A phenomenon that causes oak trees to produce large amounts of acorns in certain years has yielded a bounty of acorns. Credit: Monika Maeckle for the San Antonio Report

That clonk on your car hood or thump on the roof is happening more often this fall as area oak trees experience what’s called a mast year.

The term “mast” comes from the old English “mæst,” meaning a bumper crop. In botany, mast means any kind of fruit or nut that falls to the ground to produce fodder for wildlife. A mast year denotes a season in which various species of trees synchronize their reproduction and drop large amounts of fruit and/or nuts – in this case, acorns.

Mast years for oak trees occur periodically when weather, genetics, and available resources converge to encourage reproduction. They are typically followed by seasons with few acorns. What’s less understood is how organisms communicate with each other to synchronize the effort.

“Masting is observed among many tree species, but what causes it is as yet an unanswered question,” said Chris Best, Texas state botanist for the U.S. Fish and Wildlife Service. Best, based in Austin, acknowledged that it’s a big year for acorns and considers the boom-and-bust cycle a “happy coincidence” for trees, since it keeps herbivores in check during off years. 

“My own hypothesis about oak population dynamics is that this group of trees co-evolved with a certain herbivore that loves to eat acorns, stashes the acorns in hidey-holes all over the place, and has a very short life expectancy: the squirrel,” he said. “Every squirrel that gets eaten by a hawk or coyote leaves scads of stashed, uneaten acorns, some of which germinate to become new trees.”

Residents of San Antonio and the Hill Country have noticed the abundance of acorns carpeting yards, driveways, and patios.

Aimee Holland of San Antonio shared a photo of a mound of Burr Oak acorns that she found in Dignowitty Hill Park. “These are BOMBS! They are HUGE!” she wrote.

The bed of Craig Chew’s pickup after parking overnight under an oak tree Credit: Courtesy / Craig Chew

“You should see my driveway, yard and the bed of my pickup,” said Craig Chew of Pflugerville in a post on the Mason County Community Facebook page. “Covered in acorns.” 

Chew sent a photo of his pickup bed, pointing out “this is from this morning, after I cleaned it out yesterday.”

Cynthia Jipson, a resident of Mason, a Hill Country deer and pig hunting destination each fall, claimed she hasn’t seen that many acorns “because the deer are seeing them first.”

Deer benefit from mast years, said Michael Nentwich, an arborist for the Etter Tree Co. and San Antonio’s former city forester. Nentwich pointed out that hunting could suffer in this mast year because corn typically provided by hunters will not be as effective in luring deer because they can have their fill of acorns. 

“This is assuming there are abundant crops of acorns the deer prefer,” he explained, adding that white oak acorns are considered tastier since they have less of the tannic acids of red and live oaks. Tannins make acorns taste bitter. 

Nentwich contended that deer definitely prefer fruit, nuts, acorns, and forbs over corn. Best isn’t so sure.

“Do they know that acorns are better for them than a surplus of nutrient-poor corn?” asked Best. “Who knows? It’s kind of a philosophical question. Deer will walk right out into a sendero during hunting season to eat free corn off the ground.”

Burr Oak acorns in Dignowity Hill Park Credit: Courtesy / Aimee Holland

Deer and squirrels aren’t the only creatures that gather and consume acorns. Henry David Thoreau was known to eat white oak acorns, finding them “unexpectedly sweet and palatable.” He compared their taste to chestnuts. The acorn was considered a reliable staple food for thousands of years and in North America was collected and stored each fall to provide nutrition before wheat was cultivated.

Acorns are making a comeback in foraging circles. A recent Wall Street Journal article headlined “Humans Are Gobbling Up Acorns, Driving Squirrels Nuts” and datelined Seoul, South Korea, explained how acorns’ newfound status as a “superfood” that can fight diabetes and obesity is challenging the squirrel population there. Humans are gathering so many acorns that the squirrel population in Seoul is dwindling.

Accumulating all that nutrition requires effort, however. Foragers must collect the oak nuts, then crack, peel, and leech them to make a nutritious acorn flour. The process can take days, weeks, or months, depending on the acorn species. Since different species of oak contain higher levels of bitter tannins, soaking the shelled acorn meat in a water bath for multiple days is required for good-tasting acorn meal.

In her recently published book Eating Acorns: Field Guide-Cookbook-Inspiration, author and “oakmeal” activist Marcie Lee Mayer waxes nostalgic and poetic about the lost appreciation of acorns as food for people. Citing its high iron, protein, healthy fat and antioxidant content, Mayer calls acorns “a vital nutrition source for thousands of years” and includes recipes for acorn grits, muffins, and soup as well as acorn tortillas, brownies, and cookies. Acorn recipes are also available online.

But some prefer to leave the nut meat on the ground where it lands.

“My favorite acorn recipe?” asked botanist Best. “No. 1, let acorns fall on ground. No. 2,  let deer eat acorns. No. 3,  slay deer. No. 4, cook and eat deer.”

San Antonio Report co-founder Monika Maeckle writes about pollinators, native plants, and the ecosystems that sustain them at the Texas Butterfly Ranch website. She is also the founder and director of...