Effects of Aggressive Pruning of Mature Incense Cedar Trees
The north border of my city lot in Hillsboro is lined with incense cedars planted 5 feet apart 40 years ago. Unfortunately they were planted directly under a high electrical voltage line about 25 feet above the ground. Of course PGE has to keep topping them off every few years for fire safety reasons. Thus the trees with their lateral growth have taken over the backyard. We need the privacy that these trees supply however not at the sacrifice of our shrinking usable backyard space. I would like to aggressively cut back the lateral growth so that the lateral growth extends only a maximin of 2 feet away from the trunk into our yard. In doing so no green leaves will be left on the tree. What is the probability of green leaves returning? How many years will pass by before we gave return of our privacy? Can you steer me to your resource in answering my question? Sincerely, Jim Sohriakoff 503 730-7220 Jamesrsohriakoff@hotmail.com
Washington County Oregon
Thanks for your question. First of all, I would suggest consulting a certified arborist to help you in deciding how to handle the incense cedars, as they will be able to make a site visit and give more precise guidance. That said, I can offer some general advice regarding pruning. For one, if you cut back the the branches such that no leaves are left on the tree, chances are that you will kill the tree. A rule of thumb is to leave at least half of the live crown when pruning. Wait until late fall through late winter to prune, when the trees are dormant and less likely to attract insect pests. And make sure to prune the entire branch (do not leave behind a branch stub), though not flush with the tree (leave the branch collar). This will help the tree heal over the wound and keep disease from infecting the cut surface. For more background on pruning conifers, I suggest reading this Tree Topics post.
Do I understand you to say that if I prune all my branches all the way to the trunk of my incense cedars, but leave all the green leaves with their branches at the top 5 feet (crown) of these 20 foot trees...then there is a good probability that I will eventually get new lateral growth on the lower part of the tree that was severely pruned? If so, how many years before I will see these new leaves of any significance on the new lateral growth? How close to the ground would this new lateral growth with leaves occur?
Typically when you prune branches back to the trunk, that lateral growth will not return. You may get a small adventitious branch here and there, but not like the branches that were present before. Again- I encourage you to consult an arborist that can see the specific situation that you have on the ground. As a forester, I can only offer the pruning guidelines we use for opening up a forest understory. An arborist can show you where to trim back to the trunk and where to prune to train the lateral growth away from your yard, and can help you avoid making some irreparable pruning mistakes. If you would prefer not to hire an arborist, you could look into pruning manuals or publications like this one from Oklahoma State Extension. Page 3 has some specific advice regarding conifers with scale-like leaves (e.g. incense cedar), and the whole publication does a good job of outlining how to prune, how much to prune, when to prune, tools to use, etc.