Leeks are known for their mild, delicate onion flavor. This member of the Allium family requires a long, cool growing season, typically needing 120 to 150 days to reach full maturity. Because of this extended development period, the planting schedule must be carefully planned to ensure the plants are ready for harvest before extreme heat or a deep, permanent freeze sets in. Starting the growing process indoors maximizes the available cool-season window, giving the plants the head start they need for a substantial harvest.
Starting Leek Seeds Indoors
Sowing seeds indoors occurs approximately 8 to 12 weeks before the average last expected frost date in your region. This early indoor start ensures the seedlings are robust enough to transplant into the garden during the brief window of early spring. Germination occurs best when the soil temperature is kept consistently warm, ideally between 70 and 80 degrees Fahrenheit.
A consistent soil temperature of 77 degrees Fahrenheit yields peak germination rates within 5 to 14 days. After the seeds sprout, move the containers to a cooler location, preferably between 65 to 75 degrees Fahrenheit during the day and about 55 degrees at night. This temperature differential helps promote sturdy growth rather than leggy seedlings. The young plants require significant light exposure, ideally 12 to 16 hours daily, which is often best provided by supplemental grow lights.
Transplanting Seedlings Outdoors
The time to move the young leeks into the garden is generally in early spring, around the date of the last expected hard frost, or several weeks prior, as leeks are quite tolerant of cold temperatures. Before this final move, the seedlings must undergo “hardening off.” This involves gradually exposing the plants to outdoor conditions, including direct sun and wind, over a period of five to seven days.
Transplanting should happen when the seedlings are between 10 and 15 weeks old and have reached a height of 6 to 12 inches. A physical measure of readiness for some varieties is when the stem has reached the thickness of a pencil. Once the plants are acclimatized, they are set into the garden bed, which should be prepared with loose, fertile soil.
The planting technique directly relates to the desired outcome: a long, white, tender shaft. Gardeners often plant the seedlings in a trench about six inches deep, or dropping them into deep holes made with a dibber. Only a few inches of the green foliage should show above the soil line. This deep planting reduces the amount of hilling, or mounding soil around the base, required later in the season to achieve the blanching effect.
Timing for Overwintering and Late Harvest
A secondary planting window exists for overwintering leeks. This strategy involves planting later in the season to allow the leeks to mature slowly and be harvested throughout the fall and winter. The optimal time for this planting is mid-to-late summer, typically during July or August, depending on the local climate.
Planting in mid-summer gives the leeks sufficient time to establish root systems and develop before the deep cold sets in. This timing is especially effective in milder climates where the ground does not freeze solid for extended periods. Selecting cold-hardy or “winter” varieties is necessary for this late-season approach, as these types are bred to withstand heavy freezes and can be left in the ground for continuous harvesting.
Some of these hardy varieties, if well-mulched, can be harvested from late fall and throughout the winter, sometimes even into the following spring. By timing the planting for a winter harvest, the leeks are exposed to consistent cool temperatures that enhance their flavor and texture. This late planting ensures a fresh supply of leeks long after the typical growing season has concluded.