Protecting Our Portland Birch Trees from Bronze Birch Borer
Birch trees marked for removal by the City of Portland due to bronze birch borer.
Birch Trees Dying from Bronze Birch Borer
Many developers, builders and home owners picked the Himalayan White Birch (also called Jacquemontii,) for its crisply white bark and over planted them. They even planted them in parking strips with no irrigation, in full hot sun, which is not a good place for a birch. My Vancouver client’s neighborhood had over 200 mature infected trees removed. They had already lost 2 birch trees and I made tree replacement suggestions as part of their Landscape Design in a Day.
Back in the 1980’s the Himalayan White Birch was touted as the new success story because it had been hybridized to repel the Bronze Birch Borer (BBB). At that time I was a student learning about trees at a local community college. The European Weeping White Birch had been decimated by the BBB so everyone was very excited about the new Himalayan White Birch. Over the next 20 years, the bronze borer changed its preferences and became attracted to the over planted Himalayan White Birch. It makes sense from an evolution perspective; why not change to fit the food that is available? Smart bug!!!
30 years ago Himalayan White Birch was used because it repelled Bronze birch borer.
Recently I have noticed the dreaded yellow tape of death tied around birch trees in the city. I create my Landscape Design in a Day drawings on site so I am in every conceivable neighborhood. The Bronze Birch Borer is now all over Portland and has spread south to Klamath Falls.
Today when I see my client has a birch tree, I give them the current research and it’s mostly bad news. I often include in their design a potential replacement tree for when, not if, their tree is devastated by the Birch Bronze Borer.
Heritage River Birch in winter. River Birch (Betulus Nigra) is a safe replacement tree – Bronze birch borer does not feed on this birch tree.
Protecting Your Portland Birch Tree
My research says watering your trees regularly before they are infected is a huge step toward preventing the disease. If you have a birch tree that is thriving or only has minimal borer damage, consider starting to irrigate it ASAP. Start by deep watering it every week to two weeks starting in early summer into mid to late fall. Don’t let your tree get stressed. (Deep water is a long slow soak with your hose.) Under no circumstances should you water your tree every day – that is not helpful. (See my watering tips blog).
I’m also reading that more people are using a chemical treatment (which will help your tree) than they were initially. I’m not very happy about that because the treatments will harm bees. They are mostly drenches that are systemics (bad for bees) or injections done by tree services which are also systemic in nature (and so bad for bees). Apparently the timing of the treatment and how it is done can make it less lethal to bees but isn’t this backward of saving the bees and therefore our food supply? If it were my tree, given my very strong feelings about protecting bees, I would try watering deeply and regularly and not treat the tree with pesticides. If the tree is too far gone I would have it removed, grieve and plant a new tree that is resistant to disease and insects and prefers little summer water.
Katsura tree at Lan Su Chinese Garden in Portland, Oregon
In short, if you love your tree, start taking care of it. The first trees that died were neglected, poorly sited and in neighborhoods chock full of white barked birch trees.
Weeping Katsura in one of my clients gardens in Willamette Heights.
Signs of Bronze Birch Borer
The first signs are yellowing foliage in the top of the tree. As the insect infestation continues, small branches and tips die. It moves on into the larger branches. Declining to the point of death usually takes several years. There are other signs of borer; ridges in a lightning pattern and a distinctive D shaped hole in the bark. There can be a kind of stain coming from the holes, a sort of reddish liquid which looks as bad as it sounds.
Katsura tree with beautiful fall color.
New Resistant Varieties-Maybe
I am hesitant to trust that new resistant white barked birch varieties will stay resistant if we over plant them as we did the Jacquemontii/Himalayan white birch. I offer the river birch which has a brown peeling bark and typical birch leaves. Alternatively my favorite replacement for birch trees is the Katsura tree also called Cercidiphyllum. The Katsura has the graceful shape somewhat reminiscent of a birch tree with heart shaped leaves that flutter in the breeze. I feel it is a safer choice since it is not related to birch at all but alas no white bark!
Selecting trees that have the best chance to become mature old trees is my way to contribute to my clients and our community. Keeping up to date on the best trees to use and keeping my selection diverse will make the best urban forest for the future.
Kym Pokorny, (now writing for OSU’s Extension Service), says these are good replacement choices; ‘Heritage’ river birch (Betula nigra ‘Heritage’) and ‘Whitespire Senior’ gray birch (Betula populifolia ‘Whitespire Senior,’ which has the whitest bark of the replacement tree ideas. I suspect if we over plant these borer resistant birch trees, the borer will change its tastes to the available food so the best thing to do is plant lots of different trees.
Katsura ‘Red Fox’ is a smaller tree that is getting used in irrigated parking strips.
I came across a lovely old white birch tree just the other day in the Buckman neighborhood and gave my new client, who had just purchased the home, some information on how to care for the tree. The tree seems untouched by borer and is situated where it gets some afternoon shade. He will start to summer irrigate. Perhaps some birch trees are unique individuals because they were grown from seed and this unique genetic combo may cause them to be unattractive to the BBB. We can only hope that some of these remaining individual trees, if irrigated, will remain to grace our landscapes and homes. In 2010 Kym Pokorny, my favorite garden writer, warned that our graceful white bark birch trees might become a tree of the past in Portland. Boy was she right!!