Oaks are a strong, long-lived tree and a valuable wood. Beyond those facts, however, lie confusion and unproven folklore.
In fall, for example, oak owners worry about the trees' cap-wearing acorns, which turn yards into a minefield of woody marbles: Acorns are poisonous--aren't they? Or are they just toxic to dogs? Or is that cows?
Gardeners also worry about the oaks' multitudes of falling leaves: Don't they acidify the soil and change its pH? Will they leach out enough tannin to kill grass?
"All of those ideas contain an element of truth," said Ward Upham, K-State Research and Extension horticulturist. "How true they are, however, depends on where you live, the time of year and the amounts you're talking about."
Oaks are one of nature's best sources of tannin--a substance long used in making leather (tanning hides), as well as dyes and ink. Because it's a phenolic compound, tannin is a corrosive and poisonous acid, Upham said.
Yet, diluted tannin can be a good antiseptic. The compound is important in clarifying wine and beer. Tannin also gives tea its color, astringency and some of its flavor.
"So far as Kansas landscapes are concerned, none of that matters much, so long as you're willing to handle acorns and oak leaves correctly," Upham said. "If nothing else, our soils are so alkaline that a little tannin won't have much of an effect."
For certain animals, however, oaks' most dangerous, high-tannin times of year are spring, when oaks' tender buds and young leaves emerge, especially if a storm downs a limb, and fall, when the acorns fall, particularly while they remain somewhat green.
Without doubt, acorns can be toxic. Ironically, they're also an important food source for many kinds of wildlife, Upham said. Any yard with an oak tree also has multiple squirrels, jays and more.
In the wild, black bears and white-tail deer will shift their range to take advantage of a good crop. Other acorn fans include voles, foxes, mice, rabbits, raccoons, mallards, quail, turkeys, wild hogs and woodpeckers.
Apparently, tannin poisoning is mainly a problem for animals that can't roam to find alternate water and food sources, Upham said. For example, dogs and cattle both have been poisoned by drinking water in which oak leaves had soaked for a while.
Acorns' tannin can affect horses and chickens. But, cud-chewing animals (sheep, llamas, goats, cattle) seem to be most at risk--especially when other food options are limited. For poisoning to occur, oak edibles must make up a significant part of the livestock's diet over a period of time.
"Given our region's 2011's range conditions, that could be a possibility now where oaks are growing wild," he said. "Providing supplemental feed should help, though, as would fencing livestock away from the trees."
Unfortunately, playful puppies can be an exception to the cud-chewing rule. In a recent case, Chihuahua puppies chewed on a few acorns, developed bloody diarrhea and rapidly died of kidney failure. Swallowing an acorn whole could be life-threatening for pups with a small throat or narrow intestines.
For humans, the biggest problem with oak trees can be somewhat the same. Children seem fascinated with acorns. And when toddlers are still putting everything in their mouth, acorns can cause choking, he warned.
Nonetheless, unraked acorns can lead to slips and falls. Nuts left in place can lead to a backbreaking job: pulling up a generation of oak tree seedlings.
"U.S. human poisonings are kind of a moot problem. The work to make acorns edible hasn't been a common practice for a century," Upham said. "They're literally a hard nut to crack. Tannin makes the nut meat bitter, as well as toxic, and getting rid of it is time-consuming. Then, depending on the tree, the nut may be quite fatty."
Because Kansas soils tend to be so alkaline, Upham said, "What kills plants under our oak trees is the dense shade they create and the trees' highly competitive roots."
Mulching can be the best solution for such bare spots. Oaks don't like having their roots disturbed. They don't want as much water as a cool-season lawn might need. The weed part of weed-and-feed products can make them ill. They don't like irrigation water's hitting their trunk and/or pooling at their feet.
"In other words, what you've got under a healthy oak tree is dry shade--a category without many plant options," Upham said. "For planting soil, you can work with any unoccupied pockets you find. Or, you can use filled pots that you sink between the tree's well-developed and well-established roots."
Many gardeners like to use fallen oak leaves as a long-lasting mulch or rich compost ingredient.
"In either role, oak leaves can become a good source of soil nutrition. But, they're not going to decompose as fast as most tree leaves do," Upham warned. "So, if you don't remove or process them while they're still dry, they'll be likely to mat down into a layer. That kind of layer can kill a lawn that hasn't entered dormancy. It can shed water around ornamentals or create an 'uncooked' area in a compost pile that lasts until the pile is turned."
Some gardeners process oak leaves with a chipper/shredder, he said. Others mow as often as needed to break the leaves into fine pieces. Sometimes using a mulching blade improves results. Several major brands of mower sell special leaf-shredding attachments that can cost from $20 to $60.
"Because you can put them to better use elsewhere, you'll need to catch or rake up the pieces if you're mowing," Upham said. "Any grass clippings you happen to pick up, too, will be just fine."
For a while, tannin will leach out of the leaf pieces, he said. That's why they'll contribute most if used as mulch around acid-loving plants--evergreens, azaleas, rhododendrons, and oak trees.