Bur oak

Description

Bur oak is a slow growing, long-lived, deciduous tree rising on a single trunk from a taproot and widely spreading roots. The root system is massive—the weight of the root system equals the weight of the above-ground growth of the tree. In Minnesota mature trees are usually 50′ to 80′ tall and up to 43″ in diameter, though individuals can reach over 100′ in height.

In open areas with deep soil the trunk is straight and is distinct to the upper crown. In less favorable conditions the trunk splits into heavy, gnarled, ascending branches.

The branches are ascending to spreading; ascending in the upper part of the crown, nearly horizontal in the lower part. The crown of young trees is tall and oval with a rounded top. In open areas the crown of mature trees can be very broad.

The bark on young trees is rough. On mature trees the bark is thick and ashy gray, with deep furrows and ridges that are broken into irregular, dark gray scales.

The twigs are stout, yellowish- to grayish-brown, and slightly hairy. Branchlets often develop flat, radiating, corky wings. Terminal buds are reddish-brown, hairless, round, 1 ⁄16″ to ¼″ long, and blunt at the tip. Lateral buds are 1 ⁄16″ to 3 ⁄16″ long and are closely appressed to the twig.

The leaves are alternate, leathery, 4⅓″ to 8″ long, 2¾″ to 6″ wide, and inversely egg-shaped in outline but otherwise variable in shape. They are on hairy leaf stalks that may be ⅜″ to 1 3 ⁄16″ long, but are usually ⅝″ to 1″long. The leaf blade is rounded or wedge-shaped at the base. There are 2 to 6 large, irregular, rounded, primary lobes and 2 to 10 smaller, rounded, secondary lobes or rounded teeth per side. The deepest sinuses, near the center of the blade, are usually very deep, extending 50% to 90% of the way to the midvein. Most leaf blades are fiddle-shaped, with a broad, expanded, shallowly lobed upper half (terminal lobe) above a deep sinus and a few short lobes on the lower half. The upper surface is dark green, shiny, and hairless or sparsely hairy. The lower surface is pale green or gray and densely hairy with short, appressed, star-shaped hairs, making it velvety to the touch.

Male and female flowers are borne on the same tree. Male flowers are in slender, ¾″ to 2⅜″ long catkins that hang downward from buds on branchlets of the previous year. Female flowers appear in clusters of 1 to 3 on a short stalk rising from leaf axils of the current year’s twigs. The flowers appear after the leaves in May to early June.

The fruit is a large, narrowly egg-shaped to flattened egg-shaped, 9 ⁄16″ to ⅞″ long, ⅜″ to ⅝″ wide acorn. This is the largest acorn of any of the native oaks. They are in clusters of 1 to 3 acorns on a short, stout stalk. A scaly, dome-shaped cup encloses ½ to ⅞ or more of the lower part of the nut. There is a conspicuous fringe of soft, curly, 3 ⁄16″ to ⅜″ long awns along the rim of the cup, which gives this tree its common name. The inside of the acorn cup is uniformly hairy. The nuts ripen in early August to early mid-September of the first year. It tastes sweet or slightly bitter.

Height

50′ to 80′

Record

The champion bur oak in Minnesota is on private property near Rochester, in Olmsted County. In 1999 it was measured at 68′ tall and 277.5″ in circumference (88.3″ in diameter), with a crown spread of 155′.

Flower Color

Green

Similar Species

Bur oak (Quercus macrocarpa var. depressa) is found along the western margin of the Great Plains. It is a shrub or small tree no taller than 16′ that occurs on bluffs and hillsides. The acorns are smaller with smaller, less fringed cups.

White oak (Quercus alba) bark is thin and fine-textured. The branchlets do not have corky ridges. The lateral buds diverge from the twig. The leaves are more uniformly lobed and never has a large terminal lobe. The underside of the mature leaf is hairless or has a few hairs along the main veins. The acorn cup encloses only the lower ¼ to ½ of the nut, and does not have a fringe of awns along the rim.

Habitat

Moist to wet; drought tolerant. Prairie borders, dry hillsides, river valleys, savannas, bottomlands. Full to partial sun.

Ecology

Flowering

May to early June

Pests and Diseases

Jumping oak gall wasp (Neuroterus saltatorius) appears as small, BB-sized growths on the underside of leaves and wart-like brown spots on the upper surface of leaves of oaks in the white oak section.

Oak flake gall wasp (Neuroterus floccosus) causes fuzzy, buff to light brown galls along the veins on the underside of leaves and buff, wart-like bumps on the upper surface of leaves of oaks in the white oak section.

Oak Leaf Blister (Taphrina caerulescens) is a fungus that causes blister-like galls on the upper leaf surface.

Use

Bur oak is the official State Tree of Iowa.