Washington State University Extension

Garden Tips

The Results of Last Year’s Abnormal Weather

GARDEN TIPS – written by
Marianne C. Ophardt
WSU Extension Faculty
for the Tri-City Herald, Kennewick, WA Written May 21, 2015

I wondered what the repercussions would be from our record breaking hottest-summer-on-record last year. Did you know it reached a high of 109 degrees in mid-July?  I knew this heat strained both us and our electric bills, but it also stressed our landscape plants.

One reason that many of our landscape plants are stressed by high temperatures is because they are not well adapted to our climate, growing better in more temperate regions with higher humidity, like conditions found west of the Cascades. Even in normal summers, these plants are subjected to stress, but the high temperatures experienced here last summer were even more burdensome. At the time I worried, knowing that extremely stressed plants are more vulnerable to attack by insect pests and winter injury from cold temperatures.

In addition to heat stress, many trees and shrubs also likely experienced drought stress last summer because the majority of tree owners habitually fail to provide them with adequate water. They rely on lawn watering for irrigating their large trees and shrubs. As I noted last week, large trees and shrubs should receive additional water with deep watering and moistening the soil to a depth of 12 to 18 inches at least once a week during hot weather. With the extremely high temperatures of last summer, the water needs of these plants were even greater.

The severe heat of last summer was followed by an extraordinarily mild, warm October. Nice for us, but woody plants are cued into the oncoming winter by lengthening nights (shortening days) and gradually cooler weather. The extended warm early fall weather delayed plants from “acclimating” or going through the physiological changes that make them resistant to damage from cold temperatures.

This might not have been a significant problem if temperatures had cooled gradually in late fall, but in the middle of November temperatures suddenly dropped from temperatures in the 60s and 70 into the teens. It was obvious that many trees and shrubs were not prepared for severe cold temperatures, because the leaves were frozen on the trees instead of going through the normal process of leaf fall. Because many plants were not fully acclimated, making them ready for winter temperatures, the severe freeze had the potential to damage buds, twigs, and branches.

So what have been the results of this extraordinary sequence of climatic abnormalities?  Here is a list of the plant problems I have seen so far:

– Flowering cherries that were healthy last year failing to produce flowers or leaves is attributed to cold temperature damage and heat stress.

– Dieback on a number of arborvitae is being caused by the flatheaded cedar borer that attacks heat and drought stressed arborvitae, juniper, and cedar.

– Excessive needle drop, dieback and death of mature pines and other needled evergreens is being noted. It looks like many of these trees were already compromised and on the edge of fatal drought stress from shallow watering practices, past winter droughts, compacted soil, restricted roots, or other factors that restricted water uptake. Last summer’s heat appears to have pushed a number of them over the edge.

In addition to twig dieback, some woody plants have been slow to leaf out and have undersized leaves. As the weather warms, I suspect we will see more plants start to fail. All we can do now is provide the plants with the best growing conditions possible, watering correctly, pruning out obviously dead tissues and hoping for a cooler summer and enough water to keep our plants alive.

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