Summer branch drop brings big limbs crashing down

Here's the end of a branch, 8 feet long and very bushy,
 that broke off a flowering cherry tree during the current
 heat wave. (Photos: Kathy Morrison)

Keep an eye on your big trees in the heat

By Debbie Arrington

The summer heat is relentless, the air is still. Then suddenly, crack! A huge branch falls.

What happened?

It's a perplexing phenomenon known as summer branch drop. Also called sudden limb drop, this issue can affect such large trees as sycamores, oaks, elm, eucalyptus and ash. It can happen to trees in irrigated landscapes or native woodlands. And it's happening again to big trees in the greater Sacramento area.

Why remains a puzzle.

"It tends to occur when we get these really high temperatures," UC Davis Arboretum's Ellen Zagory said during a similar heat spell last summer. "From what I can tell, no one knows exactly what causes it, but there are various theories. There are various conditions that seem to predispose trees, like being g older and over-mature. Certain species are more prone and long horizontal branches are more susceptible."

Long  branches that run parallel to the ground are the ones most likely to drop. A tree that has dropped a branch in summer before has a greater probability of doing it again.

The gap left by the broken branch.

According to the UC Cooperative Extension master gardeners, most fallen branches are brought down with the help of wind. But unlike branches broken off in storms that snap off at the trunk, summer branch drop breaks large-diameter limbs 3 to 12 feet or more away from the trunk.

Branch drop may be related to a buildup of pressure and higher moisture content in branches on hot days, but those theories have not been proved, Zagory said.

Preventive pruning to shorten and lighten those long horizontal branches can help. Also, remove any sickly branches that show signs of decay.

In the meantime, choose shady spots with care. Look up before you relax.