The Truth About Fertilizing Oak Trees

Professor Craig Harper at the University of Tennessee in Knoxville routinely repeats this advice when hunters request fertilizing tips for oaks. Harper said hunters typically want a quick way to produce fat, juicy acorns too sweet for deer to resist.

“Most deer hunters only think about this during deer season,” Harper said. “If they have oaks where they hunt, they think acorns are the deer’s primary food source, and they want an easy way to transform acorns into bait. They’ve read or heard that fertilizer is magic dust. I have to tell them they’ve been misled; that they’ll just waste their time, sweat, and money hauling fertilizer into the woods.”

Harper, a wildlife management professor and UT Extension wildlife specialist, traces oak-fertilizing falsehoods to mass advertising from the 1990s through the 2000s. Harper assumed fertilizer companies had research to back up their claims, and so he talked to a sales representative in 1998 to learn how their fertilizers sweeten acorns and boost production.

“I was seeing claims galore in hunting magazines and TV shows about the many benefits of fertilizing oaks,” Harper said. “I thought maybe I’d missed something, and wanted to learn more. I talk to thousands of people every year through our agricultural extension program, so I can’t just recommend something without studying data from research. When I got that sales rep on the phone, I soon learned his company had nothing for data. And I mean nothing. The best he could say was, ‘Well, it stands to reason that fertilizing would help.’ I had to end the conversation. I felt so naïve for thinking they’d never make claims without supporting data.”

Acorn Research That disappointment inspired Harper and several UT graduate students to launch a 10-year study in an upland oak/hickory forest in eastern Tennessee to learn what could be done to boost acorn production. After all, deer love acorns, and acorns are more nutritious than corn—they provide twice the amount of carbohydrates and 10 times the fat.

But acorn crops are also fickle. The UT study found white oaks produced abundant acorn crops two out of every five years, and some years produced none. In fact, the study labeled the mast crops near-complete failures in six of 10 years.

Farther north, Penn State University researchers report acorn crops occur almost every other year in black oaks (a red-oak subspecies), every third year in white oaks, and every four years in Northern red oaks. That report by professor James Finley noted that weather factors explain the loss of 55% of black-oak acorns and up to 89% of red-oak acorns. Finley put summer drought atop the list, followed by spring temperatures, and spring-frost dates.

Given that acorns are an unreliable food source, deer find ways to get along without them. Some of the continent’s biggest whitetails, in fact, live in Western cottonwood bottoms and Northern conifer/hardwood forests that are totally absent of oaks. Biologists also note that whitetails give birth, raise fawns, build body mass, and grow antlers during spring and summer when acorns are least available. Whitetails rely far more on green, leafy plant growth during those critical months.

But deer pounce when acorns fall, so Harper and his students set out to learn if fertilizers boost production. They also studied how opening an oak’s canopy with chainsaws and herbicides, also known as “releasing,” can help oaks grow, and how those regimens’ combined impacts affect acorn size, quality, production, and consumption by wildlife.

Collecting Data The researchers started by collecting data for five years on acorn production, which meant monitoring 120 individual white oaks before applying fertilizers or felling trees. That analysis included ranking each tree’s acorn production as poor, moderate, good, or excellent.

Next, they chose which trees to fertilize and which ones to “release” by felling, killing, or trimming surrounding trees. Mature trees won’t grow any taller, and lower branches stop when they encounter branches on neighboring trees. By releasing an oak’s canopy from competition and allowing sunlight to reach its outer branches, the tree resumes growing and generating more acorn-bearing twigs.

Harper’s team then monitored the site for five more years to assess how the oaks responded to fertilizing and releasing. Acorn production varied by tree, but the study showed that a few good trees produced most of the site’s acorns.

More specifically, the 11% of oaks classified as “excellent” producers generated 31% of all acorns, and the 41% of oaks classified as “poor” producers generated 17% of all acorns. Overall, the 39% of oaks classified as “good” or “excellent” producers generated 69% of the acorns.

Further, by comparing fertilized and released trees to “control” trees that received neither treatment, the researchers found that canopy-released trees increased their acorn production by 65%. Further, oaks that were fertilized and had their canopies released increased their acorn production by 47%.

In both cases, researchers noted the treatment effects were greatest on oaks classified as poor producers. They also learned that fertilizing didn’t affect acorn size, production, or quality.

Release treatments generated greater benefits by allowing more sunlight to reach the trees. Their crowns often increased by 25% the first year after the release treatments, helping them increase acorn production by over 50% per tree. Harper also reported that by allowing as little as 30% more sunlight into a previously closed canopy, the woods typically increased deer forage by 7 to 8 times.

Quality Acorns To assess acorn quality, Harper’s team analyzed the acorn weights, fibers, phosphorus content, crude proteins, carbohydrates, and digestible nutrients. They found little difference in acorn quality data from fertilized and unfertilized oaks. Regarding sweetness, it would take a discerning palate to detect the difference between fertilized (10.1% carbohydrates) and unfertilized (9.8% carbohydrates) acorns.

“If fertilized acorns were sweeter or of higher quality, we would have found significant differences in sugars and total digestible nutrients, but we didn’t,” Harper said.

All those reasons lead Harper to suggests that hunters spend a few years identifying poor acorn producers, and then consider removing them so good producers can expand production. “You can kill or fell up to 50% of your poorest acorn producers and still increase the stand’s overall acorn crop,” he said.

Harper knows he won’t convince all landowning hunters to surrender their fertilizer spikes. After all, the power of suggestion and the belief you’ve “done good” can blind us to harsh realities. But at least fertilizer companies are now careful about making unverified claims.

“We’ve published our data in the scientific journals, and we share it everywhere we can in the popular media,” Harper said. “All we ever asked the fertilizer companies was to show their data. Why won’t they show it? Because it doesn’t exist. It’s just that simple.”