Bur Oak (Quercus macrocarpa), a massive and majestic Oak, is found in all of Ohio except for the southeastern portion of the state, although it is most frequent in the western half of the state. Bur Oak is an Oak primarily of the Midwestern and Great Plains states, venturing into south-central Canada to the north, and Texas to the south. It is found in both continuously moist as well as very dry sites, and is the Oak that is most often found off by itself, standing alone in a farmer's plowed field or in a flat bottomland.
Along with White Oak, it is the most picturesque of the Oaks of the Midwest, especially in winter. Its fiddle-shaped leathery leaves, huge fringed acorns, thick and sometimes corky twigs, and deeply ridged bark add to its bold texture. Bur Oak has the alternative spelling of Burr Oak, and may also be referred to as Mossycup Oak, in reference to its fringed, rough-capped fruits that to some resemble acorns covered in moss. It may reach 90 feet tall by 80 feet wide at maturity, when found in the open. As a member of the White Oak group and the Beech Family, it is related to the Beeches, Chestnuts, and other Oaks.
Bur Oak is very adaptable to soils that are permanently moist, moist but well-drained, seasonally dry, or very dry. The soils may be acidic or neutral, but ones that are mildly alkaline in pH are preferred. Deep soils that are sandy, organic, average, or composed of clay are all acceptable to this Oak that, like White Oak, is especially tap-rooted and often very long-lived. Its large size makes it an excellent choice for parks and other areas with wide-open spaces. It thrives in full sun to partial sun (but is somewhat shade tolerant in youth) and is found in zones 3 to 8.
Bur Oak may rarely develop rounded galls on its twigs and branchlets, the result of chewing by the Oak rough bulletgall wasp. Unless infestations are especially dense, no long-term damage is done. Otherwise, Bur Oak will be subject to the usual array of pests and pathogens that can affect many Oaks.
Bur Oak has large, bold-textured, leathery leaves that are alternate, obovate, and usually dark green. Leaf shape is highly variable in Bur Oak, but consists of about five to nine lobes of greatly different size, with the lower lobes very small, the central lobes average, and the upper lobes very large with crenate margins. The sinuses in the central portion of the leaf may be shallow, but are usually very deep (sometimes to the midrib), giving this leaf the approximate shape of a base fiddle.
Fall color is usually chartreuse to yellow brown and ineffective, but leaf drop is usually complete. Even in summer, the bold-textured, foliaged canopy of a mature Bur Oak is an impressive sight
Bur Oak is monoecious, having pendulous pollen-bearing catkins in mid-spring that fertilize the miniature female flowers on the same tree. Its massive fruits (acorns) only take a single season to develop, and are nearly completely covered by a rough, frilled cap, ripening in early to mid-autumn. The acorns may be initially confused with those of Sawtooth Oak, but are much larger, cover more of the nut, and are its frills are not as elongated and showy as those of the imported species.
The terminal buds of Bur Oak are blunt and often large, occuring on its stout twigs.
Branchlets are sometimes corky, while at other times moderately rough, but non-corky.
Mature bark becomes highly ridged and deeply furrowed, with the long, longitudinal fissures often recognized from a distance as belonging to Bur Oak. This species has medium gray (sometimes dark gray) color to its mature bark that is characteristic of the White Oak subgroup.
The growth habit of Bur Oak is bold-textured and majestic, especially in winter, when its rugged limbs, stout twigs, and massive trunk with deeply furrowed bark can be seen. The upright growth habit of youth and middle age gives way to a broad-spreading canopy with age.