Leeks, a mild-flavored member of the Allium family, are prized for their long, tender, white stem, which contrasts with the tougher, green leafy tops. The quality and length of this desirable portion are determined by how the plant is positioned and managed in the soil. Planting depth is the single most important factor influencing the ultimate length of the marketable, edible stem, setting the stage for the blanching process.
Why Depth Matters: Achieving Blanching
The white part of the leek is the result of blanching, a cultivation technique that excludes sunlight from a portion of the plant’s stem as it grows. This physical exclusion of light prevents the stem tissue from developing chlorophyll, the pigment responsible for the green color of leaves and the process of photosynthesis.
Without light, the stem cannot produce chlorophyll and remains pale, tender, and mild in flavor. The green part of the leek, which is exposed to the sun, is photosynthetically active, resulting in a fibrous texture and a stronger, more pungent taste. By planting deeply, the grower essentially pre-blanches the initial portion of the stem, maximizing the length of the mild, palatable product.
A deeper initial planting directly correlates to a longer blanched stem harvested at the end of the season. The goal is to achieve a substantial white shaft, ideally 6 to 10 inches long, which is only possible by keeping that entire section buried. This deep planting provides a head start on the blanching process before any subsequent mounding of soil is even required.
Planting Techniques for Optimal Blanching
The standard recommendation for achieving a long, blanched stem is to transplant seedlings when they are pencil-thick and about 8 to 12 inches tall. At this stage, the leeks are ready to be set into the ground at a depth that immediately initiates the blanching process. The optimal initial planting depth for transplants is typically between 6 and 8 inches.
The Dibbling Method
The Dibbling Method is a popular technique for transplanting individual leek seedlings. This method involves using a dowel or specialized stick, known as a dibber, to create a deep, narrow hole in the soil, usually 6 to 9 inches deep. The grower then drops a single leek seedling into the bottom of this hole, trimming the roots to about an inch and the leaves to encourage new growth.
After placing the seedling, the hole is not backfilled with loose soil; instead, the plant is simply watered in thoroughly. The water settles the soil naturally around the roots, securing the plant while preventing grit from entering the tight leaf sheaths. This method is efficient and provides an immediate deep planting that maximizes the blanched portion from the start.
The Trenching Method
Alternatively, the Trenching Method is often used, especially for larger plantings. A trench is dug approximately 6 to 8 inches deep, and the leeks are planted upright along the bottom. The soil removed from the trench is reserved on the sides and will be used later for gradual backfilling, which is an ongoing process of blanching the stem as the leek grows.
Regardless of the method, proper spacing is necessary to allow for the plant’s growth and the later addition of soil. Leeks should be spaced 4 to 6 inches apart within the row, with rows separated by 18 to 24 inches to ensure adequate room for hilling the soil later in the season.
Maintaining Depth: Hilling and Watering
Deep initial planting provides a good start, but achieving maximum blanching requires continuous effort through a technique called hilling or earthing up. This process is an extension of the initial deep planting, where soil is gradually pulled up around the leek stem as the plant grows taller. The hilled soil continues to exclude light, which extends the length of the white, tender stem beyond the initial planted depth.
Hilling
Hilling should begin when the leeks are robust, typically around 10 to 12 inches tall, and is repeated every few weeks throughout the growing season. With each session, the grower gently pulls 2 to 3 inches of loose, crumbly soil up around the stalk, being careful not to bury the fan of leaves, which need sunlight to sustain the plant. Using fine, dry soil for hilling is important, as wet or lumpy soil can cause rot or create pockets that let in light.
Watering
Watering practices must also be carefully managed in relation to the planted depth and hilling. Leeks require consistent, deep watering throughout their development to ensure steady growth and large stems. When watering, care must be taken to apply water directly to the soil, avoiding the tops of the plants, which prevents newly hilled soil from being washed away and keeps dirt from being trapped within the leaf sheaths.
Deep watering helps settle the soil around the roots, especially in dibble holes, and prevents the soil from drying out, which can slow growth. Maintaining the depth of the covering material, whether it is the initial planting or subsequent hilling, is the mechanism that ensures the development of a long, high-quality blanched stem.