Getting the Skin Set Right on Your Potato Crops

Skin set, the natural hardening of the periderm, is essential in protecting potatoes from scuffing, bruising, water loss, and disease.

Understanding the Skin

The potato periderm (skin) consists of three layers. The outermost layer is the phellem, which functions as a protective barrier. Beneath that is the phellogen, a single layer of meristematic cells that produce both the phellem above and the phelloderm below. The phelloderm provides energy and raw materials needed for building the periderm.

While tubers are growing, phellogen cells actively divide to form the skin as the tubers expand. At this stage, the skin is thin, fragile, and only loosely attached. If potatoes are harvested in this condition, even minor abrasions can lead to skinning injuries.

As tubers stop bulking and the vines senesce, the process of skin set begins. Phellogen activity slows, and its cells thicken and stiffen, anchoring the skin in place. At the same time, the phellem cells become infused with suberin, a waxy, water-resistant substance that helps seal in moisture and block out pathogens. To assess skin set in the field, you can apply sideways pressure with your thumb. If the skin slips when rubbed, it’s not set.

Immature tubers are sometimes harvested early for specialized markets. However, for long-distance transport or storage, firm skin is essential.

Effects of Cultivar and Environment

The rate of skin set varies among potato cultivars. Generally, smooth-skinned varieties develop skin more slowly than russet types; however, each cultivar is unique. While genetic differences are significant, environmental conditions and vine maturity can have an even greater influence on the rate of skin set.

Skin set progresses best when soil moisture is maintained at 70% to 75% of available water, with soil temperatures between 70°F and 75°F. Conditions outside this range, especially when there is cool and wet weather late in the season, can significantly delay periderm maturation.

Periderm maturation begins in the field, but it can continue after harvest. Curing usually lasts 10 to 14 days.

Ideal conditions include warm temperatures (typically 50°F to 55°F), high relative humidity (90% to 95%), and good airflow to remove excess carbon dioxide and moisture. After curing, the temperature should be gradually lowered to storage temperatures.

Timing Matters

On average, it takes 10 to 21 days after the vines have fully senesced for the skin to set, depending on the cultivar and field conditions. Any delay in vine senescence, however, can postpone skin set and disrupt harvest schedules. One way to help time this correctly is by carefully managing nutrients, especially nitrogen. Too much nitrogen keeps the vines green. Allowing nitrogen levels to decrease toward the end of the season encourages natural vine senescence. A light frost that reduces the vines can also be beneficial if it happens at the right time.

That said, you also don’t want to leave tubers in the ground too long. Delayed harvests can raise the risk of soil-borne diseases (e.g., soft rot, silver scurf, and Rhizoctonia black scurf). They can also result in over-mature tubers that store poorly and may be unsuitable for processing. When stored, over-mature tubers respire more, leading to shrinkage, and they tend to build up more reducing sugars, which can cause dark colors in processed chips or fries. Additionally, they are more likely to sprout earlier.

Managing Vine Kill

Natural vine senescence doesn’t always align with harvest schedules. To manage this, mechanical or chemical vine kill methods can be used to trigger skin set. This practice can also help regulate tuber size, facilitate the separation of vines from tubers during harvest, and avoid problems caused by late-season foliar pests and diseases.

Mechanical options include flailing, rolling, or flaming vines. You can also desiccate the vines by applying non-selective herbicides (e.g., Reglone, Aim, Gramoxone). A combined approach using mechanical and chemical methods is common in commercial production. Each vine kill method has pros and cons that should be carefully considered before use.

Regardless of the method, timing is essential. The best results are when vine killing is done after the plant has naturally started to senesce. But if you need to kill actively growing plants, lightly flailing or rolling the vines can help initiate senescence.

Rapid vine kill methods can cause stem-end browning, a physiological disorder that affects the vascular ring of tubers. There are many ways to slow down vine kill, but a common approach is to make two applications of a chemical desiccant several days apart, using lower product rates.

Skin Set and Postharvest Success

Skin set isn’t just a harvest issue — it affects stability, disease resistance, and marketability. When timed correctly and supported by good agronomic practices, the skin set helps ensure the crop not only survives harvest but also thrives until it reaches the market.