Prolonged above-normal heat in southern Idaho and southeastern Oregon stresses potato plants and causes them to temporarily stop growing, experts say.

Varieties shown to perform well in the region’s at-times harsh conditions can benefit local processors, which could choose them over potatoes delivered at greater cost from farther away, said Bill Buhrig, Oregon State University Malheur Experiment Station faculty research assistant, during a field day July 10. Temperatures that day soared 20 degrees above the threshold at which plants shift into self-protection mode.

Near Wilder, Idaho, July 15, Rafael Luna, a co-owner of Doug Gross Farms, looked at a crop and prepared for a weekly test dig. Performance to date looked roughly normal but below that of the good 2023 season.

“It’s not what we’re shooting for, but normal,” he said. “The heat is just not helping. The heat is going to make us lose some yield.”

When temperatures exceed about 90 degrees, potato plants tend to enter a dormancy that signals “it’s too hot to work,” Luna said. “It shuts down. The tuber is not bulking.”

Potato plants are mature, their rows closed. The plants look healthy, but when a plant gets too hot and shuts down, “it stops sending the right things to the tubers,” he said.

A temperature of 86 degrees a foot above the plant canopy prompts a physiological shift, said soil scientist John Taberna, who owns Western Laboratories in Parma, Idaho. Leaves’ potassium-containing pores — key to photosynthesis — start to collapse and the plant wilts, a protection.

When the above-canopy temperature threshold is reached during the day, and how long it persists, factor into how potato plants are affected.

“If there is adequate moisture in the soil, the plant will regain turgidity (production readiness) the next morning,” Taberna said.

Soils that are too warm and dry can contribute to crop impacts such as a “heat runner” — a tuber putting out a stolon that makes another tuber depending on leaf coverage. These new tubers typically are small, low in quality and take energy from the tuber of origin.

“In current conditions, evapotranspiration in potatoes, onions, mint and hops is above the summer average,” Taberna said July 15. Evapotranspiration is water lost to the atmosphere from the plant and soil.

Heat stress severity depends on how hot it gets, the rate at which temperatures increase, the duration of extreme temperatures, and crop and soil water status, according to a Washington State University potato alert July 15. Temperatures at least 10 degrees above normal, which spike suddenly and persist under drought conditions, represent the worst-case scenario.

“What we’ve seen when it gets really hot is that the plants essentially close off so they don’t lose water,” said Rhett Spear, a University of Idaho researcher who studies new potato varieties. He is based at the university’s Aberdeen Research and Extension Center.

Temperatures at which the process starts differ by variety, Spear said.

As for variety development, “if we see something that pulls through the heat decently, we can use it” in future information about a variety’s characteristics, he said.