Hilling potatoes is an agricultural practice that involves progressively mounding soil, compost, or other organic material around the base of the growing potato plant. This technique is fundamental to successful potato cultivation, affecting both the safety and abundance of the final harvest. The process ensures that the developing underground parts of the plant are adequately covered throughout the growing season.
Protecting Developing Tubers from Sunlight
The most direct reason for hilling is to shield the newly forming potato tubers from any exposure to light. Potatoes are modified stems, and when these underground parts encounter sunlight, a detrimental physiological change begins. The light triggers the production of chlorophyll, which causes the skin of the potato to turn green.
While chlorophyll itself is harmless, its production is closely linked to the synthesis of a toxin called solanine. Solanine is a glycoalkaloid that develops in the same areas as the green pigment, and its presence is the primary concern for consumption. Solanine has a bitter taste and can cause digestive distress, including vomiting and diarrhea, if consumed in high concentrations.
Keeping a thick layer of soil, straw, or compost over the developing tubers creates an effective physical barrier, preventing this light-induced toxicity. The practice of hilling ensures that the edible part of the plant remains safely buried in darkness. Consistent hilling acts as a safety measure, ensuring the entire crop remains wholesome.
Supporting Plant Growth and Maximizing Harvest
Beyond preventing toxicity, hilling also serves to maximize the number of potatoes produced by each plant. Potatoes develop from specialized underground stems called stolons, which extend horizontally from the planted seed piece. New tubers form at the tips of these stolons.
By continually mounding material around the main stem, the gardener encourages the plant to produce more tuber-bearing stolons higher up the stalk. The covered portion of the stem becomes a new area where potatoes can form, increasing the available underground space for tuber development. This repeated burying of the stem material directly translates to a greater potential number of potatoes per plant.
The hilled material also serves a structural purpose by anchoring the growing plants. The extra mound of soil or organic matter provides physical support against wind and rain. This stability ensures the plant remains upright throughout the season. Furthermore, the thick mound helps keep the soil cooler, which is beneficial since tuber development slows significantly when soil temperatures rise above 68 degrees Fahrenheit.
When and How to Hill Potatoes
The hilling process should begin early in the plant’s life, when the emerging shoots reach a height of six to eight inches above the ground. The first mound of loose material is gently placed around the plant stems, covering the lower third to half of the visible foliage.
This is a repetitive action that continues as the plant grows upward. Gardeners repeat the hilling process every two to four weeks, or whenever the new foliage has grown another six inches above the previous mound. The goal is to always leave a few inches of green leaves exposed for photosynthesis, which provides the energy for tuber growth.
Hilling can be stopped once the potato plants begin to flower, or when the foliage becomes dense and starts to fall over. At this stage, the plant focuses its energy on tuber formation rather than vertical growth, and further hilling is less effective.
Common materials for hilling include loose garden soil or organic mulches such as straw, hay, or aged compost. Straw is advantageous because it is light, breathes well, and makes for an easier, cleaner harvest. The final hilled mound should be substantial enough to fully protect all developing tubers from sunlight.