Preventing Winter Injury

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The cold spell we had in mid-November was a good wake-up call that winter is on its way.  The question is “Are your plants ready?”  In most cases, the answer is “yes.”  Plants that evolved in the mid-Atlantic are prepared for winter by dropping their leaves or dying back to the ground completely, in the case of herbaceous perennials.  Needle evergreens have evolved narrow leaves with coatings that prevent moisture loss in the winter to help them survive.  The real problem is with broad leaved evergreens.  In a recent talk at the Delaware Ornamentals and Turf Workshop in Hockessin, DE, Jason Veil, curatorial intern at the University of Delaware Botanic Gardens, pointed out that of the 50+ broadleaved evergreens we commonly plant in Delaware, only 4 are native.  Very few broadleaved evergreens have evolved to handle our weather.

When we think about winter injury, there are a few factors in play.  First is the genetics of the plant.  It is important to know where the plant evolved.  Red maples grow from Florida to Canada.  Seeds from Florida red maples may not be hardy in northern states.   Most cultivars are selected for their ornamental characteristics, but environmental characteristics, such as cold tolerance, moisture tolerance etc. are even more important for planting success.

With broad leaved evergreens, the most common winter injury is leaf scorch–dead areas on the leaves.  This is caused by water loss during the winter.  Moisture is lost from the leaves, the wind blows the moisture away from the leaf surface and the leaf loses more moisture. Wind can’t be changed, but you can locate plants to avoid prevailing winds when planting broadleaved evergreens.  Excess sunlight is also a problem.  The sun can warm up the leaf, tricking the plant into losing more water.  So, plant broadleaved areas in spots that are protected from the wind and winter sun.  Since moisture loss is the major cause of winter injury in broadleaved evergreens, fall rains are important to build soil moisture. When you complain about that cold, wet day; remember how good it is for our plants.  If we have a dry fall, it may be necessary to water tender or newly established plants during the fall.  Fortunately, this fall we’ve had plenty of rain.

In Delaware, plants acclimate to the cold weather by “hardening off” in response to shorter days and colder fall temperatures.  Acclimation is a slow, steady process.  After a 2-month chilling period has been satisfied, deacclimation can occur pretty quickly in spring.  If we get some unseasonably warm weather in late February, plants can break dormancy before our cold weather is really over.  We’ve all seen cherries or magnolias start to bloom early and then get damaged by normal cold weather that follows. Loss of flowering is a nuisance, but deaclimation can cause even more severe injury when we get a warm spell in late winter.

Not all plant parts have equal cold tolerance.  Stem tissue is the most tolerant part of the plant.  Vegetative buds are next because the plant needs these buds to grow.  Flower buds are less cold tolerant; we want flowers but they aren’t as necessary for plant survival.   Interestingly, while they are critical to plant survival, roots are least cold tolerant because they are normally buffered by the soil.  Plants in above-ground containers are not as cold hardy as that same plant would be in the ground.

For winter hardiness, avoid top growth and promote root growth in the late summer and fall.  Avoid pruning and fertilization that would push new, tender growth.  Avoid planting perennials in the fall when there isn’t enough growing season left for them to establish a healthy root system before they have to tolerate the alternate freezing and thawing that occurs during a Delaware winter.  Evergreens don’t grow roots in the fall, so it is best to wait until spring to plant evergreens.

Salt is another important factor in winter injury.  Plants close to salted roads or sidewalks get the double whammy of cold temperature and salt injury.  For those locations, pick salt tolerant plants, like groundsel bush, eastern red cedar, bayberry and any other plant that grows well at the beach.

So, what can you do to prevent or reduce winter injury?  Start by selecting cold tolerant or salt tolerant plants.  Place those plants to avoid winter sun and wind.  Consider protecting broadleaved evergreens by planting in shaded locations, such as the north side of a building. Avoid late season fertilization and pruning and fall planting of evergreens.  Irrigate during dry spells in fall or before cold snaps.  Apply mulch to young or tender plants to help protect roots   Consider placing evergreen boughs over small tender plants for winter protection.  Some people wrap their evergreens in burlap to protect them from winter injury.  I have to admit that doesn’t make sense to me.  If you planted an evergreen, you chose it because it stays green all winter, why would you cover it up?  It is best to select plants that tolerate our weather conditions.  If you “push the envelope” as many gardeners so, be prepared to replace tender plants after a tough winter.  Who knows what the winter of 2014/2015 will bring.  We can only wait and see.