Birch trees going going gone
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. Fast forward to today. Boy was she right!! It seems to me as I look back over these past years that the birches in the subdivisions died first. Many developers, builders and home owners picked the Himalayan White Birch also called Jacquemontii, for its crisp white bark and over planted them. In 2009 I was working with landscape design clients in a Vancouver neighborhood. They had already had 2 birch trees removed and we made a replacement plan for the third as part of the Landscape Design in a Day. Their neighborhood had already removed over 200 birches. Their developer had used them extensively. 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. Over time however the bronze borer changed its preferences and became attracted to the available and over planted Himalayan or White Birch. It makes sense from an evolution perspective; why not change to fit the food that is available? Smart bug!!!
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 with my clients so I am in every conceivable neighborhood. The Bronze Birch Borer is now in North Portland to SE Portland, not just the suburbs.
These days when I have a client who has a healthy looking birch I give them the current research and bad news. From what I have read there isn’t a whole lot of good news. I often include in their design a potential replacement tree for when, not if, their tree is devastated by the Birch Bronze Borer. It’s shocking and pretty sad for them to hear that they will probably witness the demise of their tree especially when it looks just fine.
If you have a birch tree that is thriving consider starting to irrigate it if you haven’t already. You might start by deep watering it every two weeks. Under no circumstances should you water your tree every day – that is not helpful (see my watering tips blog). There are chemical protections that you can apply to your tree before it becomes infected that will typically keep it from getting the borer. General Tree Service is one company who provides this service. Sometimes the tree can be saved if you can catch the infestation at the very beginning but you will need to apply the pesticide every year. Another important issue to think about it this: The product is a systemic pesticide. Many people refuse to use any systemic pesticide because some can kill honey bees. Birch trees are wind pollinated not bee pollinated and so the systemic should not theoretically affect honeybees. If you have flowering plants of any kind under or near the birch those plants will uptake the pesticide and that can harm bees. This protection for your birch is contrary to protecting bees. I guess you could take out your hostas or other flowering plants and put in sword ferns. In short, if you love your tree, start taking care of it. The first trees that died seem to me to have been neglected trees in full sun and in areas where there were too many birch trees so the borers could move from one tree to the next door neighbors tree.
What to look for: 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 but remember last years horrid, hot and nasty summer? There were trees who seemed to get the borer early and by the end of the summer, the trees were gone. 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 hole, a sort of reddish liquid which looks as bad as it sounds………Is my tree bleeding? So this is not a happy blog but there are some great trees to consider for replacement.
New resistant varieties; I am hesitant to trust that new resistant white barked birch varieties will stay resistant since the Jacquemontii/Himalayan did not stay resistant. When I have a client who loves birch trees 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 and since it is not related to birch I am not worried about Borers. I know that the trees I place in a home landscape may be removed for capricious reasons by the next homeowners but selecting trees that have the best chance of becoming mature old specimens in their neighborhood is my chance to contribute not only to my clients well being but for my city and region. Keeping up to date up on the best trees to use and keeping my selection diverse will make the best urban forest for the future.
White birch trees that have been planted in part shade, in good soil that drains well and that get irrigation may continue to survive. Another bit of advice from Kym Pokorny’s article is to mulch over the shallow roots of your birch tree. This provides some protection from heat and also from physically damaging the surface roots. I’ve been told by an arborist in the past not to put more than 2 inches of mulch over roots. The best person to ask about these fine points of tree care is an ISA Certified Arborist.
I came across a lovely 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 doesn’t seem to be infected. Some birch trees that are individuals, seed propagated instead of cutting, may have some unique genetic protection and so we can only hope that some of these individual trees will remain to grace our landscapes and homes.