Quercus macrocarpa (Bur Oak)
|Also known as:|
|Life cycle:||perennial woody|
|Habitat:||sun; forest to open prairie|
|Bloom season:||early spring|
|Plant height:||to 100 feet|
|Wetland Indicator Status:||GP: FACU MW: FAC NCNE: FACU|
|MN county distribution (click map to enlarge):|
|National distribution (click map to enlarge):|
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Long, greenish strings of male pollen bearing anthers hang in clumps (called catkins) from buds at the tip of last year's branches. Female flowers are also green, the naked styles clusters on a short thick stalk in the leaf axils of new growth.
Leaves and bark:
Leaves are broadly club shaped (obovate), 4-8 inches long by 2-6 inches wide, narrowly tapered at the base with several deeper, rounded lobes. Typically a large, deep sinus, shallowly lobed around the edged, is at mid-leaf before expanding fan-like at the tip. The upper surface is dark green and shiny, the lower surface paler and densely covered with short, fine hairs. Leaves turn a golden brown in fall.
The trunk can get up to 40 inches in diameter at breast height (dbh), with thick, dark gray bark with deep verticle furrows and corky ridges. Large portions of the trunk may have smooth, lighter gray patches where the bark has sloughed away, caused by a sprophytic fungi that does not damage the living tissue. Branch structure is gnarly, the smaller twigs thick and course with corky ridges appearing in just one or two year old wood.
Fruit is a round to egg shaped nut (acorn), ½ to just under 1 inch long, set in a dome-like cup around the base that is fringed with thick, coarse, brittle hairs, on a stalk up to ¾ inch long. This cap encloses half the nut or more, sometimes nearly all of it.
Bur Oak is the most common and widespread oak species in Minnesota. While often present in our northern and eastern forests, it is highly shade intolerant and does not regenerate well in competition with others trees and shrubs. It is well adaptive to the open prairie where its thick protective bark is an adaptation to a fire ecology as well as tolerance to drier, sandy soils. Mature Bur Oaks can attain hundreds of years of age and in open savannas the impressive crown can be wider than the tree is tall. In east central and southeastern counties its range overlaps with two other native oaks with round lobed leaves. White Oak, (Quercus alba), can also have a large spreading crown but its bark is thinner and more scale-like without the deep furrows, its leaf lobes are typically more evenly lobed, both upper and lower surfaces are smooth, and the acorns do not sport the coarse hairs around the lip of the cup. The other oak is the Swamp White Oak, (Quercus bicolor), which has very similar bark on mature trees but its leaves have shallow, more even lobes, its acorns are borne on a long stem and their caps have just a few coarse hairs scattered about the cup.
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- Bur Oak tree
- leaf variations
- Bur Oak buds and bud scars
- gnarly branches
- gnarly branch
- a “bur”
- fall color
- tree in winter
- oak savanna habitat
Photos by K. Chayka taken in Ramsey County. Photos courtesy Peter M. Dziuk taken in Anoka and Dakota counties.
Have you seen this plant in Minnesota, or have any other comments about it?