Cedar hedge winter burn in St. Norbert
There are some important considerations to think about while preparing coniferous evergreen trees and shrubs for winter. Evergreen needles and leaves (in cedars, yews and junipers) are thick and often waxy to reduce water loss.
Very small openings called stomata on the lower side of the needles and leaves allow water to evaporate during the growing season, but in preparation for winter, stomata close up to prevent water loss and freezing injury. The needles of pine, spruce and fir normally do not freeze because of their resin content which acts like anti-freeze.
Although coniferous evergreen needles and leaves stop photosynthesizing, their cell chloroplasts (where photosynthesis takes place) are not killed as they are in deciduous leaves. That is why evergreen needles and leaves in winter are grayish-green and not the vibrant green or blue-green colour of summer. Coniferous evergreen plants naturally shed needles and leaves as a result of getting older each year. On the Prairies needle and leaf shedding are usually hastened by pests and diseases.
Dormancy, which starts in the fall, in all trees and shrubs stimulates a process called 'hardening of the tissues' which is a change in the living conducting cells in the twigs, branches and trunk to prepare the plant for the extremes in cold temperatures. Liquids in living cells, called protoplasm, become more concentrated with a mixture of sugars and salts so that it freezes at a much lower temperature. If the temperatures in the cells do become very cold, the viscous liquid changes to almost spear shaped crystals that prevent the cells from splitting apart. The cells' walls also assist by developing a greater ability to change as they adapt to the new crystals.
Coniferous trees and shrubs have the greatest problem with hardiness as their evergreen needles and leaves stay on the tree continuously for up to five years if they are healthy. Winter burning of leaves in younger cedars and junipers on the Prairies is very common because many of these plants have not been able to adapt to critical winter conditions. Often these plants are grown in warmer areas of North America which do not experience the extremes of winter cold like our area. Winter burning is caused by dehydration. The needles and leaves dry out as the sun gets warmer and higher in March and their roots simply cannot replace the water lost to dehydration. If there is no water absorption then there surely will be no oxygen absorption from the soil pores. The leaves die of thirst and suffocate so to speak. The very small rootlets want to absorb (very slowly) molecular water and air from the surface of clay soil particles even in the dead of winter. It is very important to water all coniferous evergreen plants right now this fall so that soil moisture is available later in the winter. Although there has been a lot of summer rainfall, the fall has been very dry. The roots of smaller coniferous evergreens especially need extra water before the ground freezes.
Wrapping evergreen plants with burlap is not advisable as the burlap traps late winter heat and causes a further increase in dehydration. Before the ground freezes place several firm wood stakes in the ground on which to attach the burlap as screens. The screens should be somewhat higher than the plant. Place one screen on the south side and one screen on the south-west side. Make these screens continuous leaving no gap. Keep the burlap at least one foot away from the needles and evergreen leaves. In the long run it is best to purchase the hardiest plants that you can from well established reputable nurseries.
Michael Allen is a consulting urban forester, certified ISA arborist and owner of ViburnumTree Experts. He provides advisory services on tree and shrub problems through homevisits. He also gives many tree care and pruning courses. He can be contacted by calling 831-6503 or emailed at viburnumtrees@shaw.ca. You can also mail questions to Michael Allen, c/o Newsroom, Winnipeg Free Press, 1355 Mountain Ave., Winnipeg, MB, R2X 3B6. His website is: www.treeexperts.mb.ca
