It’s easy to assume that all evergreen shrubs are tough as nails, since they don’t seem to change with the seasons. In fact, winter can be hard on evergreens.
“Keeping their leaves all winter actually makes evergreens vulnerable,” said Spencer Campbell, Plant Clinic manager at The Morton Arboretum in Lisle. Those needles can dry out in the cold and wind, resulting in brown, dead branches.
Broad-leafed evergreens such as boxwoods and some rhododendrons, whose leaves have more surface area to lose moisture than narrow needles do, are especially at risk of winter kill.
Unlike deciduous plants that drop their leaves in fall and enter a protective state of dormancy, evergreens never become completely dormant. “They are still a little bit active in winter, and that means they’re a little bit needy,” Campbell said.
Give your evergreens some attention in autumn to help them make it through until spring. You can learn more at mortonarb.org/winter-injury.
Continue to water. Water evergreen shrubs in fall until the soil freezes and can no longer absorb water. The more water the plants can store in their leaves, needles and branches, the better they can resist drying out. “Long, slow watering is best,” Campbell said. “Place a trickling hose on the soil under the shrub’s branches for an hour or more so the water really soaks into the soil.” A sprinkler may not work as well: The plant’s leaves or needles are likely to intercept much of that water so it never reaches the soil where it is needed.
Spread mulch. A layer of mulch over the plant’s roots will keep moisture in the soil and protect the roots against temperature fluctuations.
Hold off on pruning. Winter is a good time to prune a deciduous tree or shrub, but not an evergreen. Because the plant isn’t dormant, pruning in winter can stimulate new growth, prompting soft new needles and twigs that will easily be killed by the next hard freeze. “Wait to prune evergreens until spring,” he said.
Look for salt hazards. The salt used to melt ice on sidewalks, roads and driveways pulls moisture from plants’ tissues. It can dry out evergreens when meltwater soaks into the soil or when passing traffic kicks up salt spray that can reach plants far from the road. Salt damage is easy to spot on evergreen hedges when many branches turn brown and dead on the side facing a road or sidewalk. Minimize the use of ice-melting chemicals, and be careful where you dump snow when you shovel. “When that snow melts, any salt in it will soak into the soil and damage plant roots,” Campbell said.
Think about wind. An evergreen exposed to the prevailing wind will dry out more readily than one sheltered by a wind-breaking fence or a building. Use burlap as a shield. If there is a sensitive evergreen in a bad spot, such as an arborvitae hedge facing a road, consider erecting a barrier of burlap — a coarse, open-weave fabric — to intercept the salt or break the wind. You can wrap the plant loosely or stretch the fabric between poles to form a fence. “Take care to allow plenty of air circulation,” he said. “You don’t want to trap moisture, which can lead to disease.” For that reason, never use plastic to wrap a plant.
Consider more appropriate plants. You can avoid erecting protective structures by choosing a plant suited to a tough site. Juniper, for example, can tolerate salt spray better than arborvitae. The Arboretum’s plant search tool (mortonarb.orb/search-trees-and-plants) can help you find species that are appropriate for the site.
For tree and plant advice, contact the Plant Clinic at The Morton Arboretum (630-719-2424, mortonarb.org/plant-clinic, or plantclinic@mortonarb.org). Beth Botts is a staff writer at the Arboretum.