Washington State University Extension

Garden Tips

Local Evergreens Turning Brown and Dying

GARDEN TIPS – written by
Marianne C. Ophardt
WSU Extension Faculty
for the Tri-City Herald, Kennewick, WA Written June 11, 2015

Several weeks ago I noted that a number of our local landscape plants were showing signs of problems related to last year’s hot summer and other extraordinary weather events. Now hot weather has arrived much too early and local pines, arborvitae, juniper, and other evergreens are “dropping like flies” causing shock and dismay. Both old mature trees planted 20 years ago or more and younger ones planted less than five years ago are turning brown and dying.

There are a number of factors that have led to this widespread browning and dieback of local evergreens. One is the watering habit of many tree owners. Despite me frequently urging owners to deep water trees and shrubs weekly during hot weather, few actually do this. Instead they rely on lawn watering during the growing season to meet the needs of their trees. This is typically very shallow watering and has subjected a number of trees to years of chronic drought stress. The cumulative effect of this stress has weakened the trees and made them more vulnerable to the weather extremes we experienced last year.

Past winter drought stress is another likely factor in evergreen dieback. While this past winter was fairly “wet” for us, in recent years the winter weather has been mild with little precipitation. That is why I also urge tree owners to take on the burdensome task of deep watering trees, especially evergreens, in October before irrigation water is turned off and monthly during dry, mild winter weather. Again, few do this resulting in additional drought stress.

At this point we might assume that last summer’s heat was the proverbial “straw that broke the came’s back,” but other factors could be contributing to the current dieback problem.

1. Girdling roots restrict the uptake of water. They are caused by a failure to adequately loosen and spread the roots at planting time.

2. In addition to loosening roots at planting time, proper planting techniques are important. Planting too deeply smothers the roots. Leaving plastic pots, biodegradable containers, and even treated burlap around the roots can delay or restrict root growth out of the original root ball.

3. In many landscapes, soil becomes compacted from lawn use or was compacted during home construction. Soil compaction prevents air and water from getting to the roots.

4. Sandy soils and shallow soils are not capable of retaining as much water as heavier or deeper soils, limiting the availability of soil moisture.

5. Physical damage to tree trunks from weed trimmers or mowers impedes the transport of water from the roots to the top of the tree, causing water stress in the top of the tree.

6. I am a strong advocate of mulches, but rock mulches and close proximity to concrete walls put additional heat stress on a tree and increase its need for water. Bark or wood chip mulches are recommended, but when applied in layers thicker than 3-4 inches, they restrict air and water movement.

The are many lessons to be learned from our current situation, but deep watering and keeping our trees and shrubs in good health is one of the most important. There is no one cause for the widespread browning and dieback of local evergreens, but the primary factors are watering, weather, and root problems.

Next week I’ll talk about wood boring insects that are exacerbating the dieback problem by attacking stressed trees and quickening their demise.

Garden Tips, WSU Extension, Benton County, 5600-E West Canal Drive, Kennewick, WA 99336-1387, 509-735-3551, Contact Us

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