Birch (Betula spp.)

Posted by shane - December 21, 2010 - A-Z Plant Maintenance, Trees - No Comments

 

Paper birch with fall colour

Paper Birch – Betula papyrifera

Weeping Birch – Betula pendula ‘Dalecarlica

River Birch – Betula fontinalis

‘Youngs’ Weeping Birch – Betula pendula ‘Youngii’

These are common trees in older residential neighbourhoods.  Generally speaking, once proper structure has achieved through training the young tree, birches are generally low maintenance.  The biggest nuisance for home-owners seems to be raking up all the small twigs that break free from the tree during windstorms.  Unfortunately, birches are often planted in soil that is less than ideal, without irrigation.  These stresses seem to predispose birches to attack by a number of insects including: bronze birch borer, birch leafminer, and aphids.

Bronze birch borer is tough to identify from the ground because in order to see the D-shaped exit holes and sawdust, you often have to climb the tree.  Dead tops are often a sign that bronze birch borer is present.  The tree becomes injured as a result of borer larvae feeding on the cambial tissues, which prevents proper uptake of water and nutrients, and eventually girdles whole limbs.

I normally see bronze birch borer in mature trees that are likely close to the end of their urban lifecycle.  Since mature birch trees add value to the landscape, both esthetically and as a shade tree, I tend to try and keep them around for my clients as long as possible.  I prune out all the obviously infested tops, and encourage home-owners to make sure that their birch receives adequate watering and fertilizer.  A vigorously growing tree tends to resist insect attacks.

Birch leafminer is a big problem in Central Alberta, and I have seen in sporadically in other areas.  Entire neighbourhoods in Red Deer are infested with leafminers, evidenced by dead, brown foliage throughout most of the tree in late June.  The problem is that there are 5 species of leafminers, with a possible 9 generations of attack per year.  The most common species has 2 or 3 generations per season.  Repeated, severe attacks of the insect stunt the growth, and increase the stress, of the tree.  At least one nursery in Red Deer is no longer growing birches due to leafminer problems.  I treat severe attacks in late May with a systemic insecticide applied as a soil drench, with specialized equipment.  This effectively control birch leafminer and aphids throughout the growing season.  This chemical is not available to the general public, so most neighbourhood trees go untreated.

Aphids are usually not cause for alarm.  Heavy infestations can be treated in the same manner as leafminers.

Pruning Time

Early June to late July is optimum.  I have pruned birches up until about September 15th, so long as the leaves are still green, and the tree has enough energy to compartmentalize its pruning wounds.  Pruning from mid-September up until leaf emergence will allow sap to drip (or pour) freely from cut wounds.  While this is unlikely to kill the tree, it is alarming and black sooty mold will grow on the sap that has made its way down the limbs and trunk.

Pruning and Health

I don’t often get the chance to work on young birches because I’m normally called to look at the tree once its mature and problems become apparent.  I always recommend that I remove all the major deadwood (greater than ¼” diameter) throughout the tree.  This greatly improves the look of the tree.

Iron chlorosis is a very common nutritional problem of birches growing in high pH soils.  A low nitrogen fertilizer 11-26-27 plus iron is helpful, applied in May and again in September.  It is also possible to have a professional arborist inject an iron supplement directly into the trunk of the tree using a specialized syringe.

Structure

Train paper and weeping birch as a central leader standard with the well-spaced scaffold limbs starting at around 4-5’ from the ground.  Subordinate codominant leaders early to avoid included bark and unstable branch attachments.  Older trees that were never trained for proper structure may have problems with weak attachments.  It is sometimes necessary to bolt and cable the tree to mitigate potential hazards.

Irrigation

Moist, well-drained soil is best.  Plant trees in a mulched bed to avoid competition with surrounding lawns.  Irrigate during extended dry periods.

(c) Shane LePage, Wild Rose Garden & Tree Service, Red Deer, AB