The Dry Garden: How to prevent your trees from looking like this
Nobody wants to live in the house with the fallen tree, squashed sedan and news truck out front. Nobody wants to learn the definition of what arborists call the “wind sail effect” after “tree failure.”
When winds whip through Southern California, it's time to ask ourselves just how well-equipped the biggest plants on our properties are and what we should do, particularly this time of year. Winter storms may be past, but the best season for pruning is months away.
“The thing I would want to avoid is someone running out in a panic and severely pruning all their trees now,” arborist Jan Scow said.
When considering why trees fall over in high winds, experts such as Scow start their conversation with roots. “The single most important thing to keep trees from coming over in wind storms is to cultivate and protect a healthy root mass,” said city of Santa Monica urban forester Walter Warriner.
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.
This still might happen after we plant the tree. In its hurricane-battered state, a University of Florida extension sheet warns, “The usual advice 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 to use soil on site rather than potting mix when planting a tree. Amendments should not exceed one-third of the total mix.
Keeping the trees well anchored then 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 [that it's] underground,” said Carl Mellinger. Trees don’t grow like that, warned this Los Angeles arborist and former president of the Western chapter of the International Society of Arboriculture. 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,” Mellinger said.
Santa Monica's Warriner agrees.
“In general, the thinking over the past has been sidewalk work and street repair work has been done by general contractors,” he said. “What they failed to recognize is once you pull up a sidewalk and start working on roots, it becomes tree work.”
Now that his department requires arborists be called in for root pruning, he said, “We have had fewer failures this year than previous years and far fewer failures than we would have otherwise.”
Cutting roots to install irrigation adds insult to injury.
“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.”
University of California plant sciences professor Alison M. Berry recommends that those worried about how to water trees contact their local UC Extension office. Berry is involved in the California Tree Failure Report Program, an ongoing census among arborists. Berry, who has seen thousands of tree-toppling reports, added 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.”
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.”
So what are the take-home rules for homeowners who want long-lived, aerodynamic trees? Select right. Plant right. Respect the roots. Don’t over-water. Don’t water trees with sprinklers. Prune saplings judiciously (and see this University of Florida sheet for good tips). As trees mature, only use a certified arborist. To find one, go to the International Society of Arboriculture or the Tree Care Industry Assn.
When interviewing candidates, Mellinger advised, “Be sure he looks you right in the face and says, ‘No, I won’t do top chops.’ ”
-- Emily Green
Green's column on sustainable gardening appears here weekly.
Photo: A fallen tree on Clinton Street in Los Angeles earlier this year. Credit: Irfan Khan / Los Angeles Times.
California gardening: Follow the scene and get weekly advice by joining our Facebook page.