How Do You Know a Tree Limb Is About to Fall?

Over the weekend, the New York Times reported that tree limbs were unexpectedly falling on, injuring, and even killing people in New York City parks. Victims of this branch bombardment were reportedly gathering lawyers and investigators to prove that the city has been negligent in its tree inspections. But how do you know if a tree limb is going to fall?

Mark Chisholm, an International Society of Arboriculture?certified arborist, says that while it might be obvious that high winds and storms knock down branches, no one knows exactly when a tree will drop a limb. Still, expert observers like Chisholm try to predict when tree limbs will fail by looking for a few key signs, and those signs can help you monitor limbs in your own yard that might be in danger of falling.

The first signal is the most obvious: If a limb is leafless when it should be in bloom, that's usually a hint that the tree isn't doing well. "Those dead limbs usually come down in wind and rain," he says.

Other possible clues can also be spotted from the ground. If a tree has two equal-size arms branching off of the trunk from the same part of the tree (think of a Y shape), the union of the two branches could be weak. The connecting point between the branches might not be able to support the weight of both limbs, putting the tree at risk of a limb drop, Chisholm says.

A tree's structural issues can also be the result of either man-made or natural traumas. A poorly done pruning job or a strong wind event can both cause trees to develop hollow points, fractures, or splits that never heal properly. A limb will remain the same size and weight while the fractured anchor point to the trunk weakens, eventually causing a drop.

Biological invaders can also play a role. If shelf mushrooms are able to grow on a tree, Chisholm says, it's a sign of danger. "Big mushrooms growing out of the side of the limb usually indicate that the tree is rotten on the inside."

Although arborists can identify many of the issues that cause limb failure, some remain a mystery. Summer, or sudden, limb drops happen without warning on calm, windless days. The limbs usually show no sign of rot or distress, but some researchers have suggested that rapid movements of fluids through certain limbs could be the cause. Other theories blame internal cracks or microscopic changes in a tree's cells, but Chisholm doesn't think that any of these proposed explanations are very compelling.

"In most every case, the only thing they've found that's consistent is that the limb [that falls] is sticking out farther than the rest of the limbs on the tree," he says. "But that doesn't meant that all limbs that stick out further than the rest will fail."

Certain trees are also particularly susceptible to limb failures. Over-mature silver maples, for instance, are more likely to drop limbs in storms because their particularly fast growth rate can make their limbs more brittle. Black locust trees, an invasive species now found all over the world, are more likely to drop limbs from heart rot, a fungal disease that starts in the trunk and spreads to the rest of the tree.

You can try to save limbs that appear to be in immediate danger of falling, Chisholm says?structural flaws aren't necessarily a tree's death sentence. Sometimes you can save branches by cabling, or roping the two halves of a split trunk tightly together. Arborists will also try strategic breaking, or purposefully breaking a limb to repair it with a rod or glue, to get a tree growing in the right direction.

But, Chisholm says, while it's important to inspect your own trees for possible weak limbs, it's crucial to be safe. So don't be an idiot?falling branches are dangerous.

"There are some things you can do to predict limb failure on your own," Chisholm says, "but ultimately if you're really concerned, you need to get an arborist to check it out."

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