Bur Oak
Quercus macrocarpa
Beech family (Fagaceae)

Description: This tree is 80-120' tall at maturity, forming an ovoid to globoid crown with a tall stout trunk (up to 5' across). The branches of the crown are ascending to widely spreading and somewhat crooked. The thick trunk bark is gray to gray-brown with flat corky ridges and deep irregular furrows. The bark of branches and twigs is gray-brown to brown and often rather corky with flat elevated ridges. Alternate leaves about 4-10" long and 2–5" across develop from the twigs. These leaves are obovate or broadly elliptic in outline and pinnatifid with rounded lobes. Most lobes extend moderately to deeply into the leaf blade. The deepest lobes usually occur along the lower one-half of the blade. Leaf margins are undulate and irregular, lacking any bristles or true teeth. Upper leaf surfaces are dark green and glabrous, while lower surfaces are pale gray-green and densely tomentose with short fine hairs. These hairs are often stellate or clustered together (visible with a 10x hand lens). The petioles are –1" long, light green, and either glabrous or tomentose.

Bur Oak is monoecious, producing both male (staminate) and female (pistillate) flowers on the same tree. The male flowers are produced in drooping yellowish catkins about 2-5" long; individual male flowers consist of a lobed calyx with 5-20 stamens. Female flowers are either solitary or clustered together in groups of 2-3. Individual female flowers (about 1/8" or 3 mm. in length) consist of a pistil that is covered with appressed scales (involucral bracts) with red styles at its tip. Overall, a female flower has the appearance of a tiny narrow cone. Both male and female flowers bloom as the leaves begin to develop during the spring. Cross-pollination is by wind. During the summer, fertile female flowers develop into nuts that are either solitary or occur in pairs. The nuts occur on short stalks up to 1" long (shorter than the petioles of the leaves); they are often nearly sessile. Nuts mature in a single year and usually germinate during the fall of the same year. Individual nuts (including their cups) are 1–2" long and similarly across; they are initially green, but become brown to grayish brown at maturity. The distinctive cups extend at least one-half the length of the nuts, sometimes nearly enclosing them. The coarse scales of the cups are keeled and rather knobby in appearance; the outer scales along the rim each cup have soft awns up to 1/3" (8 mm.) in length, forming a conspicuous fringe around the nut. The starchy meat of the nuts is low in tannins and potentially edible. The root system produces a deep taproot and widely spreading lateral roots. At favorable sites, this tree sometimes forms colonies.

Cultivation: Bur Oak prefers full to partial sun and loamy soil that is moist and deep. However, it is very adaptable and will tolerate more dry situations and soil that contains significant amounts of sand, clay, or gravel. Limestone-derived soil with a high pH is also tolerated. This tree is relatively slow to develop and sometimes difficult to transplant because of its deep taproot. Young saplings are relatively intolerant of flooded conditions. The nuts do not require exposure to cold winter temperature in order to germinate. An individual tree does not produce nuts until it reaches at least 30 years of age. Individual trees can live several centuries (400 years or more).

Range & Habitat: Bur Oak is a common tree that is found in every county of Illinois. Habitats include moist bottomland woodlands, upland woodlands, and savannas where deciduous trees are dominant. This tree is most commonly found in bottomland woodlands a little outside of the flood zone. It also occurs in savannas and can be an invader of prairies because of its resistance to fire. Occasionally, Bur Oak is cultivated as a landscape tree, where it can become quite large.

Faunal Associations: Like other oaks, the value of Bur Oak to wildlife is quite high. The leaves are eaten by the caterpillars of several butterflies, specifically Calycopis cecrops (Red-Banded Hairstreak), Fixsenia favonius ontario (Northern Hairstreak), Parrhasius m-album (White-M Hairstreak), Satyrium calanus falacer (Banded Hairstreak), and Satyrium liparops strigosum (Striped Hairstreak); the caterpillars of the skippers Erynnis brizo (Sleepy Duskywing) and Erynnis juvenalis (Juvenal's Duskywing) also feed on the leaves. In addition, the caterpillars of probably several hundred moths feed on the foliage and other parts of oaks; the Moth Table lists several of these species. Another major group of insect feeders consists of Long-Horned beetles (Cerambycidae) and closely related beetles, whose larvae bore through the wood or bark of oaks. The Long-Horned Beetle Table lists many of these species. Bur Oak and other oaks are among the major hosts of treehoppers (Membracidae); the Treehopper Table lists species that often feed on these trees. Other insect feeders include plant bugs (Miridae), leaf beetles (Chrysomelidae), the Oak Lace Bug (Corythucha arcuata), the Northern Walkingstick (Diapheromera femoratum), the larvae of weevils (Curculionidae), and insects from other families (see Insect Table). Many of these insects are eaten by insectivorous songbirds. The acorns of oaks are an important food source of several birds; these species include the Wood Duck, Mallard, Ruffed Grouse, Ring-Necked Pheasant, Greater Prairie Chicken, Bobwhite Quail, Wild Turkey, Passenger Pigeon (now extinct), Monk Parakeet (introduced from abroad), Blue Jay, White-Breasted Nuthatch, Tufted Titmouse, Pileated Woodpecker, Red-Bellied Woodpecker, Red-Headed Woodpecker, and others. Among mammals, the Black Bear, Raccoon, Fox Squirrel, Gray Squirrel, Red Squirrel, Southern Flying Squirrel, Eastern Chipmunk, and White-Footed Mouse are all avid consumers of acorns. Because of their large size and reduced bitterness from low tannins, the acorns of Bur Oak, in particular, are an important food source for these mammals. White-Tailed Deer and domesticated cattle browse on the twigs, leaves, and acorns of oaks, while the Cottontail Rabbit browses on the bark, twigs, and leaves of young saplings. Oak trees provide nesting habitat for such birds as the Northern Parula, Blue-Gray Gnatcatcher, Yellow-Throated Vireo, Summer Tanager, Red-Tailed Hawk, and Swainson's Hawk; the cavities of older trees provide dens for tree squirrels and Screech Owls.

Photographic Location: Crystal Lake Park in Urbana, Illinois.

Comments: This tree has the largest acorns of any oak. Not only are the acorns large, but they have a distinctive appearance because of the conspicuous fringe along the rim of their cups. Bur Oak is a member of the White Oak group, which means that its acorns mature in a single year, its leaves lack bristles at the tips of their lobes, and it can hybridize with many other oaks within this group. In addition to its distinctive acorns, Bur Oak can be distinguished from other oaks by the corky ridges of its branches and the shape of its leaves. Generally, the leaves of Bur Oak have deeper lobes than those of many other oaks, and the deepest lobes usually occur along the lower half of the length of each leaf. Bur Oak can be distinguished from the similar White Oak (Quercus alba) by the tomentose hairs on the lower sides of its leaves; the leaves of the latter oak are hairless on their undersides.