Leeks are a mild-flavored biennial vegetable in the Allium family, prized for their long, edible white shanks. Unlike onions and garlic, leeks produce a thick, cylindrical sheath of tightly wrapped leaves instead of a bulb. Successful cultivation of this long-season crop relies on precise timing, which dictates the final size, flavor, and harvest window. This timeline begins well before the last spring frost and extends into the cooler months.
Timing Seeds for Indoor Starting
Starting leek seeds indoors is necessary because the plant requires a long growing season, often taking 120 to 150 days to reach maturity. Seeds should be sown indoors approximately 8 to 12 weeks before the last expected spring frost date to get a head start. This early start allows seedlings to establish a robust root system and reach sufficient size for transplanting.
Optimal germination occurs when the soil temperature is kept around 70°F (21°C). Using a heat mat helps maintain this temperature, and seeds typically sprout within 7 to 14 days. After germination, young plants require bright light, ideally 12 to 16 hours daily under grow lights, to prevent them from becoming weak and spindly.
When to Plant Leek Seedlings Outdoors
The transition from indoors to the garden bed requires careful timing. Before planting, young leeks must be “hardened off,” a gradual process over five to seven days. This acclimates them to outdoor conditions like wind and direct sun, preventing shock when they are permanently set out.
Leeks are cold-tolerant and can be transplanted outdoors around the last spring frost, or even a few weeks prior, once the soil is workable. Daytime temperatures should consistently reach at least 45°F (7°C). Seedlings should be about 8 inches tall and roughly the thickness of a pencil before planting.
For optimal development of the tender white shaft, leek seedlings are often planted deep using a method called dibbling. A hole 5 to 6 inches deep is created, the seedling is dropped in, and the hole is filled only with water. This allows the surrounding soil to wash in naturally as the plant grows. This deep planting acts as a form of blanching, yielding a longer, milder-flavored edible portion.
Adjusting Planting Time for Climate and Harvest
The ideal planting schedule depends on the grower’s harvest goal and local climate conditions. For a summer or early fall harvest, the standard spring planting schedule, following the indoor start, is used. These “summer leeks” are milder and smaller, maturing as early as 70 to 90 days after transplanting.
To achieve a late fall or winter harvest, planting time must be shifted later in the year. Planting seeds or setting out transplants in mid-summer (July or early August) allows leeks to mature slowly into cooler weather. This timing is effective for “overwintering leeks,” which are cold-hardy varieties bred to remain in the garden through winter in milder climates (Zone 7 and above).
In regions with very short growing seasons, starting seeds indoors very early is necessary, focusing on quick-maturing cultivars. In areas with mild winters and hot summers, the focus shifts to fall planting. This allows leeks to develop during cooler months and prevents heat stress that can lead to bolting. Overwintering leeks can be insulated with a heavy layer of mulch or straw, allowing them to be harvested as needed throughout the winter and into early spring.
Knowing When to Harvest Leeks
Leeks are ready for harvest when the stalk reaches a usable diameter, typically between 1 and 1.5 inches. Most varieties require 100 to 120 days from transplanting to reach this mature size. Unlike onions, leek tops do not die back to signal maturity, so the grower must monitor the shaft’s thickness.
A major advantage of growing leeks is the extended harvest window, as they can be pulled progressively as needed. The plants are tolerant of light and heavy frosts, which helps to sweeten their flavor. In many regions, leeks can be left in the garden past the first frost and harvested from late fall until the ground freezes solid.