Scientific Name
Quercus macrocarpa
Bur oak is primarily a tree of the U.S. Midwest, and its native range interestingly includes both southern Manitoba and Texas down to the Gulf of Mexico, hinting at its toughness and adaptability. The state tree of Iowa and the classic oak of the prairies, it is a somewhat slow-growing species capable of living more than 300 years. Its tolerance of drought, cold and alkaline clay soil gives it an advantage in our urban areas, where it often grows to 75 feet or more in height, with the Colorado champion reaching 97 feet. Bur oak wood is also valued as timber, being attractive and durable. As the tree matures, it develops a rugged and majestic outline, with many strong horizontal branches.
Bur oak is a member of the white oak group, characterized by leaves with rounded lobes lacking bristle tips and by acorns that mature in a single growing season. These acorns are also much less bitter than those of the red oak group and, from some trees, can even be palatable raw. The leaves vary considerably depending on geographic origin and on their position in the tree; they may be as long as 13 inches or as short as 3 inches. What they almost always share is a pair of especially deep sinuses — a “narrow waist” — and more leaf area beyond the midpoint than near the stem.
Bur oak flowers in the spring as the leaves are just beginning to form. The female flowers are inconspicuous, but the wind-pollinated male flowers appear as long, hanging catkins. The acorns are among the largest of any oak, with fringed caps that cover nearly the entire nut, giving rise to the “bur” in the tree’s name. As with many oak species, a year of heavy acorn production is often followed by a year or two of lighter crops.
An unusual feature of this tree is that the younger twigs often develop longitudinal corky wings, giving them a rough appearance. In addition, a species of gall-producing wasp specializes in attacking young bur oak twigs, producing spherical galls that can sometimes become quite numerous. Although they may look unsightly, these galls rarely cause serious harm to the tree.