A number of people have called me recently about the premature yellowing of tree leaves and the subsequent drop of many hundreds of those leaves. The trees that are the worst culprits are hybrid poplars including Swedish aspens, Amur maple and silver maple. These trees tend to have shallow root systems which are readily stressed during periods of adverse environment growing conditions. To a limited extent Manitoba maple could be added to that list.
These trees can have some difficulty in extracting nutrient energy sources such as nitrates, iron chelates and phosphoric acid salts from alkaline soils which dominate the urban regions of the Red River valley. Hot, dry weather causes a deficit in the water relationships between microscopic soil water, essential nutrients and the ability of the roots to absorb these ingredients during periods of hot, dry weather. These deficits place a strain on the chlorophyll bearing elements in the leaves causing the chloroplast cells to break down. These cells are responsible in great part for the green colour of the leaves.
Hybrid poplar leaves will often go from total smooth green leaf to a total bright smooth yellow leaf. On occasion the twigs and leaf stalks of these poplars become infested with insect pests such as the poplar bud gall mite and the poplar aphid petiole galls. These insects are very common on summer stressed poplars.
Another common feature of trees whose leaves turn yellow and fall during the summer is the obvious damage that has occurred to the roots. Long lengths of roots in older trees can often be exposed as the soil shrinks around them. Lawn mowers and weed eaters often damage exposed the bark of the roots causing damage. Wood decay and fungal diseases gets into those damaged roots causing the spread of disease and wood decay throughout the tree. Excessively damaged roots cause trees to become weak and subject to wind blow over.
Other trees that can turn colour during the summer are silver maple, Manitoba maple and Amur maple. Maples like to grow in slightly acid soils except our native Manitoba maple which will grow in just about any soil that has not been contaminated with products destined to kill plants. The leaves of these trees will often show a colour pattern indicating severe nutrient stress (especially iron oxides and nitrates) which in the tree business is called chlorosis. The leaves are often yellow green with normally darker green veins. A program of fall fertilization to supplement the nutrient deficient salts is strongly recommended for the fall for at least three consecutive years. Spring season fertilizing is also recommended. Never fertilize trees during the late spring, summer and early fall periods.
Know how to take care of your trees and enjoy them.
Michael Allen M.Sc.F., RPF (ret'd) 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.
