Leeks (Allium ampeloprasum var. porrum) are a cool-season vegetable belonging to the Allium family, which also includes onions and garlic. Unlike their relatives, leeks produce a long, thick, cylindrical shaft of tightly wrapped leaves instead of a bulb. They offer a milder, sweeter flavor profile, making them highly valued for soups, braises, and other cooked dishes. Growing leeks requires a long, consistent season, but the process is straightforward once the specific needs for starting and tending the crop are understood.
Starting the Leek Crop
Successful cultivation starts with selecting the right variety and proper indoor seed starting. Leeks are categorized into early-season types (e.g., ‘King Richard’) for summer harvest, and overwintering types (e.g., ‘Bandit’) that tolerate cold for winter and spring harvest. Starting seeds indoors is recommended in most climates to ensure plants reach maturity.
Sow seeds eight to twelve weeks before the last expected spring frost. Use a sterile, fine-textured seed-starting mix in flats or pots at least three inches deep. Sow seeds one-quarter inch deep, spacing them about one inch apart.
Maintain seedlings under bright light for 12 to 16 hours daily, keeping the soil consistently moist. Seedlings are ready for transplanting when they are six to eight inches tall and pencil-thick. Before moving them outdoors, gradually harden off the young plants over a one-week period.
Preparing the Growing Environment
Leeks require full sun, meaning at least six to eight hours of direct sunlight each day. Although the root system is not deep, the plant needs a rich environment to support extensive top growth. The soil must be deeply worked, well-draining, and heavily amended with organic matter, such as aged compost or well-rotted manure.
The most effective preparation involves digging a trench to facilitate blanching. Create trenches six to eight inches deep and six inches wide, spacing rows 18 to 30 inches apart. This depth provides the foundation for the long, white edible stem.
When transplanting, place seedlings at the bottom of the trench, spacing them five to nine inches apart. Water gently, allowing the soil to wash in and cover the roots without completely filling the trench. This shields the stem base from light immediately.
Essential Care for Optimal Growth
Leeks are heavy feeders and drinkers, requiring consistent moisture and nitrogen throughout the growing season. The soil must be kept uniformly moist; dry conditions can stress the plant and cause premature bolting (producing a flower stalk). Side-dressing the plants with a high-nitrogen fertilizer every four to six weeks promotes the development of a thick, tender shaft. This feeding, combined with initial soil amendments, supports the high demand for vegetative growth.
Blanching
The desirable white color and texture of the leek shaft are achieved through blanching, which is the exclusion of sunlight from the lower stem. Blanching is performed by “hilling” the plants. Hilling involves mounding soil up around the stem as the leek grows. Gradually pull soil from the sides of the trench and pile it around the stem, covering the new growth with two to three inches of soil every few weeks. This practice prevents chlorophyll production, yielding the long, pale, tender edible portion.
Pest Management
Gardeners may occasionally encounter pests such as onion thrips, which cause silvery streaks on the leaves, or onion maggots, which attack the young stems and roots. To manage these, cultural controls are the first line of defense.
Cover the crop with a fine mesh insect barrier (floating row cover) to prevent adult pests from laying eggs.
Avoid using fresh manure or undecomposed organic matter, which reduces the attraction for maggot flies.
For thrips, apply a concentrated spray of insecticidal soap or neem oil. Ensure thorough coverage of the entire plant, as the tiny insects often hide deep within the leaf folds. Consistent monitoring and quick action are necessary to maintain the health of the crop.
Harvesting and Post-Harvest Handling
Leeks are ready for harvest when the shafts reach a usable size, typically one to two inches in diameter. They can be harvested earlier for smaller “baby” leeks. Most varieties mature 100 to 120 days after transplanting. Unlike onions, their green tops will not naturally die back to signal readiness. Harvest can be delayed, and in mild climates, many varieties can be left in the garden and harvested as needed throughout fall and winter.
To harvest, use a garden fork or trowel to carefully loosen the soil around the leek before gently lifting it out. Avoid pulling the plant directly, as this can damage the stem. After harvesting, shake off soil and rinse the plant thoroughly to remove any grit trapped between the leaf layers.
For short-term storage (seven to ten days), refrigerate leeks. They store best when wrapped in a damp paper towel and placed inside a perforated plastic bag in the crisper drawer to maintain high humidity. For long-term use, the cleaned shafts can be chopped and frozen for cooking. Alternatively, they can be stored in a root cellar by partially burying them in moist sand near 32°F, where they can keep for two to three months.