The bur oak (Quercus macrocarpa) is a large tree also commonly known as the blue oak, mossycup oak and scrub oak. The bur oak gets its name from the appearance of its acorns, which have a burred fringe on their caps; its acorns are also larger than those of all other oak species native to the United States. Bur oak is winter-hardy in U.S. Department of Agriculture plant hardiness zones 4 through 8.
Growth Habit and Size
A mature bur oak tree has a broad, rounded canopy and usually reaches a height of 70 to 80 feet. Under ideal growing conditions, the tree can get much taller, more than 100 feet. It is a slow-grower, often adding less than 1 foot of height and 1/4 inch of trunk diameter each year. It has a long lifespan, though, and may survive more than 400 years.
Because of its large size, a bur oak tree is not well-suited to any but the most spacious gardens or yards. Litter from its falling acorns also may be a problem if the tree is close to spaces used for outdoor gatherings. In roomy landscapes, however, bur oak's hardiness and size makes it an effective shade and specimen tree.
In its native habitats, bur oak can tolerate a wide range of soil types and moisture levels. A young bur oak does well in an area that gets full sun or partial shade. The tree grows most quickly in rich, moist soil, but it may remain small or shrublike in thin, nutrient-deficient soil. Bur oak is very drought-tolerant, and its thick bark helps it to resist damage from fire. It is not particularly tolerant of excess moisture, though, and it may not survive if subjected to flooding or saturated soil for more than a few weeks.
Pests and Diseases
Bur oak is not generally vulnerable to pests and diseases that cause serious, life-threatening damage. Insects such as the oak leaf miner and the oak webworm may cause some defoliation, but natural predators and parasites usually eventually control infestations without intervention from a gardener.
Fungal diseases such as cotton root rot, which can cause the tree to die, also may arise. Chemical control of this type of fungus is expensive and not always effective. So it's best to consult an arborist for a definite diagnosis and treatment recommendations if you see signs of infection, which include leaf yellowing and wilting.
Oak wilt, a fatal and incurable fungal infection, is uncommon in bur oak, but it may spread to bur oak from the roots of nearby infected trees.