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Saturday, September 10, 2011

The Problem With Pruning Pine Trees So They Don't Blow Over


This Mondale pine was pruned probably
 in an attempt o reduce with wind load (sail effect)
 on its canopyand help reduce the potential
for blowing over

You can see that removing the inner small
branches concentrates the weight of the new
growth all on the ends of the branches causing
 them to bend and in some cases to break
 A fairly recent trend in pruning in Las Vegas has been pruning pine trees so they don't blow over. We have some wind gusts that periodically get pretty nasty when they reach speeds of up around 70 mph (112 kph) or more. The trend was to remove some of the canopy so that there is not so much leverage on the root systems. This leverage, or sail effect, by the wind can pop the trees over when gusts come up quickly and our trees can be pretty shallow rooted which does not help to anchor them.

I have some concerns about thinning pine trees to allow more wind to blow through their canopies. Not that it is done but really more about HOW it is done. Take a look at this picture of a pruned pine tree just for that purpose.

Most pine trees will not regenerate new growth on older branches once this growth is removed. Once the wood is older than about three years old most pines will not cause new growth to come from this older wood. Once this young wood is totally removed, it is gone for good. By removing these small limbs in the interior it prevents pines like Mondale from renewing the removed growth. The juvenile part of the limb is now concentrated
only in the growing tips which are all at the ends of the branches.
Pine tree with a canopy thinned a bit better so
that limbs are less likely to bend and split

This might be good in theory (the tree trimmers don't have to come back as often). But this can create a problem.Think of a fishing pole. Why is the pole diameter tapered and not all the same diameter along its length? Poles or branches which are tapered help spread the stresses of bending along its length. If a pole or branch has no taper, then the stress of bending is concentrated in a very small area of the pole or branch which will increase its chances of snapping or breaking. By removing this small wood deep on the inside it also causes these "thinned" older limbs to no longer increase in girth (caliper or diameter).They bend more and more as the weight of the new growth is added to the ends of the branches.

A better approach would be to selectively leave some juvenile wood on the interior of these larger diamater branches so that these branches will increase in caliper and taper. So instead of removing ALL the small juvenile from branches deep in the inside, remove some but leave some as well. This still results in thinning and allows for wind to move throught the canopy easier and also results in distributing this increased load along the branches rather than concentrating the load in a small area of the branch which increases its likelihood of failure (snapping or bending).

Just a thought.

2 comments:

  1. I am posting an email question regarding this posting on my blog:

    The pine trees in our common areas were recently pruned to look exactly like the Mondale Pine in your article titled “The Problem With Pruning Pine Trees So They Don't Blow Over.” I am told that the landscape company that was hired by our HOA had a certified arborist on site the entire time, and convinced our Board that everything they did was necessary and properly done. It is very difficult for me to accept that any pine tree should look like this. That is why I am writing to you. Did I misunderstand the content of your article, and the attached PDF file from the University of Idaho Cooperative Extension? The URL for the article is: http://www.cals.uidaho.edu/edComm/pdf/bul/bul0644.pdf

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  2. This type of pruning of pines started a few years back in the Las Vegas area when larger pine trees were blowing over in heavy gusts of wind. Arborists in the area began “thinning” the trees to reduce the “sail” effect that strong winds had on trees with large canopies thus uprooting them due to the leverage on a tree with a shallow root system.

    The reasoning, I believe, was that by reducing the canopy and letting more wind through it, it would reduce the leverage that strong winds would have on tall trees with extensive canopies. And this is true, it would.

    The reasoning was not questioned by me but the technique used to reduce the canopy was. All arborists should know that when limbs or the trunk of a tree has side shoots with green leaves (or in this case needles) removed totally, all of the side shoots, from the limb or trunk, this reduces the future “taper” of the trunk or limb. When the taper is reduced and you have a limb or trunk with virtually the same taper, it weakens that limb or trunk.

    This means that when a strong wind causes the trunk or limb with little taper to bend, it is much more likely to break. This is because the stress applied to the untapered limb or trunk is not distributed along it length. Limbs or trunks with poor taper will have a point along the limb or trunk where stress pressure will “accumulate” and cause it to snap. That’s when I showed what happened to pine trees that were pruned this way on my blog. See this later posting on my blog.
    http://xtremehorticulture.blogspot.com/2011/12/problem-i-warned-about-on-pruning-pine.html

    My argument was that if tree canopies are to be “thinned” then side shoots along the trunk should be selectively removed leaving behind FEWER side shoots but the remaining side shoots would still be scattered along the entire length of the limb or trunk. By leaving behind side shoots all along the trunk, but fewer of them, it still allows wind to move through the canopy better but the remaining side shoots will still contribute toward building taper along the limb or trunk. I hope this better explains the problem.

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