Bean blossom drop can occur when temperatures stay consistently over 90 degrees and/or if the plants are stressed. There’s not much you can do about the heat except wait it out. It may help to put a breezy row cover over them to provide some shade. The plants may start producing again when the weather cools down. Blossoms can also drop off if the plants are stressed from drought, wind, or low humidity.
In addition to green beans, tomatoes, peppers, and squash , too, can suffer from blossom drop once the temperatures exceed 85 degrees.
“High temperature interferes with pollination, resulting in blossom drop, crooked or deformed pods due to the lack of ovule development”, writes Henry G. Taber, Iowa State University Extension Service vegetable specialist, in the article, “Green Bean Production.”
During days of extreme temperatures and little rain, gardeners should water bean plants since even a shortage of water can cause blossom drop. Garden experts are at odds over exactly what causes blossoms to drop. Some believe it’s due to fluctuations in moisture. Some think it’s caused by fluctuations between day and night temperatures. Some think it’s a combination of both. As an extra precaution, along with keeping the plants evenly watered and only watering at the roots, you can consider adding mulch to help keep soil temperature evenly regulated.