Why Trees Fall

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Southern California has been struck by ferocious Santa Anna winds, which have toppled century old trees, tossed palm fronds around like tooth picks and sent tree branches crashing to the ground. Since the winds hit, my landscaping crews have been chain-sawing their way through my clients’ gardens working overtime to clean up the mess.

And while there is not much one can do about the wind, the palm fronds or the branches, there is something that can be done about what arborists call “wind sail effect” and “tree failure” that send magnificent oaks and towering eucalyptus crashing to the ground, across driveways, on top of cars or into your house.

Emily Green covered this issue on May 14, 2010 in The Dry Garden blog she writes for the LA Times. I will summarize her suggestions here, but her piece is well worth reading in its entirety, “How to prevent trees from looking like this.”

Why Trees Fall

There are two reasons why trees fall in high winds: either their root systems have been compromised or their branches have not been pruned or pruned improperly so that their canopy either acts as a sail or become excessively top heavy.

Roots are everything. When considering why trees fall in high winds, Santa Monica urban forester Walter Warriner states, “The single most important thing to keep trees from coming over in wind storms is to cultivate and protect a healthy root mass.”

When trees do blow over, often it’s because of circularized roots, the chopping of roots and, most often, the over-watering of roots.

Root health starts in infancy. So, when choosing a tree from a nursery, make sure that the specimen is not root-bound with circularized roots that will grow inward, not down and out.

And don’t follow the usual advice, which has been. “To dig a large hole, put the tree in the hole, and fill around the ball of roots with enriched soil.… This may be exactly the wrong thing to do! … The hole full of rich soil may allow the tree roots to get off to a good start, but the roots may take a long time to grow from the good soil into the poor soil surrounding it. Roots may coil around in the hole just as they would in a pot.”

Most horticulturists recommend that homeowners use soil on site rather than potting mix when planting a tree. Potting mix should not exceed one-third of the total mix.

Keeping the trees well anchored involves respecting the roots, which are not where many of us would expect to find them. “Sometimes you see drawings where the root system is a reflection of the canopy, except underground,” warned Carl Mellinger, Los Angeles arborist and former president of the Western chapter of the International Society of Arboriculture. “Trees don’t grow like that. Instead, most of a tree’s roots will be found in the top two feet of soil. A lot of trees fall because someone has cut the roots to install irrigation or utilities, or do some kind of trenching.”

Water the grass, not the trees. “I would like to see lawns pulled away from the trunks of trees,” Mellinger said. “Everyone should redirect sprinklers away from trees. A lot of trees we have should only be watered four to six times a year and they’re getting watered several times a week.”

Don’t use tree stakes. University of California plant sciences professor Alison M. Berry, who is involved in the California Tree Failure Report Program, and has seen thousands of tree-toppling reports, adds a surprising cause of tree failure: overuse of tree stakes on saplings.

“There is no need for stakes unless the tree cannot stand and support itself,” she said. “If you stake a tree, it’s immobilizing the trunk. It doesn’t sway, so it doesn’t build a strong trunk.”

Watch what you prune. Berry and the other arborists warned against radical pruning as wind-proofing.

“What happens is the tree responds to new growth at cut ends rather than naturally distributed throughout the tree,” she said. “If you can imagine, two to three branches all crowded at cut ends; over 10 years’ time those branches will be huge and very poorly attached and much more prone to break.”

She also warns against pruning from the bottom of the branch up toward the top. “Lions-tailing also destabilizes the branch and leaves all the weight on the end,” she said. “Then it makes it much more prone to have a higher wind sail value.”

The biggest no-no is topping, when someone cuts the main stem and removes the crown. This not only forces panicked growth and increases the wind-sail effect, but it also invites disease. Paraphrasing a favorite arborist of his, Mellinger said, “We don’t call it butchery, we call it mutilation, because butchery’s an art.”

What are the take-home rules for homeowners who want long-lived, aerodynamic trees?

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