By James Romer
Iowa State University Extension
Just when you thought spring was here to stay, old man winter strikes again. Forsythia, daffodils and magnolias were in bloom. The grass was vibrant green. Everything was shaping up to be a grand show of nature’s beauty. But winter’s last stand in the form of freezing temperatures and snow has spoiled the show. The mailroom has been flooded with letters of concern from gardeners wondering if all hope is lost for their esteemed trees, shrubs, and perennials. Have no fear; Dr. Grow-It-All is here with probable outcomes for your treasured plants!
Plant species differ in their susceptibility to freeze damage. As a general rule, the farther along the plants are in their development, the more likely they are to be damaged by below-freezing temperatures. The lower the temperature, the greater the damage. High winds accompanying low temperatures cause tender plant tissue to dry because the plant is losing water faster than it can replace it.
Early spring flowering trees and shrubs are most likely to have some flower damage, since some were starting to flower or breaking bud. Depending on the conditions mentioned above, the damage may be brown edges along the petals, or they may fail to bloom. Foliage buds generally are considered to be more resistant to cold damage.
The leaves on those trees and shrubs that had already begun to leaf out may shrivel and turn black. Fortunately this is only a temporary setback. The damaged trees and shrubs will leaf out again within a few weeks. Trees and shrubs that were progressing more slowly (plants that were still in the bud stage) may also exhibit damage symptoms when they leaf out. Possible symptoms include distorted or misshapen leaves and leaves with brown margins.
Hardiness is also affected by the return of warmer temperatures. A few days of warm weather in mid or late winter reduces plant cold hardiness significantly. Once cold hardiness is lost from mid or late winter warming, the plant cannot return to the same level of hardiness. If mild winter temperatures prevailed, damage would have been unlikely.
Many spring-flowering bulbs, such as daffodils and tulips, were also damaged by temperatures in the teens and low twenties. Tulips and daffodils can survive freezing temperatures in the upper twenties without much damage, but colder temperatures can damage the plant’s flowers and foliage. The record cold temperatures may have severely damaged or destroyed open flowers. Freeze damaged foliage turns whitish and becomes limp. Although tempting, do not cut damaged foliage back. Cut foliage back when it dies on its own. While this year’s flower display may be poor, the bulb’s foliage needs to manufacture food for the bulbs in order to bloom next spring.
The record cold temperatures also damaged bleeding heart, daylily, astilbe, hosta and other emerging perennials. While the freezing temperatures may have damaged or destroyed the emerging new growth, the roots and crowns of established perennials should still be alive. Perennials planted within the last year are most at risk of serious damage. The damaged perennials should send up a second flush of growth in a few weeks. Good care this spring and summer should help the perennials recover.
Many gardeners ask if they should cover their plants to protect them from the cold. Blankets, sheets or tarps may be able to keep the plants a few degrees of warmer. However, when temperatures drop into the teens, covers are unlikely to prevent damage. Snow is an effective insulator, but if it is heavy and wet, the weight of the snow can break tender vegetation.
It is important to remember that this is not at all unusual for Iowa. Few plants should suffer more than a temporary setback. Some foliage may look tough and we may lose some of spring’s beautiful flower show, but the plants should recover. We often get a mid-winter warm spell followed by a return to normal freezing temperatures. Remember, our frost-free date in much of the state is not until early to mid-May.