In Iowa, spring weather is often erratic. The weather in 2010 has been no exception. Many areas in northern and central Iowa saw temperatures in the upper twenties or low thirties early on the morning of May 9th. Most perennials, trees, and shrubs likely suffered little or no damage. However, the freezing temperatures may have damaged some vegetables and annuals. Fruit trees may have also suffered damage.
The freezing temperatures should not have damaged most cool season vegetables, such as cabbage, lettuce, and spinach.
Freezing temperatures will damage potato shoots. Symptoms may vary from blackening of the leaf margins (minor damage) to death of all aboveground growth (severe damage). Fortunately, severely damaged potatoes will send up new growth within 10 to 14 days. There is no need to replant the potatoes.
After freezing temperatures, some gardeners express concerns about the edibility of rhubarb. Rhubarb is a tough plant. Temperatures in the upper twenties or low thirties usually cause little or no damage. A hard freeze (temperatures in the mid-twenties or lower) is usually required to cause serious damage. Rhubarb damaged by freezing temperatures will have black, shriveled leaves and soft, limp leaf stalks. It's safe to harvest rhubarb if the plants show no signs of damage 2 or 3 days after the freeze event. Damaged rhubarb (blackened foliage and limp stalks) should be pulled and discarded. New stalks that emerge after the freeze are safe to harvest.
Warm season vegetables, such as tomatoes, peppers, and snap beans, do not tolerate freezing temperatures. Plants that have suffered minor damage should recover within a few weeks. Replant those warm season vegetables that have suffered major damage.
Cool season annuals, such as pansies and sweet alyssum, should not have been damaged by the freezing temperatures. However, most annuals are warm season plants. Impatiens, petunias, marigolds, wax begonias, and other warm season annuals that have suffered major damage will need to be replanted.
The freezing temperatures may have damaged the blossoms or developing fruit on apples, cherries, and other fruit trees. As a result, this year's fruit crop may be smaller than normal. The extent of damage will be determined by the plant species, stage of plant development, and temperature. At petal drop on apples, a temperature of 28 degrees Fahrenheit will kill approximately 10 percent of the developing fruit while a temperature of 25 degrees will kill approximately 90 percent of the developing fruit.