Identifying characteristics:
A picturesque oak with stout, rugged branches, the bur oak grows
to 80 feet tall or more and has a broad, spreading, rounded crown. The 6-
to 12-inch long leaves are variable
in shape, but are usually broadest above the middle and shallowly lobed,
with a pair of deeper lobes in the center. Leaves are dark, lustrous green
above and gray or whitish and hairy below. Fall color is a dull yellow-brown.
Male and female flowers are produced
in separate catkins on the current year's growth. Male catkins are yellowish,
slender and hanging. Female catkins occur in reddish clusters on a short
stalk. The broadly ovoid acorns are
0.8 to 2 inches long with the cup covering one-half or more of the nut.
A distinctive fringe of long, soft bristles protrudes from the rim of the
cup and the tip of the nut has a short beak. The species name "macrocarpa"
refers to the relatively large size of the acorns. The reddish-brown, imbricate
buds are conical to broadly ovate and are densely covered with fine,
gray hairs. Bark is thick, dark gray-brown
and separated into irregular, scaly ridges.
Similar species:
- white oak (Q. alba)
- swamp white oak (Q. bicolor)
- overcup oak (Q. lyrata)
Ecology:
Distribution: The bur oak is native to the eastern US, extending
into south central US. It is the most widely distributed native oak
in Iowa and the most abundant tree in the forests of the western and north
central parts of the state.
Habitat: Due to its thick bark, the bur oak was able to withstand prairie fires and could grow on the edges of the plains, making it a forest fringe species. It is usually an upland species, but is sometimes found near streams and on floodplains. Although it prefers limestone soils, this species tolerates dry, clay soils and also grows in sandy soils or alluvial bottoms. It is tolerant of drought, is moderately shade tolerant and is more tolerant of city pollution than most oaks. Bur oak is susceptible to a number of fungal diseases, galls and insect pests, but is seldom seriously damaged. Although considered to be resistant to oak wilt, the disease can kill large limbs, disfiguring affected trees.
Uses:
Wood: The bur oak is a considerably important timber tree
in the US. The wood is hard, tough, close-grained and moderately
heavy. It is usually sold as white oak and is used for flooring, furniture,
decks, cabinets and interior finishes.
Wildlife: Oaks are very important to wildlife. The bur oak provides deer, squirrels, wood duck, turkeys and rodents with acorns. Trees also provide cover and nesting materials.
Horticulture: Although it makes an excellent shade tree, the bur oak is seldom planted in urban areas, as young trees are difficult to transplant, slow to grow and the mature tree has an extensive root system. It is an impressive tree well suited to larger areas such as parks, campuses and golf courses.
