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Does your cedar have a sunburn?
By Melanie Mathieson
As humans we often suffer from sunburn during the summer months. Did you know that trees and shrubs are susceptible to sunburn too? Sunburn on most woody plants usually occurs during the winter, although your garden vegetables and tender flowering plants can suffer from sunburn as well.
The winter season can be especially tough on evergreen shrubs and trees, particularly cedars. You may notice that the sides of your evergreen shrubs that face east, south and west may have brown foliage. This is browning is an indicator of winter occurring damage called sunscald and/or windburn. This past winter had a very low amount of snow cover for most of the winter. Snow is actually beneficial for the winter survival of shrubs and perennials in this region. We also had a fair number of warm days with a lot of sun followed by prolonged severe cold weather. Many days had a significant wind as well. The combination of these factors made for very damaging conditions for your shrubs.
During the fall an evergreen tree prepares for winter by going through a process called hardening off. The tree shuts down its growth mechanisms and slows down its rate of transpiration and consumption of nutrients. This process is obvious in deciduous trees as they lose their leaves in the fall. This is less obvious with evergreen trees and shrubs as they retain their leaves throughout the winter. But rest assured the evergreen trees in this region do go through the hardening off process. Winter damage occurs on trees and shrubs such as cedar, during days when the sun provides continuous heat throughout the day. The cells of the foliage of the plant thaw out during the day, even if the air temperature is below freezing. When night comes and the temperature falls, the cells refreeze. This causes the cells in the leaves to burst and eventually die off. This creates the dead foliage you see in the spring. This process can be magnified if the sun reflects off of the snow or the windows of your house, creating an intensified heat, accelerating the death of the foliage. Wind can also cause damage to plants like these as well. The continuous drying cold winds of winter can evaporate any moisture the tree has preserved for the winter months, from its leaves. Over time the foliage will dry out and wither also causing areas of brown and dead foliage.
In winter seasons, when the snow is deep this damage often occurs above the snow line. In winters with little snow, the damage can occur over the full length of the shrub. Trees and shrubs bought at out local nurseries are winter hardy for this area, but we cannot always predict what the winter season may bring. Sometimes we need to assist our shrubs and trees in order for them to maintain their aesthetic quality. To prevent sunburn in future winters I recommend constructing a barrier around the susceptible sides of the tree. You need not wrap it tightly in burlap just create a sun screen by constructing a burlap fence around the shrub. For a cedar hedge you will have to run your burlap fence the length of the hedge on the side that sustains damage. If you have trouble constructing a protection screen then you could wrap the entire tree/shrub. Small evergreen shrubs such a juniper, yew and cedar can be mounded completely with snow throughout the winter. If you are unsure of the amount of snow you will have available in your yard, a coverage of loose leaves will offer protection.
In order to correct the damage from last season you can trim out the dead leaves from the cedar, if the damage is not too extensive. Do not completely trim the damage out if it will create bald spots, or make the shape of the tree lopsided. If this is the case leave the damaged foliage on the trees and see if time will allow for new foliage to develop and replace the damaged foliage. If the evergreen damaged is a juniper or yew, for instance, you can prune the branches as needed to remove the damage. Once you have the tree cleaned up ( June to mid-July is the best time for evergreen trimming) you can apply a fertilizer to the ground around the tree or shrub. Purchase a good quality fertilizer for evergreen trees (this is a fertilizer with a higher nitrogen content). Follow the instructions carefully and reapply throughout the growing seasons as per the recommendations on your fertilizer product. As with your perennial garden, you would prepare the trees and shrubs for winter by making sure the ground is very moist. If the fall season is dry, you may have to water your trees and shrubs with your hose to ensure the ground is saturated with moisture prior to freeze up. By doing this you will ensure that the roots are embedded in the frozen ground for the entire winter and will prevent frost heaving. It is also important that the roots stay frozen completely throughout he winter to prevent damage to the roots from continuous thawing and freezing.
Following these tips will help to ensure that your cedars and other evergreens will look their best summer and winter for many years to come.