GardenLine | Trees and Shrubs | Guide to Birch Trees

Prairie home owners love birch trees. Although these trees are not particularly well suited to dry prairie conditions, where they are given proper care, birch trees provide attractive long- lived additions to our landscapes.

The three most common birches grown on the prairies are the European weeping birch, the paper birch and the river birch.

European weeping birch (Betula pendula var. gracilis) has a graceful weeping form and is the largest of the birches. Mature height of these trees can reach 16 to 18 meters. The weeping form is a result of the youngest twigs draping at the ends of the branches, and the deeply cut leaves give the tree a lacy appearance. `Purple Rain' is a newly available cultivar of this tree with purple leaves. While this new introduction survives on the prairies, it may be somewhat more prone to insect attack than other weeping birches. All types of European birch are highly susceptible to the insect known as the bronze birch borer.

Paper birch (Betula papyrifera), is the native birch of northern forests and river valleys. A mature paper birch will reach a height of 10 to 12 meters and has an upright growth habit. Paper birch is often grown as a clump with three or more main trunks. The bark on mature trunks is white, papery and peels away naturally to create an attractive contribution to both the summer and winter landscape. Because it is native to our area, paper birch has greater resistance to the local population of insect and disease pests than does the European birch.

River birch (Betula nigra) is less frequently seen in landscapes but is probably the more resistant to insect and disease pests than any of the other birches. Its attractive bark has a strong tendency to peel in long horizontal strips and is darker in colour than the white bark of other birches.

All birch trees thrive where conditions are sunny and the soil is well drained, somewhat sandy but high in organic matter. Birches do not usually thrive on heavy clay soils where air movement to the roots is restricted. On a healthy birch, the roots will spread to a distance of at least twice the tree's height. This means that the roots of a mature tree may cover an area about one third the size of a football field. To permit proper root spread, trees should be planted as far as possible from any obstruction that may interfere with root development. Sidewalks, driveways, patios and building foundations will all limit root development. Where a tree is growing on heavy soils, aeration holes will help maintain root vitality. These holes can be made with a garden fork, or a power drill with a wood auger bit. The holes should be as deep as possible in a 20 centimetre grid pattern from the trunk to well beyond the canopy of the tree.

The high moisture requirement of birch tree is most obvious if you notice that in nature they only grow along lakes and rivers, or on cool north-facing slopes. Where a birch tree is growing on a lawn you should not be fooled into believing that watering the lawn will provide enough moisture to satisfy the birch. Since a large portion of the water applied will be consumed by the lawn, substantially more water should be applied around the roots of the tree. To visualize the amount of water a birch tree can consume, consider that during a typical spring day, a large birch will extract enough water from the soil to fill about ten bathtubs.

To properly water a birch, place a soaker hose upside down around the perimeter of the canopy or drip line of the tree and let the water run very slowly for several hours. The water should be applied slowly enough to filter into the soil and not run off. Once the soil is moist do a depth of 60 to 100 centimetres enough water has been provided. A birch tree should be watered in this way every two or three weeks, and more frequently during hot dry windy weather. Reduce the amount of water applied in September and October and then provide one deep watering just before Halloween. This will help to provide adequate moisture in spring. It is important that adequate springtime moisture be present because the sap of a birch tree begins flowing in early April, long before most of us are thinking about watering the garden. A mulch of wood chips, coarse peatmoss, or grass clippings 12-15 centimetres in depth will reduce water losses from the soil, cool the roots and reduce the need for weeding.

An evergreen fertilizer such as 30-10-10 may be applied in mid- May to the aeration holes described earlier. Acidified evergreen fertilizers are suggested as birch trees prefer a slightly acidic soils.

Pruning

While most other trees would be pruned in early spring, birches and maples should only be pruned in late spring after the leaves have reached full size. A tree that is pruned before the leaves have developed will bleed sap depleting the tree of moisture and sugar. Late summer pruning is also discouraged because, although there will be no bleeding immediately following pruning, the non- healed cuts will begin to bleed as soon as sap begins to flow in spring. When a birch tree is bleeding about all you can do is keep it well watered so the moisture loss is minimized. Commercially available pruning paints will not even slow the flow of sap from a bleeding wound. Since they tend to have an acceptable form, most birch trees require only a very small amount of pruning.

When a branch must be removed, there is a specific way that the cut should be made. In the past it was recommended that limbs and branches be removed by making all pruning cuts flush with the trunk. Recent research has shown that flush cuts cause structural damage to the tree, and do not close properly thus providing an opportunity for disease infection.

The current recommendation is to make the pruning cut at the point at which a tree limb is naturally lost. This is a swelling called the "branch collar." Pruning cuts should be made directly outside of this swelling leaving the branch collar intact. The collar is especially easy to identify on a birch tree because there is very frequently a blackened line running down the trunk at the point where the branch is joined to the trunk. This line indicates the location of the branch collar and should be left intact.

Birch Tree Care Part II - Pests and Problems

Russell Friesen

Like most plants, birch trees fall victim to the problems caused by insect and disease. A large percentage of the problems we face while trying to grow birch trees can be traced back to cultural practices. A birch tree that is well watered and properly pruned will only rarely show problems associated with the attack of pests.

Because of drought, soil conditions, and age many birch trees on the prairies are declining. A decline is a gradual weakening of the tree. Throughout Saskatchewan birch trees are dying, the majority due to an outbreak of an insect known as the bronze birch borer.

Bronze Birch Borer

First becoming noticeable during the drought of the late 1980's, this insect targets old and stressed birch trees, boring underneath the bark and feeding on sap-conducting tissue. The borers start at the top of the birch tree and eat their way down. A sure sign of a bronze birch borer infestation is a leafless dead tree top. Unfortunately, once a tree is infested with bronze birch borer, controls are largely unsuccessful, and death of the tree usually results. Prevention of infestation by maintaining the tree's health is the best course.

Damage

Adult bronze birch borers are rarely seen. The damage to trees is caused by the feeding activities of worm-like larvae which girdle tree trunks and limbs cutting off the sap flow. The larvae tunnel underneath the bark causing a ridge. This ridging may be apparent visually, but is more likely to be found by feeling along the normally smooth bark and noting a slight rise. The bark covering this raised area can be gently sliced away down to the sapwood revealing a serpentine trail of darkened material. (Do not pare bark on the main trunk). The darkened material is excrement and can be flicked out of the trail with a knife.

Emerging adults chew a hole in the bark shaped like the letter "D". This distinctive "D" hole is about the same size as the "D"s on a newspaper page.

Life Cycle

The bronze birch borer has a two year life cycle. The adult borers chew their way out of the tree bark in June and immediately lay eggs near the top of the tree. The eggs hatch and the new larvae chew their way into and underneath the bark. The larvae feed and grow, working their way down the trunk, where they over winter and pupating in the spring. The pupa mature into adults and the cycle is repeated. With every new generation, egg laying takes place on lower parts of the tree.

Control The best control for bronze birch borer is to maintain a healthy tree as the borer larvae cannot live in healthy vigorously growing trees. This means sufficient watering! Because of the birch tree's high water use, fall and spring pruning are not recommended for birch trees. The wounds resulting from improperly timed pruning may bleed enough sap, that branches higher on the tree will be drought stressed. Yellow-bellied sapsucker and woodpecker injury may also induce drought condition, resulting in top dieback if their holes girdle the trunk.

Once a birch tree is infested by the bronze birch borer, control is very difficult and the chances for a cure are very slim, perhaps only 1 in 10. Remove dead or infested branches and trunks below the last sign of a borer infestation (the ridging of the bark and the adult's "D" emergence holes). Trees with a great deal of dieback from the bronze birch borer should be removed. There are no regulations regarding the disposal of wood from infested trees, but because the larvae overwinter underneath the bark, firewood can serve as a source of infestation for other birch trees. Dispose of the wood or burn it as firewood before the next spring.

It is possible to prevent the adults from laying eggs by spraying the top and trunk of the tree with chlorpyrifos (Dursban) in June. Areas sprayed with chlorpyrifos should not be entered for at least 24 hours without protective clothing. For this reason and its high toxicity to wildlife, we are reluctant to advise homeowners of this control method. Precautions should always be taken with any chemical. Applicators of chlorpyrifos should wear an approved respirator, goggles, and rubber clothing including gloves and footwear. Be sure to read and understand the label instructions.

Birch Leaf Miner

Birch leaf miner is rarely lethal to birch trees. Because it is highly visible and disfiguring, it is the birch pest most people are (mistakenly) concerned about.

Damage

A birch leaf miner infestation is first signalled when the centre of the leaves appear to be a lighter green than the edges. As the leaf miner continues to feed on the inner leaf tissue, brown spots near the centre of the leaf become apparent. These resemble blisters with small black specks of pepper (excrement) distributed throughout the damaged tissue. Damage is usually noticed in June.

Control

The most common method used for controlling birch leaf miner is a soil drench using undiluted dimethoate (sold as Cygon 2E and Lygon 2E). This is poured into several small holes made in the soil directly under the drip line. The total amount to be used will depend on the trunk diameter of the tree measured at 1.2 meters above ground level. In a multi-trunked tree add all the trunk diameters together. Read the product label directions very carefully to determine the correct amount. Never add more chemical than is recommended. Only one soil drench per year is required to control the miner and is applied in early to mid May while the tree is leafing out.

The soil drench method is considered the most effective and preferred method of application. It avoids foliar spraying and drift and the leaf miner is controlled before any damage can occur. If for some reason, the soil drench is inappropriate to a particular situation, the label does include directions for spraying or bark painting.

Although, bark painting is stated on the pesticide label, dimethoate should not be painted on the trunk on an annual basis. The chemical will weaken the tissues under the bark and induce the same sort of conditions as drought. Many more birch trees have died from the over-use or misuse of dimethoate than from birch leaf miner. If the tree was leaf miner free this year, consider skipping next year's application of dimethoate. The Horticulture Information Service has noted an increase in reports of symptoms similar to dimethoate toxicity.

Birch Canker

This is a fungal disease of birch trees. Not much is known about birch canker at this time, but it is becoming increasingly common. The canker appears as an unusual swelling near bad pruning cuts or broken branches. As the swelling expands it cuts off the water supply to the main trunk or branch, causing the death of the branch. A canker infection mimics the die back associated with the bronze birch borer. Cankers in branches may be removed by proper pruning. Just as with the bronze birch borer, control is in maintaining proper tree health.

Minor Pests

Any pest of a tree that is not lethal or disfiguring can be considered minor. Birch trees have a number of minor pests. The least significant is the paper wasp which will peels a strip of bark causing a few twigs to die back. Spittle bugs and aphids do more damage than wasps but are still minor. Both insects are sap feeders and will cause leaf loss. Spittle bugs protect themselves in a frothy spittle. Aphids are often found in large numbers under leaves. Sometimes aphids will cause leaf distortions. Of the two aphids more frequently warrant control measures. The insects can be controlled with contact insecticides.

Cultural Problems

Two cultural problems manifest themselves quite are regularly. The most common of the two is herbicide damage. Birches have a wide and shallow root system. Some of the chemicals in lawn herbicides remain active in the soil and are absorbed by the tree's root system. It is preferable to use spot sprays of herbicides for weed control rather than a widespread spray or broadcasting a herbicide-fertilizer combination.

The other cultural problem is subtle than any of the other commonly encountered. Trees a decade or so old sometime seem to die in a period of a few weeks. Typical pre-death symptoms include dwarfed wilted leaves, poor growth rate and trunk abnormalities at the soil surface. The cause of death in these cases is often root girdling. Girdling roots are roots which wrap around the trunk choking off sap flow and water uptake.

Container grown trees are often root bound. The roots encircle the outside edge of the pot and may lead to girdling roots. Using a technique called "butterflying" at planting will eliminate the problem of girdling roots. After removing the tree from the container, quarter the root ball with a sharp knife. Spread out the root mass and plant the tree into a cultivated hole five times the diameter of the root ball. Addition of peat, manure or top soil is not required or desirable.

Although, controlling pests is a major activity in caring for a tree, proper watering and fertilization is far more important to the health of a birch tree than any chemical intervention.

© 1995 Russel Friesen

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Sustainable horticultural information, offered free of charge to the public with the support of the University of Saskatchewan Extension Division, the Department of Plant Sciences and the Provincial Government.