Yard and Garden: Freezing Spring Temperatures Can Affect Landscape Plants

Cool nighttime temperatures can pose a risk to landscape plants, but they may recover

April 23, 2020, 10:47 am | Richard Jauron

AMES, Iowa -- In Iowa, the weather in spring can be quite erratic. Recently, nighttime temperatures were well below freezing. This brings up an important question about plant survivability, according to horticulture specialists with Iowa State University Extension and Outreach.

How do plants cope when winter makes a late visit?

The newly emerged growth of most perennials can tolerate temperatures in the low 30s and upper 20s Fahrenheit. Freeze damage may occur if temperatures drop into the lower 20s or teens. Most well-established perennials that have been damaged by freezing temperatures should survive. The roots and crowns of severely damaged perennials should send up a second flush of growth in a few weeks. Good care (for example, watering weekly during dry weather) this spring and summer should help damaged perennials recover.

The developing leaves on many trees and shrubs can also tolerate temperatures in the low 30s and upper 20s. Freeze damage is most likely when temperatures drop into the low 20s or teens. 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.

rhubarb plant in snow.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.

However, flowering trees and shrubs may not bloom as well in spring, as the cold temperatures may have damaged or destroyed some of their flower buds.

The recent freezing temperatures may have damaged the developing flower buds on apples, cherries and other fruit trees. Damage is most likely in the southern portion of the state. If flower buds have been damaged, the fruit crop may be smaller than normal. The extent of damage is determined by the plant species, cultivar, stage of flower bud development, and temperature. For example, at the tight cluster stage on apples, a temperature of 27 F will kill approximately 10% of the developing flower buds while a temperature of 21 F will kill approximately 90% of the flower buds.

Is rhubarb still edible?

After freezing temperatures, some gardeners express concerns about the edibility of rhubarb. Rhubarb is a tough plant. Temperatures in the upper 20s or low 30s typically cause no damage. Temperatures in the low 20s or teens may cause visible damage. Rhubarb damaged by freezing temperatures will have black, shriveled leaves and soft, limp leaf stalks. It is safe to harvest rhubarb if the plants show no signs of damage three or four 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.

To have additional questions answered, contact the Hortline at 515-294-3108 or hortline@iastate.edu.

Photo Credit: Rhubarb plant in snow by JGade/stock.adobe.com

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