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Quercus macrocarpa Michx. Bur oak

Leaf with typical pinnate lobing

Also known as: Blue oak, mossycup oak.

Habitat

Bur oak is rare in New Brunswick. It occurs only in the lower St. John River valley where it occasionally grows with other hardwood species on rich bottomland sites.

Form

Mature tree

Bur oak is a tree that can grow up to 18 m in height with stem diameters up to 50 cm. The crown is broadly rounded and open. Upper branches are angled obliquely upward but middle and lower branches are mostly horizontal and zigzagged.

Morphology

The leaves are deciduous, simple, and alternately arranged. They are 10–20 cm long, obovate, and pinnately lobed, usually with a broadly expanded end part with rounded teeth separated from the lower, tapered, slightly round-lobed part by two or three deep, rounded sinuses, and rounded lobes; sometimes, leaves are irregularly round lobed throughout.

Twig showing leaf scars raised from the twig surface

The upper surfaces are shiny and deep bluish green and the lower surfaces are pale, dull, and slightly hairy.

The twigs are stout, yellowish brown, slightly hairy, and often somewhat ridged. The lateral buds are conical to ovoid, blunt tipped, brown, and hairy. They are borne on slightly projecting portions of twig surface and tend to be appressed to the twig above. The buds near the twig ends are close together, sometimes abutting the terminal bud. Among these clustered buds are short, narrow-pointed, arching scales. The pith of a twig, when viewed in cross section, has five points.

Male catkins at the base of a new shoot and female flowers in the axil of the second new leaf along the shoot

Male flowers are borne scattered along limp, hanging catkins, up to 8 cm long, that emerge rapidly at the bases of newly expanding shoots. Once pollen is shed, the male catkins are shed. Female flowers are borne in small clusters on short stems in axils of new leaves farther along vigorous new shoots. The fruits, which are a kind of nut that is called an acorn, develop quickly from the female flowers. Each developing acorn is almost covered by a greyish-green acorn cup formed of a mass of closely packed scales with extended tips that form a bur-like structure with a distinct fringe around the portion of the acorn that is exposed. The originally green acorn gradually changes to yellowish brown as it matures in the fall.

Side view of a fruit-the acorn is mostly covered by the bur-like acorn cup

The bark is smooth and light ashy grey when young. When older it is deeply dark grey, furrowed with light-grey, flattened ridge tops.

Top view of fruit

Notes

Bur oak is in the “white oak” group of oaks that have round-lobed leaves and acorns that develop to maturity in one season. The “red oak” group, in contrast, have pointed-lobed leaves with bristle tips to the lobes and acorns that take two seasons to mature. Thus, a red oak commonly has acorns of two sizes in the fall, 1 season old (small ones) and 2 seasons old (mature ones).

Bark of stem, 76 cm in diameter

Bur oak wood is ring porous, hard, and strong, and can be used for furniture, interior trim, boat building, and, because it is of the white-oak group, barrels for storing liquid (woods of the red-oak group cannot be used to store liquids).

Bur oak withstands city environments well, so is useful for streetside plantings and other ornamental uses.