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Oak Trees in Nature and for the Garden

Text by Brian Barth for the July/August 2017 issue of Horticulture.

oak trees oak sapling

Throughout the mountainous regions of California it is not uncommon to come across small bowls hollowed out in the surface of granite outcrops. I’ve seen them myself while hiking in the Santa Lucia Mountains of Big Sur. Circular and worn smooth, they are clearly not the work of Mother Nature, at least not entirely. These are the mortars where native Californians pounded acorns into flour for millennia.

Foods such as acorn mush, acorn bread and acorn soup were primary staples sustaining the indigenous groups of the state, just like in other regions of the world where oak trees are prominent in the landscape. Making food from acorns is extremely laborious—they must be shelled, dried, roasted, ground and then leached in running water for a period of days before they are edible. This helps to explain why oak trees resisted domestication as a nut crop in modern times and remain a beloved symbol of nature’s majesty instead.

Yet the oak tree does have a role in modern cuisine, just not as food. Acorns are high in tannins, the extremely bitter compounds that indigenous people learned to remove with leaching. But in low concentrations tannins, which are found in a variety of plants, lend desirable flavors to certain foods—the bitter flavors found in chocolate, coffee and wine, for example. While the seeds and skins of grapes contain some tannins, the primary reason that wine (as well as whiskey and many other alcoholic drinks) is aged in oak barrels is to further boost the tannin content to enhance flavor.

Oak barrels not only imbue such libations with the rich, complex flavors revered by connoisseurs, but the tannins in the wood are also high in antioxidants, giving credence to wine’s reputation as a healthful elixir. Herbalists refer to bitterness of oak tannins as astringent—a medical term for substances that cause the body’s tissues to dry out and contract—and they have long prescribed decoctions of oak bark for ailments ranging from intestinal inflammation to acne to infected wounds.

Not coincidentally, astringent, tannin-rich oak bark is one of the items that tanneries historically relied on to make leather soft, supple and water-resistant—the phrase “tanning a hide” refers to the tannins used in the leather industry. Cork is another example of how the unique qualities of oak trees have benefitted humankind; until recently bottle corks were made exclusively from the pliable, waterproof bark of Quercus suber, an oak native to southwestern Europe. Tannins and other compounds found in oak lumber also make it resistant to insects, rot and decay. Today oak is used primarily in fine furniture, though it was once the principal building material for ships.

Gardening With Oaks
The water-resistant nature of oak wood also comes into play in the ecology of oak forests, and thus in how oak trees must be cared for in garden landscapes. The 600-plus species of oaks worldwide are primarily upland plants—that is, they prefer dryish, well-drained sites on ridges, slopes and plateaus, rather than valley bottoms (though there are notable exceptions, such as Q. bicolor, the swamp oak). Oak trees by and large resent excessive moisture, which some gardeners, myself included, have learned the hard way after planting an oak in a poorly drained site or in a lawn where it receives constant inundation from sprinklers.

Newly planted oak trees certainly require modest irrigation during dry spells, though once established most species are quite drought tolerant. Gardeners who are fortunate enough to inherit a mature oak on their property are wise to plant species with minimal water needs in the understory, ideally drawing from the native plants associated with the oak in question. Frequent irrigation around mature oaks is often a catalyst for disease.

Oak trees are climax forest species, meaning they have evolved to thrive in well-developed native soils where the beneficial fungal organisms associated with them are present in the root zone. These are not the conditions typically found in the “disturbed” sites most gardeners confront around their homes, where the land has been bulldozed and graded for construction purposes. It’s not that you can’t grow oaks in typical garden conditions, but if you have the option to plant one in a more “naturalized” (i.e. less disturbed) corner of the landscape, your chances of success are vastly improved.

Oak saplings may grow slowly at first, but mature specimens are known to live for hundreds of years or more. The ancient Greeks revered oaks as a symbol of strength and longevity and considered them sacred to Zeus, the most powerful deity in their pantheon. In other ancient traditions, the oak is the archetypal Tree of Life, a symbol of wisdom and harmony with nature. In modern times, to plant and care for an oak is to participate in a long and glorious history—and to make a gift to future generations.

Brian Barth is a writer and landscape architect whose work can be found in Horticulture, Modern Farmer, Landscape Architecture and other publications.

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