Hilling involves progressively mounding soil or organic matter around the base of a growing potato plant. This process is necessary because potatoes are not root vegetables; they are underground stem tubers that form on specialized horizontal stems called stolons. The main confusion for new gardeners is whether they should bury the entire plant, including the leaves. Successful potato cultivation requires balancing protection for the developing tubers with the plant’s ability to photosynthesize and grow.
Understanding Why We Hill Potatoes
Hilling serves a dual purpose that directly impacts the quality and quantity of the potato harvest. The first reason is to prevent sunlight from reaching the developing tubers. Potatoes exposed to light turn green due to the accumulation of chlorophyll, and simultaneously produce a toxic alkaloid called solanine.
Solanine is a natural defense mechanism produced by the plant, and its presence makes the green parts of the potato bitter and potentially harmful if consumed. Mounding creates a deep, dark environment, ensuring the potatoes remain white or yellow and safe for eating. The second reason for hilling is to increase the total surface area where new tubers can form. Since potatoes develop along the buried stem, adding more material encourages the plant to produce more stolons, resulting in a larger yield.
The Specifics of Covering Foliage
The answer to whether you cover the leaves when hilling potatoes is yes, but only the lower leaves and stem, never the entire plant. The uppermost leaves must remain exposed to the sun and air for photosynthesis. This top foliage converts sunlight into the energy required to grow and produce tubers.
A good rule of thumb is to cover about two-thirds of the visible plant height with hilling material. When the plant is 6 to 8 inches tall, gently pull soil or mulch up around the stem until only the top few sets of leaves are visible. This action forces the buried stem section to convert into the tuber-producing zone without starving the plant of the light it needs. Mounding must be done carefully to avoid snapping brittle stems or disrupting the root structure below the surface.
The Hilling Schedule and Materials
The hilling process should begin once the potato plant reaches a height of 6 to 8 inches above the ground. After this initial mounding, the process is repeated every two to three weeks, or any time the plant grows another 6 to 8 inches of new foliage. The goal is to continuously build the hill, maintaining a deep layer of material over the developing tubers until the plant begins to flower.
Gardeners often use loose garden soil, compost, or straw as hilling materials. Soil offers a dense, supportive environment and is easily accessible when growing in traditional rows. Lighter materials like straw or aged compost are popular for container or raised bed gardening because they are easier to handle and make the final harvest much simpler. Using straw or compost also helps keep the soil cooler and retain moisture, which are beneficial conditions for potato growth.