The cold temperatures last week in Iowa may have damaged some of the blossoms (ovaries) on fruit trees. The extent of damage depended upon the stage of flower bud development and minimum temperature at the site. Flower buds become less resistant to cold injury as they progress from the silver bud stage to full bloom. Many of the fruit trees in central Iowa were in full bloom last week. At the full bloom stage, previous scientific studies have found that only 10% of apple blossoms are killed when the temperature drops to 28oF. If the temperature drops to 25oF, 90% of the flowers on an apple tree in full bloom are killed. The degree of damage to pears, cherries, and plums would be similar. Since low temperatures across most areas in Iowa were in the upper 20's and low 30's, the damage to the fruit crop may be relatively minor. Home gardeners may still have a good crop if weather conditions are favorable the remainder of the spring and summer. (Only 20% of the blossoms on a heavily blooming apple tree need to develop into fruit for a good crop.) The greatest damage to the fruit crop likely occurred in northwest Iowa where temperatures dropped into the middle 20's.
Additional information on the effect of cold temperatures on the buds and blossoms of fruit trees can be found in FG-151, "Critical Temperatures for Fruit Flower Buds." Copies can be obtained from the Horticulture Department at Iowa State University.
This article originally appeared in the May 4, 1994 issue, p. 59.