Bur Oak


Bur OakIn early October 2012, the UMD Arboretum planted bur oak trees to take the place of the Chapel Oak, our iconic white oak tree that died and was removed in the summer of 2012. The bur oak, or Quercus macrocarpa, is native to North America from the Appalachians to the Great Plains, south to Texas, and east to the coast. It is very drought tolerant, and common in dry uplands, sandy plains, and prairie grasslands. It has also displayed excellent tolerance to urban pollution and was named the 2001 Urban Tree of the Year by City Trees, the online magazine of the Society of Municipal Arborists. Bur oaks are often used for shade trees, and are very slow-growing and long-lived. They can eventually reach 100 feet in height with a long, clear bole and wide-spreading form. They typically grow in the open, away from the forest canopy, and are more common in the Midwest than in the east, because the forests are less dense and there is less competition for sunlight.

Our new bur oaks are 5 inch caliper trees, moved with 90 inch root balls, and were raised by nurseryman Joe Kraut at Pope Farm, the Maryland National-Capital Park and Planning Commission’s tree nursery in Derwood, Maryland. A member of the white oak group, the bur oak has alternate, simple leaves, 6 to 12 inches long, that are roughly obovate in shape, with many rounded lobes. The leaves are dark green above and lighter green and slightly fuzzy underneath. The thick bark is ashy gray to brown in color and scaly with vertical ridges. The extreme thickness of the bark enables bur oaks to survive forest fires. On this monoecious tree, the male flowers are yellow-green and borne on catkins, and the female flowers are green tinged with red, and appear as short spikes. The twigs are stout and yellow-brown, often with corky ridges.

The name macrocarpa means “large fruit,” and the acorns of the bur oak are the largest of all North American oak species—about 1.5 inches long. The fruit is half-way enclosed in a cap with long-fringe. The acorns mature in the same season, typically germinating soon after falling, and do not withstand drying. The size of bur oak acorns is determined by how far north the tree is growing. Trees in the southern part of the range produce larger acorns than those growing further north. Research has shown that this is likely due to environmental factors, such as temperature and rainfall (Koenig, et al., 2009).

The fruit is an important food for many mammals and birds, but heavy nut crops are borne only every few years in a cycle called masting. This is thought to be an evolutionary strategy to ensure the survival of some seed for the growth of new trees. The bur oak is the sole host plant of the Bucculatrix recognita caterpillar.

To develop the above description, we used many sources, including Michael Dirr’s Manual of Woody Landscape Plants (Champaign, Illinois: Stipes Publishing, LLC, 1998), and the following websites: Dendrology at Virginia Tech (dendro.cnre.vt.edu/dendrology/main.htm), the University of Connecticut Plant Database (www.hort.uconn.edu/plants/), Wikipedia (www.wikipedia.org), and the U.S. Forest Service (www.fs.fed.us). The research on acorn size was published by NRC Research Press in Botany volume 87, 2009, pages 349–356; and written by Walter D. Koenig, Johannes M H. Knops, Janis L. Dickinson, and Benjamin Zuckerberg.