Tree of the Season: Bur Oak
This Quercus tree is native to the Great Plains of the Midwest, and even a few successful cases have found a home here on the Maryland Piedmont in parks and urban settings. Quercus macrocarpa literally means large-fruited. The commonly used name Bur oak refers to the long-fringed structures found along the margin of the acorn cap.
Photo by Steve Houser, Arborilogical Service, Inc.
Bur oak grows to be a large tree with its crown often reaching wider than tall. Featuring a deep taproot, thick bark, and twigs armored with corky ridges, this tree was found thriving in a fire-dependent ecosystem known as savanna. A savanna is a mixed woodland-grassland ecosystem where the trees are spaced out enough allowing sufficient sunlight to fall upon a continuous grassy ground cover. In a text book example of savanna, the 50% tree canopy of a bur oak grouping never closes in completely, allowing an unbroken sprawl of grasses on the ground layer. These areas are believed to have been repeatedly burned by Native Americans. Oak savannas have been diminishing due to a long history of fire suppression and human development eventually converting many of these habitats into agricultural lands. Today, much effort is put forth to restore these rare habitats with the use of prescribed fire. Intentionally burning the chosen landscape, land managers are attempting to mimic this ecosystem as it might have been found in pre-Columbian times.
One adaptation exhibited by bur oak that makes it suitable for some urban planting sites is its rapid rate of taproot growth. According to USDA Forest Service, “researchers thought that rapid taproot growth allowed bur oak to tolerate the temperature and moisture fluctuations characteristic of prairie habitats.” This too may hold true for trees planted on urban sites where reflected sun light, minimal pervious ground surface for water collection, and drought conditions are all too common. Once established, this bur oak may prove to be a street-wise species selection if enough horizontal and vertical space is available. The bur oak is suitable for parking lot buffer strips, with that wide canopy creating shade and abating storm water. It is especially impressive in the landscape as a specimen tree. So keep an eye out for an open space in your parks.
417 E Oliver street tree example of an adolescent Bur Oak
As always, we must practice “right tree, right place” when figuring a tree species’ suitability in the landscape. Any plant’s sun, soil, and water requirements, growth habit, and tolerance to urban stressors must be considered before choosing a location. Or maybe you have a location already, you just need to identify a list of suitable species for that location. Either way, your tree will live and grow for many years to come if you plant it in the right location from the start.
Unlike many other oaks species, acorns are produced in the same year as the flowers. Propagating bur oak from its acorn is relatively easy. No pretreatment is necessary although storing seed for a month or two in the refrigerator in a moist sand is suggested. Next time you visit the Maryland Zoo, you might try to collect seed from the Baltimore City Champion Quercus macrocarpa. Go in late summer or early fall and get them before the squirrels do. According to John Bennet at mdbigtrees.com, this celebrated tree is the larger of two fine examples along the boardwalk and beside the bird aviary and gazelle pen.
Bur oak’s long taproot makes it difficult to transplant, but tolerant of drought, once established. According to Dirr’s Manual of Woody Landscape Plants, this tree is “very adaptable to various soils and is found on sandy plains to moist bottomlands. It succeeds well even in dry, clay soils; more tolerant of city conditions than most oaks.” When selecting a planting site, make sure to plant it in full sun.