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written by Marianne C. Ophardt WSU Extension Faculty for the Tri-City Herald, Kennewick, WA It seems like it wasn’t that long ago that I wrote a column about why all the birch trees were dying, but it was actually several years ago. With your forbearance I’m going to address the topic again, because more birch trees are failing and dying in area home landscapes. The two main factors in their failure are stress and the bronze birch borer. Birch trees are not well adapted to our climate. When placed in a harsh urban environment, especially in a region with very hot summer temperatures, they suffer from heat stress and often fail to thrive. They can also often suffer from winter drought in our region. If you want to plant a birch tree, there are some things you can do to help promote a healthy, vigorous tree. First, site it where it will receive full sun most of the day, but be protected from the heat of the afternoon sun and heat. The east and north sides of a building are best. Second, birches like to have cool roots. They’ll do better if mulched with a shredded bark, wood chips, or coarse compost. These organic mulches moderate soil temperatures and keep the soil moist… essentials for a happy birch tree. Definitely, do not mulch your birch with a rock mulch. Yet another ‘must’ is to provide the tree with enough water during the spring, summer, fall, … and even the winter. Adequate water is a key factor in birch tree health. During the summer be sure the trees are watered deeply, moistening the soil to a depth of 12 to 18 inches once a week. If a tree is situated in a lawn, shallow watering every day will not provide the water that’s needed. A deep soaking in the root zone is required. Weekly deep watering is not necessary once the trees go dormant, but the birch roots still need some moisture if the temperatures are mild and the soil isn’t frozen. In our region, natural precipitation during fall and winter months is usually not adequate for birch trees. This has been especially true during the past several winters that were very mild and extremely dry. (Yes, this means hauling out the hoses once the irrigation water has been shut off. ) The other factor in the demise of many birch trees in our region is the bronze birch borer. Like a vicious animal can sense your fear, bronze birch borers can sense that a birch tree is under stress and vulnerable to attack. This boring beetle is a native pest that attacks birch trees. It’s a problem throughout this country and Canada where native birches grow in woodlands and in urban landscapes where native and non-native birches are planted. It’s not a new pest problem, records of its destruction date back to the late 1800

s and early 1900

s. Birch trees that are subject to some sort of stress are prime candidates for attack by the bronze birch borer. Stress can be brought about by extreme heat, drought, old age, soil compaction, physical injury, severe pruning, poor growing conditions, or improper planting. All efforts should be made to plant birches correctly in a favorable location and to promote good growth. One of the first symptoms of a bronze birch borer infestation is dieback at the top of the tree… where the leaves on the top branches start to yellow and then die. The dieback is progressive from the top of the tree downwards. However, this can also be a symptom of tree decline due to drought stress, root problems, trunk injury, or other poor cultural conditions. The key to determining the presence of the dreaded bronze birch borer is lumpy bark. That’s because the larvae feed under the bark creating extensive winding galleries. The tree responds to the ‘wounds’ created by the borer’s feeding by producing callus tissue that results in bumps and lumps beneath the bark. After the larvae mature and turn into adults, the beetles chew their way out. They leave behind a hole that’s D-shaped and about 3 to 5 mm wide. However, these characteristic holes are usually only observed in badly infested trees. The bronze birch borer kills trees by damaging the conducting tissues (phloem) under the bark that transport carbohydrates up and down the tree. Because of this disruption, the tree roots aren’t able to get the ‘food’ or energy they need to survive and they starve to death. Once the roots die, the top of the tree is unable to get water and the tree starts to die. It’s a slow, but progressive death. If more than 50 per cent of the crown is affected with dieback, it’s unlikely the birch tree can be saved. We’ll talk later about control measures that can be taken if the tree is just beginning to show signs of a bronze birch borer infestation. One of the best ways to control bronze birch borer is to plant birch trees that are resistant to attack by the borer. Unfortunately, the most popular types of birches are highly susceptible to attack. This includes the whitebarked Himalayan or Jacquemonti birch (Betula jacquemontii), the European white birch (Betula pendula) with its graceful pendulous branches, and Young’s weeping birch (Betula pendula ‘Youngii’). The Whitespire birch (Betula playtyphylla japonica ‘Whitespire’), known for its resistance, is moderately susceptible to the borer. The river birch (Betula nigra) and the Heritage river birch (Betula nigra ‘Heritage’) are the only birches known to be fairly resistant (but not immune) to attack. Once a tree is attacked by bronze birch borer what steps can be taken to control it? In the past, chemical insecticide sprays were applied to the bark and leaves of the tree as preventatives to kill adult beetles before they have a chance to lay eggs after emergence and to kill young larvae before they enter the tree. Once the larvae are beneath the bark, external surface spray applications are not effective. Because beetle emergence occurs over several weeks or more, repeated sprays are needed. This is costly and not extremely effective. Because the larvae feed on the phloem tissues just below the bark, systemic insecticides have been found to be somewhat effective in the control of existing bronze birch borer infestations. These systemics are applied by injecting them into the tree trunk or by applying them to the soil for absorption by the roots. Some of these materials can only be applied by licensed commercial applicators, but one product is available for application by home gardeners. That product is Bayer Advanced Tree & Shrub Insect Control Concentrate containing an insecticide called imidacloprid. However, it’s expensive and some price conscious gardeners with large trees have found that it may be less costly to have a commercial applicator apply the same chemical. Injection into the trunk is preferred by some applicators, but this should only be used to control a severe existing bronze birch borer infestation. Repeated use of trunk injection as a preventative measure can be harmful to the tree’s vascular system and compromise tree health. Keep in mind that relying exclusively on the use of chemicals for control of the bronze birch borer is not environmentally sound gardening and it’s expensive. If you want to plant a birch tree, select one of the moderately resistant species. If you have a birch tree in your landscape, keep it as healthy and vigorous as possible. That’s what I’ve done! My three ‘’Heritage’ birches are vigorous, attractive, and free of bronze birch borer… knock on wood. Special Note: It’s important to note that while use of systemic insecticides does seem to provide control of bronze birch borer in birch trees, these chemicals are generally not effective for control of other types of borers that bore deeper into the wood of trees and don’t stay primarily in the area of the phloem just under the bark.

Published: 7/9/2005 11:46 AM



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