The vast majority of damage we see on spruce trees in Montana is not caused by diseases, but rather by environmental causes such as drought and freeze damage. The damage may not become apparent for many months, which often results in misdiagnosis. Symptoms are usually dieback that begins from the needle tips and moves back toward the branch, in a rather uniform pattern. Sometime whole branches or portions of the tree may be killed. Color varies from brown to yellow to reddish-brown.
Heavy fall moisture can result in flushes of growth susceptible to winter damage. In addition, warm periods during winter, including chinooks, can cause spruce needles to loose moisture through transpiration and because the ground is frozen, roots can not absorb more moisture from the soil. Drying out (desiccation) of tissues can result.
Trees that go into winter under dry conditions also account for much of the damage we see every year. Most tree roots, of even the tallest trees, are in the upper 18 inches of soil, meaning that trees can become drought-stressed much more quickly in both winter and summer than we would suppose. Without a deep watering that soaks into the ground at least two feet in late fall, just before the ground freezes, the tree is very susceptible to drying out, resulting in dead needles and branches the following spring or summer. Windy, sub-zero temperatures increase moisture lost through transpiration, making the problem even worse.
To help prevent dieback, stop watering all your trees, including spruce trees, after midsummer. Then give them all one final deep watering after deciduous trees have changed to autumn color, but before the ground freezes. Water the entire area under and around the tree half again beyond the dripline of the evergreen trees. For example, if the branches extend six feet from the tree trunk, water at least nine feet around the trunk. (Roots of deciduous trees extend 2-3 times farther than the branches, so water that extended area for those trees.) Allow the water to soak down a minimum of two feet. A good, deep watering again in spring, as soon as the ground thaws can provide the tree with important moisture it may have lost during winter. In years when fertilizer is needed by the tree, you may wish to apply a appropriate granular product, then follow with this deep fall watering.
Fertilizing trees can also result in similar problems, especially to the flush of new growth that results. In most parts of Montana, trees need no fertilizing. This is particularly true in turf areas that receive occasional applications of fertilizer. In areas where soil tests show nutrients ARE lacking, fertilizing only once every 3-4 years is usually sufficient. If you do not conduct a soil test, do not apply fertilizers unless trees are showing symptoms of nutrient deficiency. Avoid annual applications of tree fertilizer. When fertilizers are necessary, apply them in fall, before the leaves of deciduous tree turn autumn color, but before the ground freezes, then apply your deep fall watering to help carry the fertilizers into the root zone. Research has shown that in spring trees use nutrients they stored the autumn before; spring-applied fertilizers will not be absorbed until early summer, about the time deciduous trees are fully leafed. If you wish to make tree fertilizer applications in spring, wait until this time.
In areas with heavy clay soil or very shallow water tables, another cause of dieback can be soils saturated with water, leading to oxygen starvation of the roots. If needles three or more years old are dying, the problem is not too serious. If new growth is affected, chances of recovery are not as good. Deep sampling of soils with a soil probe in these areas can help determine if drainage improvement is needed.
To prevent dieback in areas where heavy soils are not a problem, stop watering spruce trees after midsummer, until one final deep watering after deciduous trees have changed to autumn color, but before the ground freezes. Water the entire area under and around the tree half again beyond the dripline of the evergreen trees. For example, if the branches extend six feet from the tree trunk, water at least nine feet around the trunk. Allow the water to soak down a minimum of two feet. A good, deep watering again in spring, as soon as the ground thaws can provide the tree with important moisture it may have lost during winter. In years when fertilizer is needed by the tree, you may wish to apply a appropriate granular product, then follow with this deep fall watering.
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Written by Sherry Lajeunesse, Extension Urban Pest Management Specialist. Sept., 1997