Why Are All the Birches Dying?
Marianne C. Ophardt
Washington State University Cooperative Extension
Area Extension Agent
Even if you don’t own a birch tree, it would be hard to ignore the fact that so many birches in our area are dying. There are various factors involved, but the causes involve stress, winter injury, and a devastating insect borer.
Birch trees are generally not well adapted to our region. They’re better adapted to areas where the summers don’t get quite so hot; where the soils are more acid and have more organic matter; and where there is snow over the roots during the cold parts of winter. In their native habitat you’re most likely to find them at cooler, higher altitude locations growing close to water.
Birch Tree Roots Need Air
Birch trees have a rather large system that can spread twice the distance of the tree’s height or more! A mature tree’s root system can be as large as one-third the size of a football field. Obviously, birches need room to grow and shouldn’t be planted in areas that restrict their growth. Small planting areas or sites close to sidewalks, driveways, and building foundations should be avoided.
Compacted soils limit air getting to roots. When planting birches you need a soil that has not been compacted by construction equipment. Compacted soils should be “ripped” or deeply cultivated before planting birches. (Keep in mind that you need to loosen the soil in an area about one-third the size of a football field.) If the soil becomes compacted after planting due to traffic from people, pets, or vehicles or due to the use of impact sprinklers, aeration is a must. Aeration will help get air to the roots and keep the trees healthier.
Birch Trees Need Water
Remember, in their native habitat you will most likely find birches associated with a water source... rivers, streams or lakes. Watering a birch tree just with your lawn irrigation is inadequate. Birch trees need much more water than the leftovers they receive from lawn watering. This means watering the trees once a week during the hot part of summer and once every two to three weeks during the cooler parts of the growing season. Watering deeply with a soaker hose placed at the dripline (the perimeter of the branches) is what’s needed. After watering deeply the soil should be moist to a depth of 18 to 24 inches. Watering should continue until the end of October. If the winter remains mild and warm, then it’s also recommended to water at least once a month if the soil becomes dry.
While it would be very hard to mulch the entire root system of a mature tree, young birch trees benefit from mulching the root zone with bark chips or compost. This helps maintain soil moisture and keeps the roots cooler.
Birch Trees Need It Cool
“Coolness” or mild summer temperatures are not something we can control, but we can avoid planting birches in sites where they will be under additional heat stress. Don’t plant them on south or west facing slopes. Don’t plant them where they’re surrounded by paving or asphalt. Don’t mulch them with stone or rock mulches. Avoid planting birches where they’ll also be exposed to strong, drying winds. Shelter them from wind and sun on the north sides of buildings or with other trees.
Is It Decline or Is It Borers?
Too many birch trees in our area are declining and dying back because they’ve experienced considerable stress. This stress and subsequent decline of the birches has been the result of inadequate or improper watering, compacted soils, and damage to the roots from cold winter and hot summer temperatures.
Identifying Bronze Birch Borer
The weakened state of local birches has made them very vulnerable to attack from the bronze birch borer. This is a wood boring beetle pest that is decimating many local birches. The first symptom of bronze birch borer attack is the wilting and dying back of the top portion of the tree. However, this is pretty much the same symptom that results from the general decline of the tree.
To determine if the bronze birch borer is at fault, an examination of the main branches and trunk at top of the tree needs to be made. The bark will be bumpy and ridged from the borer larvae feeding beneath the bark. Slicing into the area right below the bumpy bark will reveal serpentine trails filled with the dark excrement of the feeding larvae. Once the feeding larvae mature they pupate and turn into adult beetles. The adults exit the tree leaving characteristic D-shaped holes in the bark. These holes are sure signs of bronze birch borer activity.
What can be done to control bronze birch borer in our birches? Well, the very first thing is to plant trees that are more resistant to attack from the borer. Unfortunately most of the types of birches planted in our area are very susceptible to bronze birch borer attack. This includes the European white birch (Betula pendula,) Jacquemonti Birch (Betula jacquemontii,) and Young’s weeping birch (Betula pendula ‘Youngii’.) The River birch (Betula nigra) and the Heritage birch (Betula nigra ‘Heritage’) are probably the most resistant birches available. Unfortunately, the River birch and Heritage birch don’t have the much-desired white bark. Instead they have an attractive peeling apricot colored bark.
The Whitespire senior birch (Betula platyphylla japonica `Whitespire Senior’ is moderately resistant and does have white bark. However, it’s important to get `Whitespire Senior’ that has been propagated from the original, superior tree in Wisconsin. Other trees that are simply designated as “Whitespire” may be seedling trees that are probably not resistant to the borer.
Managing Bronze Birch Borer
If you chose to plant a tree that isn’t resistant to bronze birch borer, your next line of defense is to keep the tree as healthy as possible and growing vigorously. Plant the tree in a suitable location where the soil can be kept cool and moist. The north or east side of your home is best. Be sure to water deeply during the growing season, especially during the hot part of summer. Mulch the roots with shredded bark, wood chips or coarse compost. Don’t use rock mulches!
Fertilize the tree to maintain vigorous growth if needed. Use a slow-release tree fertilizer and be careful not to over-fertilize. Don’t prune birch trees unless there is a very good reason to prune. When you prune, avoid pruning in late winter or early spring because the sap will bleed. Absolutely do not prune until July during the growing season because adult female bronze birch borers are attracted to trees with fresh pruning wounds.
Chemical control of bronze birch borer is difficult. Pesticide applications are best applied to the bark to prevent reinfestation by adult borers that have already emerged. By the time a borer attack is evident, chemical pesticides will have limited or no effect on borers feeding under the bark. To prevent reinfestation the recommended sprays are applied to the bark and the leaves. These should be applied in late spring and early summer (May through June.) They must be applied with thorough coverage to the bark because the adults lay their eggs under loose bark and inside bark cavities. Timing is critical to get the adult beetles and the baby larvae before they eat their way back under the bark where they’ll spend the rest of the summer feeding. In the fall they pupate or transform into a resting stage just underneath the bark for the winter. In the spring they mature and transform into adults and then emerge... leaving D-shaped holes.
Systemic insecticides applied to the soil for uptake by the roots or injected into the tree usually have no or only limited effectiveness on the borer larvae in the tree. The systemic insecticides which do provide some control are only effective when the larvae are actively feeding in the summer and early fall. Spring and fall applications of these systemics is not practical.
If a birch tree has been infested by bronze birch borer, it’s also important to prune out infested wood as soon as possible. Pruning cuts should be made properly well below the infested area. Infested wood should be destroyed or disposed of as soon as possible.