Garden Tips: Heat tough on plants not adapted to our summers

Two years of record heat is just too much. I am tired of seeing samples of crispy brown plants and being asked the cause of the problem. Quite simply, it has been hot and difficult for plants to survive, especially plants that are not well adapted to our climate.

Several weeks ago, I took a look at some local Norway and Colorado spruces that were turning brown and dying. While not always the best clue to the origins of a plant, these spruces’ common names do give us a hint that they may not be well suited to the hot, dry summer weather of the Mid-Columbia. Norway spruce is native to northern and central Europe and prefers a cool, moist, humid climate. Colorado spruce, native to the central and southern Rocky Mountains, is also best adapted to a cool, humid climate.

In their publication on Spruce Problems, the University of Illinois says spruces as a group “prefer locations with acidic, well drained soils” and that “spruces are not well adapted to hot, dry locations and often suffer when planted in the warmer regions of the U.S.” It also discusses heat injury of spruces, saying that high temperatures can cause damage, especially when the high temperatures are preceded by cool weather. Heat damage on spruces shows as browning and dropping of new needles, leaving dead branch tips. After two or three years of this, a spruce is pretty much dead.

Just because spruces are not well suited to our climate does not mean that you ca not find healthy spruces growing locally. However, our climate does stress them and makes them more vulnerable to attack by pests, like spruce mite, bud scale and needle miner. I did find spruce bud scale on one of the browning Norway spruces, but the infestation was not severe enough to kill the tree. At least we can be thankful that our dry climate keeps fungal diseases from being a problem here.

Flowering dogwood is another tree that suffers when faced with hot, dry, windy summer weather. In its native habitat, it is an under-story tree that grows in the filtered shade of other forest trees. It grows best in a moist, well-drained, slightly acidic soil that is high in organic matter.

Dogwoods become stressed when planted in full sun and subjected to high temperatures, reflected heat and light from pavement and structures, wind, compacted soil, and poor watering practices. This stress shows up as curled leaves and crispy brown leaf edges. Nevertheless, many of these trees still put on a beautiful display of flowers each spring. To help dogwoods better cope with mid-Columbia conditions, plant them where they will have filtered shade and will not be subjected to a southern-western exposure or drying winds.

Dogwoods and spruce have very shallow root systems, making them more subject to drought and heat stress. It is advisable to keep the roots cooler and moist by mulching the roots with wood chip or shredded bark mulch. Apply a 3 to 4 inch layer of mulch all the way out to the tree’s dripline and beyond. Keep the soil moderately moist, but avoid excessive soil moisture. Be sure to water the spruces during the winter if the weather stays dry and mild.

This story was originally published September 13, 2015 at 4:47 AM with the headline "Garden Tips: Heat tough on plants not adapted to our summers."