Why is My Old Mountain Ash Tree Falling Apart?

Posted by shane - November 12, 2013 - Insects, Diseases & Other Problems, Pruning Techniques - No Comments

I just moved into a 1950’s neighbourhood in Red Deer and the only tree in my yard is an old American mountain ash (Sorbus americana).  It, like many or most mountain ash of the same age, is decayed, has very poor structure, and without considerable help, will self-destruct within a few years, or perhaps in the next major windstorm, or late spring blizzard.

Why do mountain ash trees fall apart?

The answer is very simple: poor branch structure.  On a mature tree, however, the solution is not so easy.  Mature, poorly structured trees can be managed, however, and other steps can be taken to maintain the health of the aging tree.  But first, lets look at why the tree is poorly structured in the first place.

Typical mature mountain ash branch structure.  R. LePage photo 2013.

Typical mature mountain ash branch structure. R. LePage photo 2013.

The tree in the picture has multiple main stems originating from the same position on the main trunk.  This is a structurally inferior arrangment.  As the stems become crowded over the years, the branch unions develop what is called “included bark”, and a noticeable split appears.  This weak branch attachment is where the tree tends to fail in severe weather, such as windstorms, and under heavy snow load. Poor structure can be prevented early on in the life of a mountain ash, first in the nursery, and then by a pruner knowledgeable in structural pruning techniques.  Basically, such pruning produces a tree with a strong central trunk, with evenly distributed main limbs up and around the trunk.  Over time, the tree will take on its characteristic mountain ash form, but the framework of the tree will be much stronger, and will resist storm damage.  Unfortunately, I rarely, if ever, see a mature mountain ash with strong structure.


So what can we do with our mature tree that has problems? Well, the fun part about being an arborist and a homeowner is that I get to try out all the fancy arboricultural techniques on my own stuff that many homeowners might be reluctant to pay for.  In the case of my tree (which is not the one in the picture, but looks similar), I have several large limbs with included bark and weak branch attachments.  One in particular is a large limb that, if it fails, will do damage to the corner of my house.  So I’m going to give it the following treatment:

1) Full and thorough pruning.  I will remove all dead and diseased branches, and reduce the end-weight of heavy, over-extended limbs.  This should ease the burden of some of the poorly attached limbs.

2)  I must install 3, 1/2″ bolts to stabilize the unions of 3 main trunks.

3) On the bolted limbs I will install a synthetic tree cable (TreeSaver), at about 2/3 the height of the tree, for additional support.  The cable is rated for 10,000lbs, so wind and storms shouldn’t be a problem.

4) To further suppor the health of the tree I’m going to eliminate the competition with the turf within the dripzone of the tree.  That means I will kill the turf from the trunk out to the edge of the crown, all the way around the tree.  I’m not going to strip the sod, but instead will either kill the turf with Roundup, or suffocate it with a temporary layer of black plastic.  Alternatively, I could use stacked newspapers but that’s not my style.  Once the turf is dead, I will add a thin layer of good wood chip mulch, 2-3 inches max.  This will effectively reduce much of the competition this old tree has with the surrounding lawn.

I have to admit that mountain ash was not one of my favourite tree species until recently.  However, now that it is the only tree in my front yard, directly outside my livingroom window, I’ve had a couple weeks to have a good look at this tree and I see it in a new light.  The red berries contrast beautifully with the snowy lansdcape, and I’m looking forward to Spring when I can, for the first time, see this tree in full leaf.  My first impulse was to remove this tree and replace with something new, but now I feel inclined to do all that I can to preserve its life for as many years as possible.  It is also on the south side of my house and will provide much needed shade to the livingroom.

Those are some tips for working with a moutain ash with poor structure.  If you are thinking of planting a new tree, or have planted one within that last few years, I highly recommend that it be pruned properly to start the process of developing a strong framework.  It will contribute to the longevity of the tree and reduce maintenance costs years later.

(c) 2013 Shane LePage, Wild Rose Garden & Tree Service, Red Deer, AB.