Hilling potatoes in a raised bed directly influences the quantity and quality of the harvest. This practice involves gradually adding soil or organic material around the base of the growing potato stems as they lengthen. Hilling manages the environment where the edible part of the potato—the tuber—will develop. The goal is to maximize the space for new potato formation and protect the developing crop from environmental damage.
Why Hilling is Essential for Potato Health
Hilling is necessary because potato tubers do not form on the plant’s root system. They develop from specialized horizontal underground stems called stolons, which emerge from the buried portion of the main stem. By continually covering the lower stem with material, you encourage the plant to produce a greater length of stem underground. This increases the number of sites where new stolons and tubers can form, which is the primary driver for increased yield.
The technique also serves a safety function by keeping light away from developing tubers. Sunlight exposure causes potatoes to produce chlorophyll, turning the skin green. This greening indicates increased production of solanine, a toxic glycoalkaloid. Solanine is naturally present in the plant as a defense mechanism, but elevated levels can make the potato taste bitter and cause digestive distress if consumed. Hilling maintains a protective, dark environment to ensure the harvest remains safe and palatable.
Initial Planting Depth and Timing
Hilling in a raised bed requires an intentional, shallow initial planting depth to leave vertical space for subsequent material additions. For a tall raised bed, plant seed potatoes only about four to six inches deep. This depth ensures the developing plant has room to grow upward, allowing for multiple layers of hilling material to be added throughout the season.
The visual cue for the first hilling should be monitored closely. Begin the process as soon as the potato foliage emerges and reaches a height of approximately six to eight inches above the initial soil level. This height indicates the plant is actively growing and ready to be covered. Hilling is a repeated action that continues throughout the early to mid-growing season.
Step-by-Step Hilling Technique
Once the foliage reaches the target height, the hilling technique involves carefully adding material around the base of the stems. Because you are working within a raised bed, you must bring in outside material instead of pulling soil from the row sides, as is done in a traditional garden. Loose, well-draining materials work best and may include fresh compost, aged leaf mold, or light straw.
To perform the hilling, gently pile the chosen material around the stems, creating a new mound that is two to four inches deep. This new layer should cover almost all of the visible stem, leaving only the top one-third of the foliage exposed. The exposed leaves are necessary to continue photosynthesis, which provides the energy and starch needed for tuber development.
Repeat this hilling action every time the plant grows another six to eight inches tall. The walls of the raised bed are beneficial because they contain the hilling material, allowing you to achieve a greater final depth than might be practical in an open garden. Continue adding material until the plants begin to flower, which signals that tuber initiation has slowed, or until the raised bed is nearly full. Using a lighter material like straw for later hilling layers can make the final harvest easier, as the potatoes will be formed in loose, uncompacted media.