Reduced light intensity and declining air temperatures at the beginning of the fall season causes green leaves of most deciduous trees (leaf-dropping trees) to turn colour and eventually drop from the tree. Most people are unaware that during the summer months these trees are preparing for winter by producing over-wintering buds. These buds will open in the following spring to produce new leaves, twigs and flowering parts. Gardeners refer to this process as "budding out." These buds typically occur under the parts of the leaves that are attached to the woody twigs. Remember, all tree buds are produced during the previous summer.
Green leaves, before they change colour, send all of their sugars and starches produced by photosynthesis to storage cells in the sapwood of branches and trunks, and in the roots. The high sugar content, as well as other by products, keep those cells from freezing as long as those woody plants are hardy to our winters. The bark provides a protective layer for those cells.
Below the bark is an important region called the cambium layer. The specialized cells of the cambium layer are important for two reasons: They are involved in the production of new wood on the inner side of the layer (xylem cells), and the production of specialized cells on the outer side (phloem cells). Those specialized cells allow the transport of sugars and other carbohydrates produced by the leaves to be stored in other living cells of the tree, especially the roots. Cork-like tissue in the bark gets thicker with each passing year, and acts as an insulation barrier to the penetration of severely cold temperatures.
Coniferous evergreen trees such as spruce, pine, fir, cedar and juniper should not drop all of their needles before or during winter. They prepare for winter differently. Evergreen needles and leaves (or bracts) are thick and usually waxy to reduce water loss. Very small openings called stomata on the lower side of the needles allow water to evaporate during the growing season, but in preparation for winter, stomata close up to prevent water loss and injury from freezing. Needles normally do not freeze, because their resin content acts like antifreeze. On the prairies, coniferous needles and bracts stop photosynthesizing later in the fall. This is why evergreen needles and leaves in winter often look greyish-green rather than the vibrant green or blue-green colour of summer. Evergreen needles and bract-like leaves do not persist for more than five to six years on average. They will typically turn rusty brown as they die and fall from the tree.
As I have mentioned many times before, use a burlap screen placed on the south and southwest side of smaller evergreen trees and shrubs before the ground freezes hard. This will help prevent needle browning from winter burn.
Although it is too late in the season right now, trees and woody shrubs can be fertilized in the early fall. They should also be fertilized the following spring. The nutrient elements such as slow release nitrogen, phosphorus, potassium and iron compounds along with a number of micronutrients in tree fertilizers are essential for healthy tree growth especially for coniferous evergreen trees 'forced' to grow on Red River valley soils.
Michael Allen M.Sc.F., RPF (ret.) is a consulting urban forester, tree diagnostician and certified arborist. He owns Viburnum Tree Experts. He can be reached at 204-831-6503 or 204-223-7709 viburnumtrees@shaw.ca His web site is www.treeexperts.mb.ca
Michael's new book, Dr. Tree's Guide to the Common Diseases of Urban Prairie Trees is available from the author or from McNally Robinson, Chapters book stores and from many local garden centres.
