I continue to receive many questions about the viability of plants that suffered severe damage from winter’s subzero blasts. Boxwood, English ivy, pachysandra, rhododendrons, a variety of needled evergreens, hydrangeas, roses and subshrubs such as butterfly bush all took a hit.
Severe winter weather causes plant damage in a number of ways – freezing, winter sun and wind, deicing salts and hungry animals. The condition of plants going into winter is an important factor; those that were well hydrated and healthy most likely fared better than those stressed by drought and/or insect and disease damage. Another consideration is the winter hardiness of affected plants. Plants rated hardy to USDA Zone 6 and even 7 have been creeping into our plant palettes, and the polar vortex was not at all to their liking. When Michael Dirr spoke in Pittsburgh in April, he mentioned that crape myrtles in Georgia were killed outright as the cold weather pushed deep into the southeastern United States.
The subzero temperatures we experienced over the winter can damage plants generally hardy in our climate. That may mean dead flower buds, vegetative buds, twigs, branches or even entire plants. Some may be slow to leaf out, while others may not bloom or may only bloom low on the plant where the flower buds were protected by snow cover. Some of the deciduous plants showing damage include flowering cherry, hydrangea, Japanese maple, redbud and rose.
Winter burn on broad-leaved and needled evergreens happens when sunny or windy winter weather causes plants to lose moisture from their leaves through transpiration. Since they cannot take up moisture from frozen soil to replace what they are losing, they become dehydrated. That results in the brown foliage we see in late winter and early spring. The severity of the damage can range from simple browning of leaves and needles to the death of the plant. If buds and twigs are still green, they should push out new growth and the affected plant will eventually recover. Some of the evergreens showing damage include arborvitae, boxwood, English ivy, holly, juniper, Oregon grapeholly, mountain laurel, pachysandra, rhododendron, Southern magnolia, white pine and yew.
Deicing salts cause damage by accumulating in the soil or by being splashed or drifting onto leaves, needles and twigs. Salt in the soil dehydrates the roots while salt spray on foliage draws moisture out of leaves and needles, resulting in dehydration damage similar to winter burn. Salt damage is more likely to occur on one side of a plant – the one facing a road — while winter burn may be more uniform on the plant. The abundant rain we have been having should help flush much of the salt from the soil, but additional irrigation may be necessary if new growth continues to show scorching.
Last, but far from least, animals such a voles and rabbits often strip bark from stems of shrubs and trees when other food sources are buried under snow and ice. If the damage impacts the plant’s vascular system, affected stems and even entire plants can die. Deer browsing was extremely severe in areas where their populations are high. While affected plants may not die, they may take a long time to regain their ornamental appeal.
Even though spring has arrived, we may not have seen the last of the damage from the polar vortex. Some plants that were damaged by subfreezing temperatures may leaf out and seem to be fine until they are stressed by hot, dry summer weather. Then you may notice branch dieback or the plant may die back to the snow line. Plants that have been slow to leaf out should be given a little TLC through the growing season. Do not be in a hurry to fertilize injured plants because they may not be able to support the extra-succulent new growth that results. Make sure to provide supplemental water to them when we get into hot, dry summer weather.
I continue to counsel patience when it comes to removing winter-damaged plants. I thought one of my expensive tree peonies succumbed to the polar vortex, as it looked completely dead while the other tree peony leafed out normally and is now in full bloom. I was planning to dig it out when I noticed that it was finally breaking new growth. Wait until you see new growth from the base of plants, such as English ivy and pachysandra, before trimming out the winter damage.
Send questions to Sandy Feather by e-mail at email@example.com or by regular mail c/o Penn State Extension, 400 N. Lexington Ave., Pittsburgh 15208.