rising and the shank going woody
When a stiff, round stalk pushes up through the center of the leaf fan in the first year, the leek has bolted, tricked into flowering after a long cold spell chilled a large transplant as though a winter had passed. Drought and transplant shock can tip it the same way. That woody core runs straight down the shank and never softens, so lift those plants at once and eat them before the rest.
Fix:
- Harvest bolted plants immediately and use them; quality only declines
- Snap out very young stalks and use those plants first
Bright orange dust dotting the leaf blades
Those raised, powdery orange specks strung in lines down the blade are leek rust, a fungus that flares in humid air and among plants crammed close. Strip away the most freckled leaves, spread the planting so a breeze can slip between the fans, and pull back the nitrogen that swells the leaves into soft, tender targets. Circling your alliums through a 3 to 4 year rotation leaves the spores no old ground to cling to.
Fix:
- Remove and destroy heavily infected leaves
- Thin or widen spacing to improve airflow
- Avoid overhead watering late in the day
Pale silver scratches dulling the leaves
Those glinting silver-white flecks and the gray film they leave over the blades are the tracks of onion thrips, slender insects that scrape at the leaf surface and swarm through hot, dry spells. A parched, stressed plant shows the scarring worst, so a well-watered, mulched bed keeps the leaves firmer, and a hard spray of water flicks the thrips loose. Where they crowd thickly, insecticidal soap worked into the tight inner folds reaches the ones tucked out of sight.
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 left pencil-thin instead of stout
A shank no wider than a pencil, still slim where it should be swelling into a stout white column, usually means crowding, a late start, or a bed run thin on food. Leeks take the full 120 to 150 days and draw on the soil the whole time, so thin the stand, scratch the weeds out, side-dress with nitrogen, and water deep and even. No need to lift them in a hurry either, since leeks sit for weeks in cool soil and the slender ones keep widening as 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 and a real hazard for animals. Like every allium, the whole plant, white shank and green blade alike, carries organosulfur compounds that destroy red blood cells and cause hemolytic anemia in pets and livestock, raw or cooked. Keep the 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 fragile of all here, and even a small nibble can do damage. The first signs are vomiting, drooling, weakness, and gums gone pale, and then breathing quickens and the urine runs red or brown as the red blood cells rupture into Heinz-body hemolytic anemia.
Dogs Moderate
A dog that chews any part of a leek can end up with vomiting, diarrhea, a tender belly, and a slow slide into weakness. A larger or repeated helping carries it into the same hemolytic anemia, the gums bleached pale, the heart racing, and the urine darkened.
Horses Moderate
A horse that grazes a real quantity of leek grows dull and heavy-footed, its gums pale or tinged yellow and its urine dark as the blood cells give way.
Rabbits Toxic
Leeks and every other allium are wholly off the menu for rabbits. The same organosulfur compounds tear at their red blood cells, so keep all parts well clear of the hutch.
Livestock Toxic
Cattle, sheep, and goats are all at risk, and the harm can gather slowly as they graze alliums over time. Weakness sets in alongside pale gums and dark urine as the anemia deepens.
Birds Toxic
Birds, backyard hens among them, should never be handed leek scraps. The toxin shreds their blood cells just as it does in mammals.
People Safe
For people, leeks are a kitchen staple and perfectly safe to grow, handle, and eat. The only catch is the cleanup, so 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, some paler and some deep blue-green in the leaf. These are the ones worth knowing by name.
King Richard
An early leek that draws out long, slender shanks made for a fall harvest, though it stands among the least winter-hardy of them all.
American Flag (Broad London)
A classic open-pollinated heirloom with thick, sturdy shanks and the cold tolerance to keep standing right through to spring.
Bandit
A very hardy late leek clothed in dark blue-green leaves, bred to hold in the ground all winter for fresh harvest in the cold.
Lancelot
A trim, upright mid-season hybrid whose long white shanks rinse clean with little effort.
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 bleach 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 rise from seed alone, each one starting as a single thread-thin, looping green shoot. Sow indoors 8 to 12 weeks before the last frost, and the seed breaks in a week or two at 65°F to 75°F (18°C to 24°C) before the seedlings slowly stiffen toward transplant size. Once they thicken to about the width of a pencil, sink them 6 inches (15 cm) deep into dibble holes or a trench, water them in without pressing the soil tight, and keep drawing soil up the stem all season to bleach a long, sweet white shank.
Myths
Leeks form a bulb like onions and are ready when the tops fall over.
A leek never rounds out into a true bulb. The pale white shank you eat is a tight cylinder of leaf bases wrapped one over the next, bleached to white by deep planting and by soil hilled up the stem. Read the harvest from the thickness of the shank, not from any flopping of the tops.
Cooked leeks are safe to share with cats and dogs.
Heat, drying, or a grind to fine powder leaves the organosulfur compounds inside an allium untouched. A cooked leek is every bit as dangerous as a raw one, and in cats even a small taste can bring on Heinz-body hemolytic anemia.