The adjective that is most often used to describe bur oak is “majestic” and it fits. This long-lived native oak reaches a height and spread of up to 90 feet, hence it is not the tree that you would want to plant in your average urban back yard. But if you have space, it is an excellent shade tree for a sprawling landscape.
The fall foliage of bur oak is unremarkable, but the tree makes up for it with its deeply ridged bark and corky twigs that stand out, especially during the winter. The gray bark has deep furrows and grooves that become more distinct as the tree ages. Bur oaks are long-lived; some specimens are estimated to be 300 years old.
Landscape architects value bur oak as a shade tree because of its symmetrical canopy. It’s also very tough. Bur oak is not affected by air pollution, it adapts to all kinds of soils, even compacted types with poor drainage, and it is drought-tolerant.
In common with all oaks, the bur oak has acorns—whose bur-like covers gave the tree its name. The large acorns attract wildlife who cart them off and gobble them up. Blue jays, deer, squirrels, and other small mammals eat the acorns, and the acorns taste better to wildlife than those from red oaks because they contain less tannin making them less bitter.
|Botanical Name||Quercus macrocarpa|
|Common Name||Bur oak, mossycup oak|
|Plant Type||Deciduous tree|
|Mature Size||70 to 90 height, 60 to 90 feet width|
|Sun Exposure||Full sun|
|Soil Type||Sandy, silt, clay, loamy|
|Soil pH||5.5 to 7.5|
|Native Area||North America|
Bur Oak Care
The most important thing to successfully grow bur oak is to make sure that there is sufficient space, both for the tree’s significant height and width and for the roots. Bur oaks have a deep taproot and their expansive roots grow more deep than wide. Though they are as a result of their wide roots less likely to damage sidewalks, it's still possible because the absorbing roots occur in the top 12" of soil.
Another important consideration when choosing a location for a bur oak is that while the tree is tough, it does not tolerate salt spray well. For that reason, it should not be planted next to a road or driveway with winter maintenance.
Bur oak prefers full sun with at least six hours of direct light.
The tree can adapt to many different soils, both in terms of texture and structure. It grows in sand, silt, and clay, and can withstand compacted soils and poor drainage.
Occasional flooding of the planting site is not a problem. However, the tree won’t do well when there is repeated and extended flooding, especially during the growing season.
Bur oak does best in alkaline soils with a high or neutral pH.
A newly planted bur oak needs to be watered deeply and regularly for at least two years after planting. In the absence of rain, continue to water the tree also during the third year growing season to make sure it develops a strong root system.
Once the tree is established, it is drought-tolerant thanks to its taproot that lets it draw water from the deeper layers of the soil.
Temperature and Humidity
Bur oak is well adapted to the continental climate of its native range with its humid summers and subzero winters. The tree can be found as far north as Nova Scotia and as far south as Texas. It is one of the most cold-tolerant oak species.
If at planting time, the soil is amended with organic matter, or the tree starts out in soil with a good nutrient balance, bur oak does not need extra fertilizer. In fact, a high-nitrogen fertilizer will harm the tree.
Propagating Bur Oak
If you don’t mind tending to it for a few years before the seedling is large enough to be transplanted, you can grow bur oaks from acorns.
Inspect the acorns for cracks and holes and choose only fully intact ones. Soak them in water for 24 hours, which helps you identify hollow ones that float on top. These should be discarded. Remove the caps if they haven’t fallen off on their own yet.
Fill a one-gallon planting container with a drain hole with the same soil as the location where you intend to plant the tree and mix in about one to two trowels full of organic matter or compost. The container might seem much too large at first but keep in mind that bur oaks are finicky to transplant because of their tap root so you want to avoid transplanting the seedling to a larger pot when it outgrows the original one.
Place two to three of the soaked viable acorns in the container, laying them on their sides. Cover with one to two inches of soil. Water regularly and deeply. The acorns will start to sprout in about one month. Once the seedlings emerge, only keep the strongest one and gently pull out the others with their acorn. Protect the young roots from root burn by shielding the exterior of the container from the sun either with wood, heavy fabric or a second, larger pot.
Keep the container outside year-round on a patio or in a location where it is easy to monitor. During the growing season, water the seedling regularly and make sure the soil never dries out.