Hilling, also known as mounding, is a cultivation technique used to increase the productivity of potato plants. This practice involves building up a ridge of material around the lower stem of the growing plant. Performing this action creates ideal underground conditions for a larger, healthier harvest. Consistent application of this method throughout the early growing season is key to a successful potato crop.
The Purpose and Timing of Hilling
The primary reason for hilling is to prevent sunlight from reaching developing tubers, which form close to the soil surface. Light exposure causes tubers to photosynthesize, turning them green and producing solanine, a natural toxin that makes them unsafe to eat. Hilling also stimulates the plant to produce more potatoes. Covering the stem encourages the formation of additional underground stems, called stolons, where new tubers form. This process creates more surface area for potato production, boosting the final yield by 20 to 30 percent.
The correct timing for the first hilling is when the emerging potato sprouts have reached a height of six to eight inches above the ground. Performing this task too early can bury the tender growth points, while waiting too long risks sun exposure to shallow tubers. The process must be repeated as the plant grows, typically every two to three weeks, or whenever the new growth reaches another six to eight inches above the previous mound. Continue hilling until the plant begins to flower, which signals that the period of maximum tuber formation is nearing its end.
Step-by-Step Traditional Soil Hilling
The traditional method of hilling relies on using the native soil already present in the garden bed or row. Start by using a garden hoe to gather the loose soil from the space between the rows of plants. The goal is to avoid disturbing the young root systems while accumulating enough material for a mound.
Once the soil is gathered, gently pull it up and around the base of the plant’s stem. The soil should be mounded high enough to cover about two-thirds of the visible stem, leaving only the top few leaves exposed. This action buries a portion of the stem, encouraging it to produce new stolons and more potatoes.
Create a steep, firm ridge or mound four to six inches high around the plant. This structure helps to shed excess water away from the stem and provides a deep, dark environment for the new tubers to develop. This initial mound sets the stage for future hilling efforts as the plant continues its vertical growth.
Repetition of this process ensures a generous harvest throughout the growing season. As the plant resumes growth and reaches six to eight inches in height, repeat the soil mounding process. Continue drawing soil from the pathways to build the hill higher, maintaining the goal of burying the lower two-thirds of the new growth until the plants begin to bloom.
Hilling with Alternative Materials
For gardeners with heavy clay soil, in raised beds, or using containers, alternative materials substitute for traditional soil hilling. One popular method uses light, organic materials such as straw, hay, or well-decomposed compost. These materials are layered around the base of the stem instead of mounding soil.
Hilling with straw or hay is often referred to as a no-till method, offering easier harvesting since the crop can be uncovered by pulling the mulch aside. The organic material also helps suppress weeds and acts as a moisture-retaining layer, keeping the soil cooler during hot weather. When applying straw or hay, ensure the layer is thick enough (at least six inches deep) to completely block sunlight from the developing tubers.
Growing potatoes in containers, such as grow bags or barrels, requires an approach that follows the principle of stem coverage. Instead of pulling soil from an adjacent row, the gardener adds new material (such as a mix of compost and soil) directly into the container as the plant grows. The container sides hold the hilling material in place. Material is added whenever the plant reaches the six-to-eight-inch growth threshold, covering the lower stem until the container is full.