The required soil depth for growing potatoes is unique because the edible tubers are not true roots but modified stems (rhizomes). These tubers develop along the underground portion of the main shoot, positioned above the initial seed piece. This upward growth habit means that soil depth directly impacts potential yield, as more stem coverage allows for more tuber-forming sites. Sufficient soil coverage is also the primary mechanism for preventing tubers from being exposed to light, which triggers a defense response that makes them unsafe to eat.
Preparing the Bed and Initial Planting Depth
Successful potato cultivation begins with preparing a deep, loose growing medium that accommodates the upward development of the crop. Potatoes thrive best in well-draining, slightly acidic soils, such as sandy loams, that are rich in organic matter to support growth and water retention. Before planting, the bed should be thoroughly worked to loosen the soil to a depth of at least 12 to 18 inches. This deeply prepared foundation allows the plant roots to establish easily and ensures that the soil used for mounding later is soft and friable.
For the initial planting, certified seed potato pieces are typically placed in a trench or hole that is 4 to 6 inches deep. This depth ensures the seed piece is protected while allowing the emerging sprouts to reach the light efficiently. For most garden conditions, a 6-inch depth is standard to provide a foundation for the subsequent soil additions. Seed potatoes should be spaced approximately 9 to 12 inches apart within the row to allow adequate room for tuber development and to reduce competition between plants.
The Hilling Technique and Achieving Final Depth
Achieving the final necessary soil depth for potatoes is an ongoing process known as “hilling,” which is the practice of repeatedly mounding soil around the base of the growing stems. This technique serves two primary functions: it encourages the plant to produce more tubers along the newly buried stem section, and it shields the developing tubers from sunlight. The first hilling should occur when the potato plant foliage reaches a height of about 6 to 8 inches above the ground. At this point, the surrounding soil is gently pulled up and around the stems, burying the lower two-thirds of the plant.
This process must be repeated two to three times as the plant continues to grow throughout the season, effectively building a continuous mound or ridge around the row. The final required soil depth is determined by the need to maintain complete darkness around the developing tubers, which form at various levels along the buried stem. By the end of the growing cycle, the final hill should be substantial, creating a mound about 10 to 12 inches tall above the initial seed piece planting level. An inadequate final depth allows light to penetrate the soil, triggering the formation of chlorophyll, which causes the tuber surface to turn green.
More importantly, light exposure also dramatically increases the production of the toxic alkaloid solanine in the tuber. While the green color itself is from non-toxic chlorophyll, it serves as a visual warning sign that solanine may be present in unsafe concentrations just beneath the skin. Ensuring the final soil coverage is deep enough prevents this harmful photochemical reaction, protecting the quality and safety of the harvest. The repeated addition of soil also provides structural support to the foliage as the plants become larger.
Vertical Space Requirements for Container Growing
When potatoes are cultivated in containers, such as grow bags, barrels, or raised beds, the vertical dimension becomes the limiting factor for achieving the necessary final soil depth. For optimal yields, a container should have a minimum depth of 18 inches, with 24 to 36 inches being the preferred height to allow for proper hilling. Using vessels that are too shallow restricts the overall length of the stem that can be buried, limiting the number of tubers that can form.
The container should also provide sufficient volume, with a capacity of 10 to 15 gallons often cited as ideal for accommodating a healthy number of plants. A general rule is to allocate about five gallons of volume for each seed potato planted. The hilling technique is fully utilized in container growing, beginning with only 4 to 6 inches of soil at the bottom to start the seed piece. As the plant grows, soil is progressively added until the container is filled, maximizing the stem length that can produce tubers. Containers must also feature adequate drainage holes, as waterlogged soil can lead to the seed piece and developing tubers rotting.