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.
Above: Bur oaks can reach great size with a stout trunk and an open rounded crown. This example has 4 trunks rising from a common base. The largest trunk is 28 inches in diameter (90 inches in circumference) indicating an age of about 170 to maybe 190 years (using the tree age calculator chart).
Below: Young Bur Oaks, as shown here have more of a cone shape. The bark with age becomes thick and rough and deeply furrowed into scaly vertical ridges. Fall color is not considered to be bright - brown to rusty brown.
Below: 1st photo - New catkins of the male flowers as they first appear. 2nd photo - Extended Male flower catkins. These arise at the tip of last year's growth. 3rd photo - The small female flowers arise in the leaf axils of new growth.
Below: Twigs are stout, yellow-brown with corky ridges. 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.
Below: 1st photo - On older branches the corky ridges can become very pronounced. 2nd photo - A mature acorn and cap. 3rd photo - 3/4 to 1 inch long is typical acorn size in central Minnesota.
Below - Acorns: 1st photo - New acorns forming in mid summer. 2nd photo - Late Summer - note the upper scales have fringes. 3rd photo - An example of a set of 4 formed together.
Above & Below: Oak leaves can be quite variable. Bur Oak leaves are wide, ovate, broadest at the middle, (Fiddle-shaped) with many rounded lobes and a broad rounded tip that resembles a crown. Depth of the lobe sinuses is quite variable. Fall color can be a dull brown. See the comparison chart for other oak leaves.
Notes: (1) Ascent of the Oaks, Hipp, Manos, Cavender-Bares, SA, Aug 2020.
Bur Oak is indigenous to the Garden. Eloise Butler catalogued it on April 29, 1907. Bur Oak is found in the U.S. from Southeast Montana and Northeast Wyoming, over into the Dakotas and southward to Texas and then eastward to the coast except for the SE Atlantic states. In Canada its range is from Alberta to Quebec. It is the most northern of the New World native Oaks. In Minnesota it is widespread, being absent in 19 counties with most of those in the SW Quadrant.
There are six species of Oak generally found in parts of Minnesota that are not considered hybrids or rarities that have not been collected in the last 100 years. These are: White Oak, Q. alba; Swamp White Oak, Q. bicolor; Northern Pin Oak, Q. ellipsoidalis; Bur Oak, Q. macrocarpa; Northern Red Oak, Q. rubra. and Black Oak, Q. velutina. Chinkapin Oak (Chestnut Oak, or Yellow Oak), Q. muhlenbergii was native but is historical only now, last collected in 1899; Scarlet Oak, Quercus coccinea, can also grow here as can Pin Oak, Quercus palustris, but neither are native.
The Wood & Uses: Bur Oak is useful for shelter belts and the wood is somewhat valuable for cabinetry and flooring where it is marketed as White Oak, but is inferior to the actual White Oak. Francois Michaux wrote that the wood “is little esteemed in the United States.” (Ref.#26c) The acorns, while not abundant in some years, are edible although bitter and with some tannin. Bitterness is removed by leaching and the tannin by boiling. Native Americans made use of most acorns for acorn-flour. Fernald (Ref. #6) has some detail on how this was done.
It is difficult to visually identify a piece of oak lumber as Bur Oak but it can be at least isolated to the White Oak group rather than the Red Oak group. In the White Oaks the tallest of the largest rays are greater than 1-1/4 inches and the latewood has numerous small pores that grade into invisibility, whereas the Red Oak group has the tallest rays less than 1 inch and the latewood pores are few and distinct. A small hand lens is necessary to see this clearly.
Bur Oak Blight: This blight causes wedge shaped discoloration areas on leaves leading to dead tissue. Veins on the underside of the leaf turn to purplish-brown. The overall effect is as though the leaf was scorched. The leaf base has black dots. Severe infections result in dieback of the affected branches. Affected trees will show symptoms in July and August, starting in the lower canopy and working upward. After several years of infection the tree will be weakened and fall prey to other diseases which actually kill the tree. Within Minnesota the blight has progressed northward from the southern counties and has now (2013) reached Hennepin and Ramsey counties. The blight is caused by a fungus that is new and as yet un-named but thought to be a species of Tubakia. Affected trees keep the damaged leaves overwinter - which a Bur Oak normally does not do. More details in this pdf from USDA.
References: Plant characteristics are generally from sources 1A, 32, W2, W3, W7 & W8 plus others as specifically applied. Distribution principally from W1, W2 and 28C. Planting history generally from 1, 4 & 4a. Other sources by specific reference. See Reference List for details.
Identification booklet for most of the flowering forbs and small flowering shrubs of the Eloise Butler Wildflower Garden. Details Here.
Friends of the Wild Flower Garden, Inc. Text and photos are by G. D. Bebeau unless otherwise credited. "www.friendsofthewildflowergarden.org"