Leek Care: Long-Season Alliums for Winter Soups
Growing Allium ampeloprasum var. porrum from seed to harvest — the long season timeline, trench-and-hill technique for blanching the shank, and variety selection by days to maturity.
USDA hardiness and when leeks grow
Allium ampeloprasum var. porrum is a cool-season biennial grown as an annual in all zones 3–9. Unlike most cool-season crops, leeks occupy garden space for most of the year — transplanted in spring, they are harvested in fall and early winter. Per Penn State Extension, leeks are "among the hardiest vegetables grown," surviving temperatures down to 0°F in some late-maturing varieties. In zones 6–8, leeks left in the ground with light mulch can be harvested through December or January.
In zones 3–5, earlier-maturing varieties (95–100 days to maturity) are preferable; in zones 6–9, longer-season varieties (120–130 days) with thicker shanks are feasible. Per University of Minnesota Extension, late-season varieties like 'Bulgarian Giant' and 'American Flag' overwinter best in northern zones and produce the thickest shanks.
Varieties
Early season (90–100 days): 'King Richard', 'Titan'. Thinner shanks, harvest summer through early fall. Good for zone 4–5 gardens where fall comes early.
Mid-season (100–110 days): 'Lancelot', 'Tadorna'. Good general-purpose varieties for most zones.
Late season/winter types (115–130 days): 'American Flag' (Broad London), 'Siegfried', 'Bulgarian Giant'. Thickest shanks, best cold hardiness for overwintering. The varieties most often seen at farm markets in November and December.
Light
Full sun — 6 or more hours of direct sunlight daily. Leeks in shade produce thin, weak plants with poor shank development. They are a full-sun crop without meaningful exception. Consistent sun across the long growing season is needed to build the thick, heavy shanks that make leeks worth growing.
Watering
Leeks need consistent moisture throughout their long season — they do not tolerate prolonged drought. Per Penn State Extension, leeks need "approximately 1 inch of water per week." The root system is relatively shallow, meaning moisture loss is detected sooner than in deeper-rooted crops. Mulching around plants (once they are established, being careful not to push mulch into the developing shank area) conserves moisture and reduces irrigation needs during summer.
Irregular watering — particularly drought followed by heavy rain or irrigation — causes cracking in developing shanks and promotes leaf disease. Drip irrigation is ideal for maintaining the consistent moisture leeks need without wetting foliage excessively.
Soil and pH
Leeks prefer fertile, well-drained soil with a pH of 6.0–7.0. They are heavy feeders over their long growing season. Per University of Minnesota Extension, "well-amended soil rich in organic matter produces the best quality leeks." Work 3–4 inches of compost into the top 10–12 inches of soil before planting — this is more important for leeks than for many shorter-season crops because they have more time to benefit from the organic matter and are in the ground when summer heat can stress poorly prepared soil.
Planting
Starting indoors: Start leek seeds indoors 10–12 weeks before the last frost date. Per Penn State Extension, "leeks are almost always started indoors and transplanted" because direct-sowing produces plants too small to transplant at the optimal time. Sow seeds 1/4 inch deep in flats or cell trays at 60–65°F. Germination takes 10–14 days. Thin to one seedling per cell when they're 2 inches tall.
Maintain seedlings under grow lights (16 hours/day) at 60–65°F. Trim tops to 3–4 inches every few weeks to encourage thick stem development — per University of Minnesota Extension, "trimming leek tops during indoor growing produces sturdier transplants." This counterintuitive step produces thicker, more vigorous transplants compared to letting them grow unchecked.
Transplanting: Harden off transplants for 7–10 days before setting out. Transplant to the garden 4–6 weeks before the last frost date — leek transplants tolerate hard frost (20°F) with minimal damage. Two transplanting techniques:
Trench method: Dig a trench 6 inches deep and set transplants 4–6 inches apart, 4–6 inches deep in the trench. Do not fill the trench at planting — as plants grow, gradually fill in the trench with soil, which blanches the lower shank and produces the white, mild-flavored portion. Per Penn State Extension, this method "blanches the shank over time as soil is gradually added."
Dibble method: Make a 6-inch deep hole with a dibber or stick, drop the transplant in, water in without filling the hole (the soil naturally settles around the roots over time). As plants grow, mound soil against the shanks with a draw hoe. Simpler than trench digging but requires more ongoing cultivation.
Fertilizing
Leeks benefit from a structured fertilizer approach across their long season. Work balanced fertilizer (10-10-10) into the planting bed before transplanting. Sidedress with nitrogen fertilizer (blood meal or ammonium nitrate) when plants are 12 inches tall, and again 4–6 weeks later. Per University of Minnesota Extension, "nitrogen sidedressing midseason supports shank development." Stop fertilizing by late August to allow plants to harden off for fall and winter conditions.
Harvest
Harvest when shanks are 1–2 inches in diameter — typically September through December. Leeks can be harvested as needed rather than all at once; in zones 6–8, plants left in the ground through fall and early winter can be harvested over an extended period as needed, since cold temperatures don't damage them. Per Penn State Extension, "leeks can be left in the ground until needed, as cold temperatures improve their flavor."
Loosen soil with a garden fork before pulling to avoid breaking shanks. Clean soil from between leaf layers promptly after harvest, as grit is a persistent issue in leeks. Refrigerate for up to 2 weeks or store in a cool root cellar (32–40°F) for 2–3 months.
Common problems
Leek rust (Puccinia porri)
Orange-yellow pustules on leaves, most common in wet summers with poor air circulation. Per University of Minnesota Extension, rust rarely kills plants but reduces vigor and leaf quality. Remove infected leaves promptly. Improve plant spacing for air circulation. Avoid overhead watering in the evening. There is no practical fungicide for home garden use that is both effective and economical for leek rust.
Thrips
Silvery streaking on leaves from thrips feeding — particularly onion thrips (Thrips tabaci). Per UC IPM, spinosad-based insecticide applied when thrips are first detected provides good control. Row covers during the first weeks after transplanting reduce initial infestations. Hot, dry conditions favor thrips population build-up.
White rot (Sclerotium cepivorum)
Fluffy white growth at the base of the bulb and shank, causing rapid plant collapse. Per Penn State Extension, white rot is a soilborne pathogen that persists for 20+ years in soil. There is no practical in-season treatment. Long crop rotations (7+ years away from all Allium crops) are the only management strategy. Once white rot is confirmed in a bed, do not grow any allium (leek, onion, garlic, chives) in that location for many years.
Frequently asked
How long do leeks take to grow?
From indoor-started transplants, most varieties mature in 100–130 days after transplanting to the garden. Starting indoors in late January or February for late April transplanting typically means harvest from late August through December. Early varieties shorten this to 90–100 days; winter storage types extend to 130+ days. Per Penn State Extension, "leeks occupy garden space for most of the growing season" — plan planting schedules accordingly.
How do I blanch leeks?
Blanching (excluding light from the lower shank) produces the white, tender portion that most recipes specify. Trench or hill soil against developing shanks as they grow — the soil-covered portion stays white and mild-flavored while the green, exposed portion is somewhat tougher and more pungent. The trench method, dibble method, or simple draw-hoe hilling all achieve the same result. Per University of Minnesota Extension, "gradual hilling as plants grow" is the simplest technique for most gardeners.
Can I direct sow leeks?
Yes, but it requires early sowing and produces smaller transplant-sized plants that are then set out, which eliminates much of the advantage. Direct sowing is more practical in mild-winter zones (8–10) where a fall sowing produces harvest-ready plants by late spring. In zones 4–7, indoor starting is strongly recommended to gain the 10–12 weeks of growth needed before outdoor planting.
How do I store leeks?
Refrigerate unwashed leeks in a plastic bag for up to 2 weeks. For longer storage, leave them in the ground in zones 6–8 (they are more cold-hardy than most other vegetables) and harvest as needed through early winter. For root cellar storage, trim leaves to 4 inches, leave roots intact, and store in moist sand at 32–40°F for up to 3 months. Blanch and freeze blanched sections for 6–12 months of frozen storage.