green beans

HERSHEY, Pa. — Farmers in the Delmarva region are used to heat stress affecting their snap beans. Some Pennsylvania farmers are just far enough north to avoid the problem, but those in the southeastern part of the state could see the same issues.

A nighttime temperature of 68 degrees or higher causes heat stress in beans, said Emmalea Ernest, a vegetable and fruit educator with University of Delaware Extension.

Ernest presented her snap bean heat stress trial research at the Mid-Atlantic Fruit and Vegetable Convention on Jan. 31 at the Hershey Lodge.

Heat stress drastically reduces snap bean yields, Ernest said. The beans are susceptible to heat stress 10 days before the plants flower.

These hot nights cause anther indehiscence. In other words, Ernest said, the heat stops the anthers from opening and subsequently they won’t release pollen.

Even if pollen is released, the quality can be reduced from the heat.

“It can make the pollen inviable,” Ernest said. “So it won’t do its job. Even if you get pollen landing on the stigma, you don’t get pollination.”

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Emmalea Ernest, a fruit and vegetable educator with University of Delaware Extension, presents at the Mid-Atlantic Fruit and Vegetable Convention in Hershey, Pa. on Jan. 31, 2023.

This can cause a variety of problems, Ernest said, including fewer seeds per pod, misshapen pods, short pods, pod abortion and delayed harvest.

Farmers do have some options to try to avoid the yield problems associated with heat stress.

Through four years’ worth of trials, Ernest has found that certain varieties stand up better to heat than others.

Though the Caprice seed variety is the standard, it didn’t perform well when under heat stress in any of Ernest’s trials.

“Caprice is the standard, but you’re going to see it doesn’t look particularly great,” she said. “We keep growing it though. Somebody please tell me why?”

After compiling data from her four years of trials, Ernest reported four seed varieties that consistently performed well, even when stressed from heat.

Bridger, a variety from HM Clause, and PV857, a variety from Crites Seeds, both showed high yields, despite heat stress. Additionally, Jaguar from Crites Seeds, and Byrd, a new variety from HM Clause, also yielded high in the trials.

Though heat stress can be somewhat combated through variety, Ernest said a lot of Delmarva growers have decided to plant later to avoid the hot summer nights.

In Delmarva, growers can plant snap beans from April to August, and then harvest July to October.

But growers who plant in the middle to end of June will likely see heat stress, Ernest said.

“You can overcome some of that with variety selection,” she said. “Choose heat-tolerant varieties for plantings that are likely to experience heat stress.”

Other than choosing heat-tolerant varieties or changing up the planting times, there’s not much growers can do if the nighttime temperatures are too high.

The buds and flowers are the parts of the plant that experience heat stress, Ernest said, so if the air temperature is hot, it will have an effect.

Irrigating the plants or mulching the soil won’t change the impact of the air temperature on the flowers.

“You can’t make up for that temperature impact with water,” Ernest said. “There’s really nothing you can do except select varieties that are heat-tolerant.”