The underside of the overcup oak acorn. Photo by Chris Evans, University of Illinois, Bugwood.org
Tree of Merit: Overcup Oak (Quercus lyrata)
Story by Cene Ketcham, Extension Arborist, Casey Trees
Overcup oak (Quercus lyrata)—sometimes known as swamp post oak or water white oak—is a tough shade tree in the white oak group. Though it can sometimes be difficult to source, its popularity for use along streets and in the landscape has been increasing—a testament to the growing interest in this species. At first glance, overcup oak is sometimes confused with swamp white oak (Q. bicolor), post oak (Q. stellata), or with its close relative, bur oak (Q. macrocarpa). Although it shares some attributes with these species, closer inspection reveals a tree with a character all its own.
In the swamp forests it inhabits in the wild, it can reach heights of 100 feet or more (the current champion, in Isle of Wight, Virginia, is 109 feet tall with a crown spread of 103 feet). However, in an urban setting it is more typically around 40 feet high and wide. The mature crown is rounded and quite consistent between individuals; in youth, it is more pyramidal-oval. It requires little pruning to develop good structure. Branches are generally well-spaced and resistant to breakage, with lower branches upswept, reducing the need for crown raising along streets and sidewalks.
Its fall color is not particularly showy and its light gray, scaly bark (similar to white oak or swamp white oak), while interesting, is not especially remarkable. It is instead distinguished by its utterly unique acorns and highly variable leaves. As you can expect from its common name, the nut of the rounded, 3/4 to 1-inch acorn is almost completely enclosed within the warty cap, leaving only a small opening at the bottom. This adaptation allows the acorns to float on the water in its swampy habitat.
Quercus lyrata foliage by Rebekah D. Wallace, University of Georgia, Bugwood.org
The specific epithet, lyrata, refers to the alleged lyre-shape of the leaves, though I have a tough time seeing the resemblance. Leaves are alternate, 4 to 8 inches long, with 3 to 5 pairs of blunt lobes. The lower two sets of lobes are smaller and separated from the upper by wide, square sinuses; the upper lobes are generally triangular. Leaves have a somewhat leathery texture, with deep green color above, pubescent and white to light green below. Twigs are gray-brown with gray or white lenticels and brown, ovoid buds are 1/8 to 1/4 inch long.
Native to the southeast United States, overcup oak is appropriate for Zones 6 to 9. A bottomland species that withstands regular inundation, it is perhaps the most flood-tolerant of all the oaks—perfect for wet sites, sites with heavy clay soil, or other poorly-drained locations. Consistent with other bottomland species in the urban setting, it is also fairly drought tolerant, making it an adaptable and versatile selection. Anecdotal evidence also suggests that it may be less susceptible to bacterial leaf scorch than many other oak species.
Overcup oak is compact, well-behaved, and susceptible to few pests and diseases. With its charming acorns and interesting leaves, overcup oak has features that can be admired by urban foresters and the general public alike.