Last week, I covered the top factors contributing to the browning and dieback of many area needled evergreens. While it is no surprise that heat and drought stress could cause problems, some of you might wonder why we are just becoming aware of the severity of the situation.
It is easy to tell when a tomato or squash plant is suffering from drought or heat stress. They wilt. This is not the case with trees, especially conifers. Browning needles and excessive needle-drop are telltale signs of stress, but a conifer may not exhibit browning until some time after the damage from stress has occurred. Why is that? Conifer needles have a very waxy coating that slows their drying out, delaying browning, diagnosis and pre-emptive action.
Mid-Columbia has had 3 years of hot summers and minimal winter precipitation
I will also point to the cumulative stress from three successive years of extraordinarily hot summers and minimal winter precipitation. Many of these conifers have been declining over that time, but the additional stress year after year has pushed them past the tipping point.
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So what about trees that are turning brown despite getting watered correctly, keeping the top 18 inches of soil moist and mulching to keep the roots cooler? Restricted or girdling roots, the result of improper planting, are often involved in helping push trees over the edge.
Container-grown plants are frequently pot-bound with circling roots or very dense, matted root masses. If these roots are not properly cut and loosened at planting time, the roots will not move out into the surrounding soil, where they have access to moisture and nutrients. Eventually, the roots will girdle, or choke, the plant, preventing the uptake of water, and killing the tree. The signs will be that same as signs of drought and heat stress.
Where trees are growing in lawns, periodic aeration can help relieve some of the compaction.
A similar problem occurs with balled and burlapped trees that are grown in the field, dug and their root ball wrapped in treated burlap. In our dry climate, the burlap does not decay quickly enough to allow for good root growth. University horticulturists, landscape professionals and the American National Standards Institute (ANSI) all say that the burlap should be removed before planting.
Also, the soil in the root ball is commonly different from the native soil where the tree is being planted. This can lead to difficulty in keeping the soil in the root ball and surrounding soil moist, but not excessively wet.
Soil compaction restricts root growth and can contribute to a tree’s demise. This is because compacted soil has less air available for the roots and roots need air to function. In addition, soil compaction physically impedes root growth. In older landscapes, soil becomes compacted over time from traffic and even sprinkler irrigation. In newer landscapes, the soil may be highly compacted due to the use of heavy construction equipment during building.
The soil in the root ball is commonly different from the native soil where the tree is being planted.
Where trees are growing in lawns, periodic aeration can help relieve some of the compaction. However, if the compaction is severe, a certified arborist may recommend more extreme measures. When planting a new landscape, the soil should be properly prepared with tilling before planting.
This week and last week, I have mostly focused on the browning and dieback of conifers in our area, but we are also seeing dieback on some local deciduous trees and shrubs. Depending on the plant’s situation, this decline may be attributed to the same factors causing the browning on conifers. I sure hope this summer is cooler.
Marianne C. Ophardt is a horticulturist for Washington State University Benton County Extension.