Ubame oak (Quercus phillyreoides) in the Culberson Asiatic Arboretum, by George Oberlander.
Botanical name: Quercus spp.
Common name: Oaks
Family name: Fagaceae (Beech Family)
Native range: Northern Hemisphere
Location in Duke Gardens: Throughout
USDA Hardiness Zones: Varies by species
Imagine an oak (Quercus sp.). Is it a towering tree or a less imposing shrub? Are the leaf edges rounded or sharp—or are there any edges at all? Are the acorns tiny or gargantuan, and how fuzzy are the “caps”? Is the bark rough and ridged, or smooth and shiny, or somewhere in between? Does it grow atop rocky mountain ridges, the dense clay bottomlands of the Piedmont, the sandy coastal plains or outside the state of North Carolina entirely?
Depending on your answers, the oak you imagine might look very different from someone else’s, but believe it or not, they are all still oaks—and ones you’ll find here at Duke Gardens, no less. With so many different kinds of oaks out there, how do botanists and horticulturists determine what an oak is in the first place, let alone distinguish them from each other?
It turns out that the first part is relatively easy: oaks produce acorns, a special kind of nut with a husk that doesn’t fully enclose the fruit (what botanists refer to as a cupule, more commonly known as the cap). When acorns aren’t present, looking at dormant leaf buds (which come in distinctive clusters at the branch tips) in winter and the long catkins shedding pollen in the spring can also confirm an I.D. Anything beyond that, however, gets tricky fast, with leaves, bark and habitat so wildly variable than it’s impossible to generalize.
However, once you’ve successfully identified a tree as an oak, the sheer variety—more than 500 species worldwide—means identifying an oak to species is challenging even for the experts. A single oak tree can produce a wide variety of different leaf shapes. In fact, some species are distinguished from each other by minute details best viewed with a hand lens. Some oaks, such as the chestnut oak (Quercus montana) and the swamp chestnut oak (Q. michauxii), are morphologically similar but distinguishable in the field since they are found in diametrically opposing habitats (mountaintops and bottomlands, respectively).
On the opposite end of the spectrum, oaks like the ubame oak (Quercus phillyreiodes, pictured above) don’t appear oak-like at all at a distance and are easily mistaken for hollies or other species. Oaks can also interbreed with each other to create naturally occurring hybrids, such as the Bartram oak (Q. × heterophylla), a cross between the red oak (Q. rubra) and the willow oak (Q. phellos).
This incredible diversity, however, is what makes the oak a keystone species across multiple ecosystems and continents. It all comes down to their acorns, which are an important food source for birds, deer, rodents and other mammals including humans—all of whom are responsible for spreading uneaten acorns across the landscape and perpetuating their lifecycle.
To ensure that at least some acorns escape so many hungry critters, different species of oaks have evolved different strategies. The first is varying levels of tannins, extremely bitter organic compounds meant to discourage predators. The second is varying life strategies, with oaks in the white oak group (section Quercus) taking a single year to mature, while oaks in the red oak group (section Lobatae) taking two years. Thirdly, oak species alternate how many acorns they produce each year, resulting in bumper crop “mast years” that overwhelm hungry animals before starving them out in the lean years that follow.
Here at Duke Gardens, visitors will encounter oaks of all ages, shapes, sizes and origins in every section of the gardens as part of our APGA Nationally Accredited™ collection, which are accessioned and labeled for easy identification. Be sure to check out an interactive virtual tour of our collection’s highlights as well.