Early spring weather sped up plant, tree and shrub growth, making them vulnerable to several nights of cold temperatures this week. Fruit plants and trees were especially hard hit by freezing temperatures. Iowa State University Extension and Outreach horticulture specialists have received many questions from gardeners concerned about damage to plants and trees. To have additional questions answered, contact the Hortline at 515-294-3108 or firstname.lastname@example.org.
Rhubarb is a tough plant. Temperatures in the upper 20s or low 30s usually cause little or no damage. A hard freeze (temperatures in the mid-20s or lower) usually is 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 two or three days after the freeze event. Damaged rhubarb stalks (blackened foliage and limp stalks) should be pulled and discarded. New stalks that emerge after the freeze are safe to harvest.
The freezing temperatures may have damaged the flowers on apples and other fruit trees. If the flowers have been damaged, this year’s fruit crop may be much smaller than normal. The extent of damage is determined by the stage of plant development and temperature. At the bloom or petal drop stage of development, a temperature of 28 F will kill approximately 10 percent of the flowers/fruits on apple trees. A temperature of 25 F will kill approximately 90 percent of the flowers/fruits on apple trees at the bloom or petal drop stage of development. Similar losses would occur on pear and tart (sour) cherry trees at 28 and 25 F at the bloom and petal drop stages of development.
Newly emerged tree and shrub foliage is susceptible to damage from below freezing temperatures. The new growth on many trees and shrubs can tolerate temperatures in the low 30s and upper 20s. Freeze damage is most likely when temperatures drop into the middle 20s or below. Symptoms of freeze damage include shriveling and browning or blackening of damaged tissue. Damaged growth usually becomes limp. Eventually, damaged or destroyed leaves drop from the tree or shrub.
Fortunately, trees and shrubs have the ability to leaf out again if the initial growth is damaged or destroyed. Damaged trees and shrubs have only suffered a temporary setback. Healthy, well established trees and shrubs will produce additional growth within a few weeks. Good care during the remainder of the year, such as watering during dry periods, should aid the recovery of trees and shrubs planted within the past two or three years.
PHOTO: Freeze damage to apple tree