General
Leek is a hardy biennial commonly grown as an annual for its long, blanched stems and mild onion flavor. There are early (summer) and late (winter) varieties; late types are more cold-hardy and store better in the ground. Blanching length and stem thickness depend on variety and technique (deep planting, dibber holes, gradual earthing-up, or using collars). Leeks tolerate frost; many winter varieties withstand -10 to -15 °C, allowing harvest well into winter in much of Sweden. For the longest white shanks, start early, grow steadily without checks, and maintain even moisture and fertility.
Light
Full sun (6–8+ hours). Tolerates light shade but yields are lower.
Soil type
Deep, fertile, moisture-retentive but well-drained loam rich in organic matter; fine tilth for sowing. Improve with compost and a little well-rotted manure before planting.
Water requirement
Consistent moisture; ~25–30 mm per week. Avoid waterlogging. Mulch to stabilize moisture.
Nutrient requirement
High nitrogen and moderate potassium. Avoid fresh, high-ammonia manure at planting; use well-rotted compost and side-dressings.
Precultivation
Sow indoors or in a greenhouse 15 Feb–15 Apr. Use modules or small pots, 1–2 seeds per cell, 1 cm deep. Ideal germination 15–20 °C; emergence in ~12 days. Thin to one strong seedling. Grow on cool and bright (10–15 °C) to prevent legginess. Pot up if roots become crowded. Optionally trim leaf tips to 8–10 cm after early growth to reduce flop and encourage sturdy plants. Harden off 1–2 weeks before transplanting.
Planting
Plant when seedlings are pencil-thick and risk of hard frost is low, usually from May. Spacing: 15–20 cm between plants, 30–40 cm between rows. For long white shanks, make 15–20 cm deep dibber holes; drop plants in and water to settle—do not backfill; soil will wash in gradually. Alternatively, plant shallower and earth up in stages as stems thicken. Keep mulch off the leaf bases to avoid rot. In windy or exposed sites, a light collar or additional earthing helps blanching and stability.
Pruning
No routine pruning. Remove scapes promptly if any plants bolt in summer to maintain stem quality. Trim ragged leaf tips only if damaged by wind or pests.
Maintenance
- Water: steady moisture is essential; mulch with compost, straw, or grass clippings.
- Feeding: side-dress with compost or a nitrogen-rich organic fertilizer in late spring and again mid-summer. Foliar seaweed can support micronutrients.
- Earthing-up: hill soil around stems 2–3 times through the season to extend blanching, keeping soil out of the leaf sheaths.
- Weeds: shallow hoeing; avoid root damage.
- Protection: fine mesh (0.8 mm) against leek moth and onion fly from transplanting until mid-summer; rotate away from previous allium beds.
Harvest
Harvest as needed from mid-August for early varieties; winter types can be lifted from late autumn through March where soil is workable. Use a fork to loosen before pulling to avoid breakage. For winter, mulch heavily or heel plants into a sheltered bed for access. Store trimmed leeks in a cool, humid place (0–2 °C) for a few weeks, or upright in damp sand.
Common issues
- Leek moth (Acrolepiopsis assectella): mines leaves and stems. Prevent with insect netting, remove crop residues, and use pheromone monitoring where available.
- Onion fly and thrips: netting, blue sticky traps for thrips, and strong plant hygiene. Avoid nearby cull piles.
- Leek rust (Puccinia allii): orange pustules on leaves; reduce nitrogen excess, improve airflow, water at soil level, and rotate 3–4 years.
- Bolting: triggered by cold followed by heat or drought stress; use variety suited to season, maintain steady growth.
- Nutrient/pH issues: slow growth and pale foliage on acidic soils; lime if pH <6.0.
Rotation schedule
Rotate allium crops on a 3–4 year cycle. Follow after legumes or well-composted beds; precede less nitrogen-hungry crops. Avoid planting after onions, garlic, or chives. Interplant with carrots or celery to maximize space and pest synergy.
Pollination
Not required for harvest (grown for stems/leaves in year 1). If seed is desired in year 2, flowers are insect-pollinated and cross readily with other leeks.
Companion plants
Carrot, celery, parsnip, beets, lettuce, spinach, brassicas, strawberries, chamomile. Carrot + leek can mutually deter pests when interplanted.
Incompatible plants
Beans and peas (alliums may suppress legumes). Avoid close succession with other alliums (onion, garlic, chives) to limit disease buildup.
