The iconic oak of Wisconsin! Bur Oak is one of the toughest oaks, tolerant of highly alkaline soils and drought. Most have interesting corky bark on young branches. The acorns make it an excellent plant to attract wildlife. Use where space is not limiting as Bur Oak is large at maturity and long-lived. May also be known as Blue Oak, Mossycup Oak.
Wisconsin Native: Yes
USDA Hardiness Zone: to zone 3
Mature Height: 70-80 feet
Mature Spread: 75-90 feet
Growth Rate: Slow
Growth Form: Tree, columnar in youth, broad with age
Light Requirements: Full Sun
Site Requirements: Adaptable to many soils and moisture levels except flooded sites
Flower: Monoecious, insignificant male and female flowers emerge in spring with leaves
Bloom Period: April-June; variable
Foliage: Dark Green
Fall Color: Yellow-Brown to Tan
Urban Approved: Yes
Fruit Notes: Acorn, 3/4 inch, brown, more than half covered by cap (involucre), good crops every 2-3 years
Bur Oak is a large, long-lived tree, so be conscious of the available space. The tree is sturdy and not likely to shed branches, making it ideal for parks or medians where there is enough space for the tree to reach its mature size. Bur Oak also functions well as a shelter belt tree due to its low moisture needs and indifference to soil types.
Bur Oak tolerates very high soil alkalinity and is one of, if not the best oak trees for tough urban sites. It is also drought and pollution tolerant, indifferent to soil quality, and requires little maintenance once established.
Bur Oaks, like all oaks in the White Oak Group, have less tannin in their acorns than those in the Red Oak Group. This makes them more palatable (less bitter) and preferable to wildlife. Blue jays and crows will flock to the tree for the acorns. Deer, squirrels, chipmunks, and other small mammals love the acorns as a food source.
All oaks are susceptible to Two-Lined Chestnut Borer during the establishment period after planting. Apply a systemic insecticide containing imidacloprid to Bur Oak when planting to protect the tree from this insect.
Do not prune Bur Oak during the growing season. While not as susceptible to Oak Wilt as those in the Red Oak Group, the disease can still damage stressed trees. Prune only during the dormant season in winter after the leaves have fallen.
We invite you to check out the Arborist For Hire lookup at the Wisconsin Arborist Association website to find an ISA Certified Arborist near you.
When healthy, Bur Oak has few significant disease or insect problems. Occasionally the leaves can be defoliated by Gypsy Moth and June Bugs. Avoiding damage and wounds during the growing season will prevent Bur Oak from being infected by Oak Wilt. Bur Oak Blight can infect the tree in periods of stress, and the native Wisconsin variety of Bur Oak is more susceptible than the southern ecotype. However, disease resistance varies between plants and some trees exhibit high resistance to this fungal disease.
The most important care for Bur Oak is to maintain health and vigor through good mulching and adequate watering. During the first two years after planting, make sure the tree has enough moisture to properly establish its roots. Use a systemic insecticide to protect it from Two-Lined Chestnut Borer.
If stressed, Bur Oak is susceptible to attack by Two-Lined Chestnut Borer, Oak Webworm, Leaf Miners, Oak Skeletonizer, Variable Oakleaf Caterpillar, Cotton Root Rot, and Strumella Canker. All of these pests and diseases can be avoided if the tree is properly cared for.
When grown naturally, Oaks develop a coarse, deep root system with a taproot. This has made them historically difficult to transplant as balled-and-burlapped (B&B) trees because much of the root system is lost in the harvesting process. Our trees are unique because they are root pruned from the start to develop more fibrous fine roots. When harvested, our B&B oaks contain more fibrous roots, making them tougher and easier to transplant than oaks not given our unique treatment process.
Bur Oak is noted for having the largest acorn of all native Oak species. In Wisconsin, however, our Bur Oaks are of the oliviformis variety, which has smaller acorns than the species. While not as large as the acorns of the southern ecotype, the acorns of our native variety are more manageable in a landscape setting and create less mess.
The wood of Bur Oak is commercially valuable for its rot resistance and strength. It is typically sold commercially as White Oak.
Bur Oak is a pioneer species at the forest edge and will invade prairies along with Northern Pin Oak. The corky bark of Bur Oak protects it against the wildfires that distinguish our native tallgrass prairies, even when young.
Of all the native North American oaks, Bur Oaks bear acorns the longest- a 400 year old tree will still reliably produce seeds. Bur Oak is a mast species- it produces acorns at irregular frequencies to help it reproduce. During years of regular production, the tree produces enough acorns to sustain a local population of small wildlife that will eat the seeds. When the trees produce a bumper crop of seeds, animals will cache large amounts of acorns but will not be able to eat them all. The uneaten acorns are thus ‘planted’ by wildlife and result in forests that have trees in similar age groups. While other oaks, like White Oak, tend to have little to no production between heavy seed years, Bur Oak is more consistent in its intervals and production, providing value to wildlife even in years of low production.
In forest margins, Bur Oak is associated with American Filbert, Smooth Sumac, Prairie Crabapple, and Coralberry.