Hilling is a cultivation practice that involves drawing soil or organic material up around the base of a potato plant’s stem as it grows. This technique is an expected part of the potato growing cycle. Understanding the proper timing for this process is directly related to maximizing the harvest and ensuring the quality of the potatoes. The decision of when to stop adding soil to the plants is based on the potato’s unique growth pattern.
The Purpose of Hilling Potatoes
Growers perform this technique for two distinct reasons that impact both the safety and the quantity of the final crop. The first objective is to prevent the potato tubers from being exposed to sunlight, which is a common issue as they develop near the soil surface. When light reaches the tubers, it triggers the production of chlorophyll, causing the skin to turn green, and stimulates the formation of solanine, a toxic glycoalkaloid.
The second reason is to increase the total yield of the plant. Potatoes form on specialized underground stems called stolons. By mounding loose soil around the base, the grower effectively lengthens the buried portion of the stem, encouraging more stolons to form higher up. This extended growing area provides additional space for new tubers to set and enlarge, resulting in a greater number of potatoes harvested.
Identifying the Critical Stop Point
The moment to cease hilling is signaled by a combination of the plant’s physical size and its reproductive stage. Hilling typically begins when the potato vines are 6 to 8 inches tall and continues every few weeks until the plant reaches maximum vegetative growth. The physical limit is reached when the vines are approximately 12 to 18 inches tall and the soil mound has reached its highest practical point.
The most reliable biological indicator that the hilling phase is ending is the onset of flowering. Flowering marks a shift in the plant’s energy allocation, moving resources away from vegetative growth toward tuber development underground. Once the plant begins to produce flowers, the primary tuber set has largely been established, and further mounding contributes little to increasing the number of potatoes.
Growers may continue a final, light hilling shortly after the flowers appear to ensure all developing tubers remain completely covered and protected from sunlight. Beyond this point, trying to add more soil becomes physically damaging to the mature, sprawling vines and the developing root system. The hilling operation is considered complete when the space between rows is entirely filled by the soil mounds and the foliage is dense.
Post-Hilling Care and Harvesting
Once the final soil mound has been established, the next phase involves waiting for the tubers to mature fully. The most visible sign of a finished growth cycle is the natural die-back of the foliage, or “tops,” which will turn yellow and then brown. This yellowing indicates that the plant has stopped drawing nutrients up and is dedicating its energy reserves to thickening the skins of the potatoes below ground.
For a main harvest of mature, storage-ready potatoes, it is best to wait two to three weeks after the foliage has completely died back. This waiting period allows the potato skins to cure naturally in the soil, which toughens the outer layer and improves the tuber’s ability to be stored long-term. Harvesting earlier yields “new potatoes,” which have thin skins and are intended for immediate consumption.