Trees and Shrubs of the Eloise Butler Wildflower Garden

The Oaks of the genus Quercus account for more tree biomass than any other species in North America and Mexico (1). Bur Oak is sometimes found as a shrub in parts of North America but more normally as a large native deciduous tree growing 50 to 100 feet high and up to 4 feet in diameter with a straight stout trunk and, when mature, a broad rounded open crown of stout but often crooked corky-ridged branches. Young trees have a more cone shape.

The largest known Bur Oak in Minnesota is in Olmsted County, measuring 68 feet high, 80 foot crown spread, 278 inches in circumference and scoring 366 points. The national champion is in Posey IN, measuring 99 feet high, 128 foot crown spread, 295 inches in circumference and scoring 426 points.

The bark is light gray and with age becomes thick and rough and deeply furrowed into scaly vertical ridges, sometimes with corky wings. There can also be smooth patches - typically of oaks in the White Oak Group.

Twigs are stout, yellow-brown, finely hairy, with corky ridges on older twigs. Buds are small but of more brown color with many at the tip of the twig and the laterals at the top of the old leaf scars.

Leaves are alternate and simple, from 4 to 10 inches long and 2 to 5 inches wide, ovate, broadest at the middle, (Fiddle-shaped) with many rounded lobes and a broad rounded tip that some say resembles a crown as the lobes appear more as large teeth instead of lobes. There is considerable variety on how deep the lobes cut to the central rib. On some the indentations are slight, on others the two middle lobes are divided almost to the central rib. One must view a number of leaves to make a correct identification. Color is dark green and slightly shiny on top and gray-green with fine hair under. The leaf stalk is short. Bases are rounded to pointed. Fall color is a rusty brown. These leaves are the largest on our native oaks in North America. The largest leaves tend to be in the juvenile part of the tree.

Flowers: The tree is monoecious, that is with separate male and female flowers. Male flowers are yellow-green and occur on drooping catkins that are up to 4 inches long, at the tip of last year's growth. Each male flower has six stamens (can very from 2 to 12), which have long spreading white filaments when the flower fully opens. The female flowers are more reddish-green and appear as small slender spikes in the axils of new growth. They have 3 (to 6) carpels and styles that are slender and short. There are no petals; both sexes have the sepals are fused together. Both flowers appear following the leaves but the male pollen is released before the female flowers on the tree are ready, thus preventing self pollination. Pollination occurs by wind from adjacent trees. Single landscape specimens will therefore not produce seed.

Seed: The female flowers mature to a large broadly elliptical acorn, the largest of our native oaks, 3/4 to 2 inches long, 1/2 to 3/4 inches wide, enclosed in a deep cap (oak acorns are highly variable as to cap coverage and acorn size by geographic location) with broadly triangular scales of which the upper group are long-pointed giving a fringe-like look. The acorn matures in late summer into autumn of the first year as do most species in Quercus Sect. Quercus - the White Oak Group. They appear in groups of 1 to 3 (4 happens - see photo) on a short stalk. The nut is light brown to grayish and can germinate immediately on falling from the tree. Dispersion is by animals. Trees require considerable age before bearing - upwards of 35 years. It is not uncommon for trees to bear heavily one year and sparsely the next.

Habitat: Bur Oak prefers rich moist soil and is frequently found in bottom-lands. It will grow on hillsides and drier spots but needs full sun as it is only moderately shade tolerant but very drought tolerant. It is the most cold tolerant and the northern most new world Oak. It has a deep taproot and wide spreading laterals. It can re-sprout from damaged trunks if not too old. Bur Oak is susceptible to a blight - details below.

Names: The genus name, Quercus, is the Latin word for Oak. The species, macrocarpa, is formed from two words and means large fruit and refers to the large acorn of this species. The author name for the plant classification - ‘Michx.’ refers to Andre Michaux (1746-1802), French botanist who made many exploring expeditions in the U.S. collecting and cataloging many species. Two important works of his are the Histoire des chênes de l'Amérique septentrionale (1801 - Oaks of North America), and the Flora Boreali-Americana (2 vols., 1803, published posthumously, in which was the description of this species). His son Francois, traveled with him and the father’s notes were later used for his 3-volume North American Sylva.

Comparisons: The oak whose leaf is closest to Bur Oak is the White Oak, but with White Oak, most of the the lobes are more deeply cleft. Also the acorns of White Oak don't have this pronounced a fringe at the top of the cup and instead have appressed hairs. White Oak bark tends to have more smooth patches. A comparison of common oak leaves is shown on the Oak Leaf Comparison sheet.