Leeks are not a quick crop, but they are one of the most rewarding. That mild, sweet onion flavor develops over a long, cool season, and the deep white shaft that makes leeks what they are in the kitchen only happens with intentional blanching. The good news is that once you get the start-indoors timing right and understand the hilling or trench method, leeks are genuinely low-maintenance for most of their long season.
When to plant
Leeks require a minimum of 120 to 150 days to mature (University of Minnesota Extension). Some newer cultivars are ready in about 90 days, which makes them more practical in short-season gardens. Either way, they are transplanted in early spring and harvested in late summer through fall.
Work backward from your first fall frost to find your latest transplant date, and from there back 8 to 10 weeks to find your indoor sow date. In northern climates, late February to early March is the typical indoor start window.
Starting leeks indoors
Leeks start exactly like onions: sow seeds 1/4 inch deep in a tray, keep the mix moist, and provide strong overhead light once they germinate. A heat mat is not required since leeks germinate fine at normal room temperature, but consistent light matters. Thin to one seedling per cell once they are a few inches tall.
Leek seedlings look like fine green grass for weeks. That is normal. When they are pencil-thick and the weather has settled, they are ready to transplant.
Harden off over 5 to 7 days before transplanting out. See how to harden off seedlings for the step-by-step.
Site and soil
Leeks perform best in well-drained soil with a pH between 6.0 and 7.0 and high organic matter (University of Minnesota Extension). Full sun is best (8 hours minimum). They are heavy feeders, so incorporate compost or a balanced fertilizer into the bed before planting.
Transplanting and blanching
Blanching the lower shaft (the part below soil level) is what produces the tender, white, mild-flavored shank that makes leeks worth growing. There are two approaches, and both work.
Two methods for a long white shaft
Furrow method
Dig a furrow 6 inches deep. Set transplants at the bottom, 4 to 6 inches apart. Water them in; do not fill the furrow immediately. As plants grow, gradually backfill soil up to the leaf base over the course of the season, always keeping the leaves above soil.
Hilling method
Plant transplants at normal soil depth (trim roots to 1 inch if needed to make planting easier). As plants grow, mound compost or soil up around the stems several times during the season, keeping the mound below the lowest leaves.
Spacing at transplant
For either method, set transplants 4 to 6 inches apart in-row, 12 inches between rows. Closer spacing (2 to 4 inches) maximizes yield but produces slightly thinner shafts.
Post-planting fertilizer
Water in with a half-strength balanced fertilizer solution to help transplants establish quickly (University of Minnesota Extension recommendation).
Watering and feeding
Leeks have shallow roots and need consistent moisture. Plan to irrigate during dry spells; 1 inch of water per week is the benchmark (University of Minnesota Extension). Water at the soil line, not overhead, to reduce disease pressure.
In midsummer, side-dress with a balanced fertilizer such as 10-10-10 at about 1 cup per 10-foot row, scratched in 6 inches from the plants. This mid-season boost supports the final stretch of shaft development.
Common problems
Leeks share most of their pest problems with onions. Onion maggot is the most serious insect threat, boring into stems and causing yellow, wilted foliage. Thrips leave silver-streaked leaves and are most troublesome in hot, dry summers. Leaf miners leave pale tunnels in the leaf tissue. Good crop rotation (do not plant alliums in the same spot more than once every 3 years) reduces most soil-borne pressure.
Harvesting
Most leek varieties are fully developed when the shaft is wider than 1 inch in diameter. A quality leek has a firm white shaft at least 3 inches long (University of Minnesota Extension). The upper leaves should be deep blue-green; if they start dying back, harvest soon.
Leeks tolerate significant cold. You can leave them in the ground through the first several frosts; some varieties survive down to 20 degrees F with mulch mounded around them. This makes leeks a valuable late-fall vegetable that bridges the gap after most other crops are done.
Harvest by gently twisting and pulling, or use a fork to loosen the soil first. Trim the roots and outer leaves. Slice lengthwise before rinsing; soil tends to lodge tightly between the leaf layers.
Related guides
Leeks fit naturally into a fall vegetable garden plan since they can stay in the ground well past first frost. If you are growing other alliums, see how to grow onions. For indoor starts, see how to start seeds indoors.
When should I start leek seeds indoors?
In northern climates, late February to early March is the typical window. Count back 8 to 10 weeks from your planned transplant date (early spring, once daytime temps are reliably above 45 degrees F). Transplants older than 15 weeks get root-bound and transplant poorly.
Do leeks need to be blanched?
You do not have to blanch them, but the white shaft that forms below soil level is milder, more tender, and what most people are actually after in a leek. Either the furrow-planting method or repeat hilling gives you a longer white shaft. Without blanching you still get a perfectly edible leek with more green and a stronger flavor.
How long do leeks take to grow?
Most varieties need 120 to 150 days from transplant. Newer short-season cultivars can reach harvest in about 90 days. They are transplanted in early spring and typically harvested from late summer into late fall, with some cold-hardy varieties surviving into early winter with mulch protection.
Can leeks stay in the ground through frost?
Yes. Leeks are one of the more cold-tolerant vegetables in the garden. Many varieties survive temperatures down to 20 degrees F, especially with some mulch mounded around them. This makes them a useful late-season crop you can harvest on demand through fall and into early winter.
What causes leeks to bulb at the base?
Bulbing (swelling at the base like a small onion) is undesirable and is usually triggered by cold stress at the wrong growth stage. Leek transplants that experience temperatures below about 50 degrees F for an extended period before they are well established can be induced to bolt or bulb. University of Minnesota Extension recommends not planting out until daytime temperatures are reliably above 45 degrees F.