I noted earlier this year that our past three summers have been abnormally hot, with record-breaking heat. Even before I started getting calls about dying arborvitae, pines, cedars and other conifers, I predicted that stress caused by extreme heat and inadequate watering practices was going to lead to the decline of many area trees. We cannot do anything about the weather, but we can try to avoid the same problem in the future.
First, let’s talk about the needles on conifers. While we call trees with needles “evergreens,” they do shed or drop their oldest needles each year. Evidence of this is the bed of needles under pine trees. This is not a problem for healthy trees that are producing a new set of needles each year. These new needles persist on a tree for a year or more. Depending on the species, pines will hold their needles for two or more years and spruces for five years or more. However, if a conifer is stressed or its health is compromised by insects or disease, the production of needles will be reduced or stop altogether. This stress can also lead to the browning and shedding of more than just the oldest needles. As a result, a tree will have fewer and fewer green needles, a sign of its decline.
Drought: We are in a region with limited precipitation, making adequate irrigation essential to growing healthy trees. Many homeowners assume trees are getting enough water, but they do not check the soil moisture in the tree’s root zone to make sure.
The water-absorbing roots of most landscape trees are within the top 12 to 18 inches of soil. It is this zone that should be kept moist with deep watering. Watering trees with the lawn for 15 to 20 minutes once or twice a day may not be enough to moisten the soil to the needed depth. That depends on how much water is being applied, weather conditions and the type of soil. The only way to know if soil is moist enough is to dig down and check.
It is also important to moisten the soil where water-absorbing roots are located. They are not close to the trunk or under the canopy of mature trees. They are at and beyond the canopy or outermost spread of the branches called the “drip-line.” It is to this area that water should be applied.
Heat stress: We can not offer much relief from the heat to established trees, but when planting new trees we should consider planting varieties well adapted to hot summers. If you plant a species that is not well suited to our climate, select the site carefully to provide it with protection from sun, heat and wind. On younger and smaller trees, mulch the root zone with 3 or 4 inches of wood chips or bark to keep the roots cooler and to maintain soil moisture.
Oh my, I have come to the end and I have not had the chance to talk about soil compaction and other factors contributing to local conifers turning brown. More next week ...
Marianne C. Ophardt is a horticulturist for Washington State University Benton County Extension.