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 backyard. But if you have space, it is an excellent shade tree for a sprawling landscape. Its 8-inch long oblong leaves have 7 to 11 lobes, are dull green above, and yellow-green and hairy underneath. 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.
|Common Name||Bur oak, mossycup oak|
|Botanical Name||Quercus macrocarpa|
|Mature Size||70-90 ft. tall, 60-90 ft. wide|
|Soil Type||Sandy, silt, clay, loamy|
|Soil pH||Acidic, neutral, alkaline|
|Hardiness Zones||3-8 (USDA)|
|Native Area||North America|
Bur Oak Care
To successfully grow a bur oak, 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. As a result of their wide roots, they are less likely to damage sidewalks, though it's still possible they could. That's because the absorbing roots occur in the top 12 inches 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.
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 during the third year to make sure it develops a strong root system.
Once the tree is established, it is drought-tolerant thanks to the taproot that allows it to draw water from 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.
Bur oak should be pruned in late fall or early spring. Start by removing any dead, diseased, or damaged branches at the base, or where they meet a lateral branch. Cut off any branches that grow downward, or those that are rubbing together. You can prune out up to 1/3 of the branches in a season to open up the canopy and allow better sunlight and air circulation.
Propagating Bur Oak
While growing an oak from acorns is the most common route, you can also propagate bur oak from cuttings. Here's how:
- With sharp, clean pruners, take several cuttings from hardwood, each at least 6 inches long, from stems that are at least the diameter of a pencil. The cutting should have at least three or four nodes.
- Snip the leaves off the bottom, leaving only a few at the top. Make a fresh, sharp cut at a 45-degree angle on the bottom of the cutting,
- Dip the cut end in rooting hormone and plant it in a container filled with potting soil made of peat moss and perlite.
- Keep the soil moist and the humidity high by wrapping a clear plastic bag loosely over the cutting. Place it in a warm area until roots form, which might take two or three months. Keep in mind that not all cuttings will "take" and grow—don't be disappointed if you lose several of them.
- Once roots do grow, put the tree indoors in a sunny, warm spot for the first year. Then it is ready to be transplanted into the ground.
How to Grow Bur Oak From Seed
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 1-gallon planting container with a drain hole with the same soil as the location where you intend to plant the tree. Mix in about one to two trowels full of organic matter or compost.
Place two to three of the soaked viable acorns in the container, laying them on their sides. Cover with 1 to 2 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.
Potting and Repotting Bur Oak
When starting a bur oak from an acorn, you'll want to use a 1-gallon container with good drainage. The container might seem much too large at first but keep in mind that bur oaks are finicky to transplant because of their taproot. You want to avoid transplanting the seedling to a larger pot when it outgrows the original one.
These are very cold-hardy trees that can tolerate freezing temperatures in their usual growing zones. Young trees that are still in containers or newly planted can benefit from a layer of mulch or straw over the roots.
Common Pests and Plant Diseases
The bur oak is affected by the same pests and diseases that are common among oak trees in the Northern hemisphere. Some of these include oak-leaf blister, which looks like small white blisters on the leaves in mid-summer, as well as leaf spot, which creates small black or brown spots on the leaves. Both can be treated with a fungicide. A white-rot fungal disease, sometimes known as canker, can affect the trees as well—and in this case, there isn't much you can do. The best treatment is the removal of the tree before the fungus affects other plants nearby.
A variety of pests make their home in bur oaks, such as caterpillars, oakworms, and scale. Introducing ladybugs into the landscape can help control the issue, as can removing caterpillars by hand. The use of an insecticide is a last-ditch effort but often works, especially for immature trees.
Common Problems with Bur Oak
Oak trees quite commonly develop lichen, which appears as a gray or green growth on the trunk or larger branches of a tree. This is usually a sign that a tree is in poor health or reaching the end of its life. The lichens don't actually cause a problem, however, and can be left alone.
Mistletoe is a parasitic plant that often attaches to oak trees. You can spot mistletoe because it remains green in the winter, long after the bur oak has dropped all its leaves. To control mistletoe, cut the affected limbs from the tree at least 1 or 2 feet below the point where the mistletoe is attached. Large infestations might require the use of chemicals designed to attack mistletoe.
How long can a bur oak live?
Bur oaks are long-lived; some specimens are estimated to be 300 years old.
Why doesn't my bur oak produce acorns?
The tree might not be old enough yet. The bur oak must usually be at least 35 years old before it produces viable acorns.
Why does wildlife flock to my bur oak but not other oaks?
In common with all oaks, the bur oak has acorns—whose bur-like covers gave the tree its name. The acorns taste better to wildlife than those from red oaks because they contain less tannin making them less bitter.