You're not dreaming, trees are producing more sap this summer

Erratic hot summer days are finally giving way to temperate fall weather, and so some of us are venturing out just a bit more on foot — be it a morning walk, an evening stroll, or those quick treks to the coffeeshop during the workday.

To the last point, the LAist HQ in Pasadena is located just minutes away from a couple of caffeine options, which requires that we traverse a short distance of Raymond Avenue.

That small stretch of sidewalk has become the talk of the office, thanks to what seems to be an unusually copious amount of tarry tree sap on the ground, creating a sticky mess.

Jason Wells is a managing editor at LAist who first noticed the ever-widening blotch of dark, sticky sap.

"This mess is giving me La Brea tar pits vibes," Wells said.

So... what's sap (get it?)

As curious journalists, we did what is in our nature and called the city of Pasadena for answers.

"Its scientific name is Quercus suber," said Garrett Crawford, public works administrator with the city and a certified arborist.

The laypeople name for this tree that's been dripping excessive sap: cork oak.

The tree, Crawford described, has standard oak type leaf, and grows out rather than up. Its fruits are acorns and the tree sometimes produces clusters of small flowers. And although its bark looks rigid, it's actually quite soft.

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Fiona Ng

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LAist

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"Any kind of natural cork comes from this tree," said Crawford. Like the one in that bottle of wine in your kitchen.

Crawford said Pasadena has hundreds — up to about a thousand — of the trees planted in the city. Those growing by the LAist building along Raymond Avenue, he said, were put in the ground around 15 to 20 years ago.

Because of the tree's hardiness — "they're pretty resistant to a lot of heat stress and drought conditions," Crawford said — the cork oak is a common sight in many municipalities in the region.

Yes, the trees really are producing more sap

Crawford said our hunch was correct. The cork oak has indeed been releasing more sap these summer months.

The culprit (wait for it): our unusually hot weather — in particular, the record-beating September heatwave that saw days of triple-digit temperatures in many parts of the region.

"Even though oak trees are fairly adapted to that climate, those sudden spikes and then prolonged heat really stressed out the tree," Crawford said.

A cork oak on Raymond Ave. in Pasadena

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Fiona Ng

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LAist

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The science of sweaty trees!

Crawford explained that during a heatwave, "the trees are massively pulling up water and just trying to keep itself cool."

"Sometimes when trees get really stressed out and overheated, they will produce excess amounts of sap," he added.

In short, the cork oaks are basically "sweating," Crawford said.

The hotter the temperature gets — and the longer the heat lasts — the more they sweat.

"Just like people do [but] through the trunk," Crawford said, noting that the secreted material is made up of starch and nutrients the tree needs. "It's just trying everything it can to survive."

Crawford said other tree species also produce more sap during an intense heatwave.

So now we know.