By Jean-Mathieu Daoust
Owner of Tree Frog Tree Care Inc.
Some of you may have noticed in your yard, or in your travels throughout Calgary, that spruce and pine trees with browning needles sometimes display a bit of a purple hue. This discolouration of needles is due to winter injury, either by desiccation or extreme cold. It is considered an abiotic plant disorder which means it’s not caused by a pest such as an insect or plant disease, therefore chemical control products can not be the solution to the problem. We have seen this type of damage on pine, spruce and fir, although the most susceptible tree appears to be the Colorado blue spruce. Weak trees are most susceptible to winter kill. Factors that can weaken trees include inadequate watering, or improper hardening off from weather related responses, or high nitrogen fertilization in the fall.
Dry hot summer weather causes drought stress, and mild fall temperatures late into the year delay the trees natural tendency to harden off for winter. Sudden extreme cold can have a freeze drying effect on conifer needles causing them to begin discolouring in late winter and early spring. There are many factors that come into play which makes this problem a little different to diagnose. Some trees are more resistant than others, depending on their growing conditions, and weakened trees are more likely to see this type of damage on an annual basis depending on the weather and seasonal changes.
Thankfully most trees will recover quite well from this damage, the buds are more resistant and will push out new growth in the spring and summer. We recommend a ‘wait-and-see’ approach for deciding what to prune out to reduce the chances of removing branches with buds that are still viable. Unfortunately the needles that have turned colour will remain that way and eventually fall off. The best thing you can do is nurture the new growth by providing the tree with water during dry periods. Fertilizer can help if applied according to current soil nutrient availability.
You can also help reduce stress to the tree by carefully creating a shallow well around the tree to avoid potential damage to the roots growing near the surface. Creating a well and adding mulch to it significantly reduces competition for the tree’s access to water. If feasible washing dust and pollution off with water will help the remaining healthy needles breathe and photosynthesize better, allowing the tree to produce more energy to direct toward growth and its stress response and recovery mechanisms. Removing dry and fully dead branches will increase light penetration through the canopy, which will inherently increase photosynthesis. Be careful when pruning and retain any flexible branches as they are probably still alive with viable buds at the tips – even if needles are purple or brown. If your tree has repeated damage, or was significantly damaged by browning, it may require removal or take several years to recover. We still recommend either having it looked at by a certified arborist or tree expert if this is the case. If you plan on doing any pruning yourself, wait until later in the summer before deciding what and where to cut.