September–October 2021

Minnesota Profile

Bur Oak (Quercus macrocarpa)

By far the most widespread tree, occurring in every county in the state.

Welby Smith

Appearance. When grown in the open, bur oak develops a stout, unyielding trunk and a broad crown supported by sturdy, gnarled branches. Grown in a forest environment, it can develop a tall, straight trunk 3 to 4 feet in diameter with a narrow crown that can tower 110 feet above the ground. With practice, all the oaks can be told apart by the shape of their leaves, but the surest way to identify a bur oak is by the signature acorn. It is the largest of any oak and has a tangled, woolly fringe, resembling a rough edge or burr, which gives the tree its name. If you don’t see acorns on the tree, look on the ground beneath the tree.

Habitat. Bur oak is one of the most adaptable trees in Minnesota. It can survive easily in the driest, poorest soils, where it will grow slowly but persist for centuries. In moist, rich soils along rivers, it will grow more quickly and become a prominent canopy tree. Because of its resilience in the face of fire, it is often the dominant tree where prairie grades into forests, a habitat often called savanna.

Range. Bur oak is a common tree over much of the eastern and central United States and adjacent parts of Canada. Of the seven species of oak native to Minnesota, bur oak is by far the most widespread, occurring in every county in the state. Although its numbers have been in steady decline since the earliest days of European settlement, it is still a common tree.
Natural History. Bur oak spends much of its early years growing roots. At the end of the first growing season, bur oak may be only a foot tall, but the taproot may reach more than 4 feet into the ground and the lateral roots may spread more than 2 feet to either side. It has perhaps the deepest, most extensive root system of any tree in Minnesota. It is often said that most of a bur oak tree is below ground. In May, before the leaves have reached half-size, the male flowers can be seen dangling from thread-like catkins. The seed is the familiar acorn, which matures in August and is dispersed by mammals and birds that will often cache the seeds for future use.

Uses. Bur oak makes a superb shade tree around houses—there may be none better for a Minnesota home. It harbors more animal life than almost any other tree, being home to birds, mammals, and an amazing variety of invertebrates. And yet bur oak resists damage from browsers and parasites as well as wind and ice. It is, however, susceptible to oak wilt, a serious fungal disease affecting all oaks. Lumber made from the wood, sold as white oak, is valued for its beauty, durability, and strength. These are qualities exhibited by the living tree as well as the furniture, flooring, and cabinetry made from its wood.