Quercus macrocarpa Michx. - Bur Oak, Mossy-Cup Oak, Scrub Oak, Blue Oak, Mossy-overcup Oak

Native? , Rare

By Steven D. Glenn

Not peer reviewed

Last Modified 01/25/2013

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Quercus macrocarpa

Common Names

Bur Oak, Mossy-Cup Oak, Scrub Oak, Blue Oak, Mossy-overcup Oak

Field Identification

Medium-sized to large tree with alternate, simple, lobed leaves; buds clustered near twig ends; catkin-like flowers in spring followed by acorns in the fall.  Acorns with loose bristled-scaled caps.

Food uses

Native American often used the acorns for a variety of food products, usually by removing the tannins by boiling the nuts or soaking them in lye water. (Moerman, 1998)

Medicinal uses

Used by Native Americans to treat a variety of maladies including cramps, diarrhea, heart and lung trouble. (Moerman, 1998)

Other uses

Wood prized for cabinet work, flooring, finish carpentry, and barrels for beer, wine, and other alcoholic spirits. (Hill, 1952)

Tannins derived from oaks have been used historically to tan animal hides into leather. (Burrows & Tyrl, 2001)

Used by Native Americans to make a black dye and to make popgun toys. (Moerman, 1998)

Oak-mast surveys may help wildlife agencies better understand dynamics of fall harvests and may be useful in harvest management models that attempt to stabilize fall harvest rates of game animals (Norman, 2003).

A 600 year regional hydroclimate was derived from bur oak tree-ring chronology (St. George & Nielsen, 2002). 


Poisonous Properties

Oak leaves, buds, bark, and acorns contain tannins which have varying degrees of toxicity in different animals.  Although oak foliage and acorns provide valuable food for many wildlife species and even some livestock, oak toxicosis, a urinary and digestive tract disease can occur when some animals are forced to subsist on oaks exclusively for several days.  Poisoning is rare in humans due to the large amounts needed to ingest to cause symptoms. (Burrows & Tyrl, 2001)


Quercus macrocarpa Michx., Hist. Chenes Am. No. 2, t. 2-3. 1801.

Cerris macrocarpa Raf. Alsogr. Am. 29. 1838.

Cerris dulcis Raf. Alsogr. Am. 29. 1838.

TYPE: unknown


Quercus macrocarpa var. olivaeformis (Mich. f.) Gray, Man. Bot. N. U.S. ed. 2, 404. 1856.

Quercus olivaeformis Mich. f., Hist. Arb. For. Am. Sept. 2: 32, t. 2. 1812.

Cerris olivaeformis Raf. Alsogr. Am. 29. 1838.

TYPE: unknown



Perennial, deciduous, phanerophytic, tree, diclinous and monoecious, 30-40 m tall (shrubby form in far western part of range).



Main stems ascending or erect, round.  Bark gray or dark gray, scaly to flat-ridged, not exfoliating.  Branches erect or ascending or horizontal. Twigs light brown or gray, fluted-terete, or often forming corky wings, 2-5 mm in diameter, smooth and lenticellate, glabrous or 1st year twigs sparsely beset with simple, fascicled, and stellate brown to light brown erect to appressed hairs.  Pith white, 5-pointed, continuous, nodal diaphragm absent. Sap translucent.  For an anatomical study of the xylem see Tillson & Muller, 1942. 



Terminal and axillary present, clustered at twig apices and scattered along stem. Terminal bud ovoid, blunt; axillary buds ovoid, blunt.  Bud scales brown, imbricate, with simple and fasciculate light brown or light gray hairs, moderately densely or densely distributed throughout. Bud scale scars bud scale scars encircling the twig. Leaf scars half-round-crescent. Vascular bundle scars numerous, scattered.



Alternate, simple, (appearing pseudo-opposite or pseudo-whorled at twig apices), crowded toward stem apex or spaced somewhat evenly along and divergent from stem.  Stipules lateral, free from the petiole, linear, caducous.  Petiole adaxially flattened, 1-4 cm long, with simple, two-five-armed fasciculate, and rotate-stellate light-brown to brown erect to appressed hairs, moderately densely distributed throughout; not glabrescent.  Leaf blades: abaxial surface light green or green, adaxial surface green or dark green, narrowly elliptic or oblanceolate or obovate, bilaterally symmetric, 10-25 cm long, 7-14 cm wide, chartaceous, pinnately veined; base cuneate or obtuse; margin lobed 1/4 - 7/8 the distance to the midvein with rounded sinuses and obtuse apices; apex obtuse.  Abaxial surface papillose and with 4 hair types: a. erect to spreading brown to light brown simple, occasionally scattered throughout  b. erect to spreading brown to light brown asymmetrical two-five-armed fasciculate, occasionally scattered throughout  c. appressed brown to light brown symmetrical 6-8(10)-rayed rotate-stellate, moderately densely to densely distributed throughout, and  d. appressed yellow to light brown minute multi-cellular bulbous glandular, moderately densely to densely distributed throughout.  Adaxial surface glabrous, (simple, fasciculate and stellate hairs on adaxial surface of immature leaves only). (Hardin, 1979a);(Thompson & Mohlenbrock, 1979).  For an overview of the phenology and the variability and density of hairs due to ecological factors and hybridization see Hardin, 1979b.  For a study of stomata size and density see Davies & Kozlowski, 1974.



Coetaneous, spike consisting of a single flower (sometimes 2-3), in axils of current year leaves, subsessile initially, becoming pedunculate in fruit, surrounded by a cupule which is persistent, accrescent, and indurate in fruit (acorn cap).  There has been debate over the years regarding the true ontogenetic nature of the cupule.  Originally thought to be an involucre of bracts, recent research suggests that the cupule is a complex partial inflorescence derived from stem tissue, see Abbe, 1974;Brett, 1964;Foreman, 1966;MacDonald, 1979;Fey & Endress, 1983.  Each cupule subtended by 3 minute, caducous bracteoles.



Perianth of one whorl, minute, fragrance absent. Calyx urceolate, of fused sepals. Carpels 3. Locules 3, each containing 2 ovules. Styles 3, each with 1 stigma. Ovary inferior. Placentation axile.



Coetaneous, compound, solitary or fascicled spikes; pendant, catkin-like; in leaf axils of previous year. Rachis moderately to densely covered with brown simple and fasciculate hairs; elongating with age, with 1-3 sessile flowers per node, each flower subtended by a small caducous bracteole.



Perianth of one whorl, 3-3.5 mm in diameter, fragrance absent. Calyx actinomorphic, campanulate, of fused sepals.  Sepal lobes 5-6, narrowly linear  with brown pilosity, moderately dense to densely distributed throughout.  Stamens (4)6-9(12), exserted, surrounding tuft of brown hairs. Anthers glabrous, basifixed, opening along the long axis.  Filaments free, 1mm long, straight, glabrous (Rowlee, 1900).  For a review of pollen morphology see Solomon, 1983b.



Acorn (glans (Spjut, 1994)) (calybium (Kaul, 1985)) pedunculate to 2 cm; maturation annual.  Acorn ovoid-oblong to subglobose, 1.5-2.5 cm long, comprised of 2 parts- a. the crateriform cup (cupule), enclosing about 1/2 to 3/4(7/8) of the base of the nut; and b. the nut, 1-seeded by abortion (occasionally double-seeded (Garrison & Augspurger, 1983)).  For a hypothesis that the first ovule fertilized suppresses the normal development of the others see Mogensen, 1975. 

Cupule exterior composed of indurate, imbricate, tightly appressed scales densely covered with brown to grey tomentum, scales at margin with soft awns distally.  

Nut olive-green to brown, with large light-colored circular cupule scar at base and apiculate at the distal end, with short appressed to spreading simple and fasciculate brown or light brown hairs, sparsely to moderately densely distributed apically, essentially smooth (minutely laterally striate).  One study found a size range of .51-15.60 cubic cm with a mean of 3.46 cubic cm and a correlation between smaller acorn size with increasing latitude see Aizen & Woodcock, 1992.



Embryo with two large fleshy cotyledons, endosperm lacking.

(Young & Young, 1992).



Quite variable, occurring in moist to dry bottomlands to uplands, forests, slopes and bluffs, often in poorly drained calcareous substrates.


Indigenous to central and northeastern North America.


United States -- AL, AR, CT, IA, IL, IN, KS, KY, LA, MA, MD, ME, MI, MN, MO, MT, ND, NE, NJ, NY, OH, OK, PA, SD, TN, TX, VA, VT, WI, WV, WY


Canada -- MN, NB, ON, QC, SK


New York Metropolitan Region -- Historically ranging in upstate New York and Pennsylvania, isolated specimens are rarely found in the New York metropolitan area.

Rarity Status

Global Heritage Rank -- G5

Connecticut -- Not listed

New Jersey -- Not listed

New York -- Not listed

Species Biology






Pollination patterns of bur oak are quite complicated and the mating system seems to be extraordinarily efficient at producing highly outbred individuals and ensuring long-range pollen flow. (Dow & Ashley, 1998b)(Dow & Ashley, 1998a)




Late summer to early autumn.

Often masting, for an investigation on the effects of weather on acorn yields see Sharp & Sprague, 1967.  For a study suggesting that masting is effected by weather in conjunction with inherent reproductive cycles see Sork, et al., 1993.



Scatter-hoarding by acorn predators Sciurus carolinensis (gray squirrel), Sciurus niger (fox squirrel) (Smith, 1972); (Stapanian & Smith, 1984) 

Small predators of acorns facilitate dispersal by dropping undamaged nuts and failing to recover cached nuts.  These include Sciurus carolinensis (gray squirrel), Sciurus niger (fox squirrel), Glaucomys volans (Southern Flying Squirrel), Tamias striatus (eastern chipmunk), Peromyscus leucopus (white-footed mouse), Peromyscus maniculatus (deer mouse) and Quiscalus quiscula (common grackle) and Cyanocita cristata (blue jay).  (Ivan & Swihart, 2000)(Smith, 1972)(Briggs & Smith, 1989)(Wolff, 1996)(Bosema, 1979)(Darley-Hill & Johnson, 1981)(Johnson & Webb, 1989)(Johnson, et al., 1993).   In addition, one study found that many predators preferred the basal end of the acorn and consumed only 30-60% of the cotyledon.  A chemical analyses of acorns from two species revealed that the concentration of protein-precipitable phenolics (primarily tannins) was 12.5% (Q. phellos) and 84.2% (Q. laevis) higher in the apical portion of the seeds where the embryo is located, suggesting that many acorn consumers consistently eat only a portion of the cotyledon of several species of acorns and thereby permit embryo survival. (Steele, et al., 1993).

Probably included in the diet of the grey fox (Urocyon cinereoargenteus) (Scott, 1955), eastern wild turkeys (Meleagris gallopavo silvestris) (Norman, 2003),  and white-tailed deer (Odocoileus virginianus) (Bryant, et al.,  1996).

For a study demonstrating the utility of microsatellite analysis for studying mating systems, seed dispersal and seedling establishment in natural populations see Dow & Ashley, 1998c.





(Young & Young, 1992)

(Schopmeyer, 1974)

(Schroeder & Walker, 1987)


Acorns of members of the white oak group have little or no dormancy and germinate naturally soon after falling, germination is hypogeal.  For germination to occur the moisture content of the acorns must not drop below 30-50%.  It is usually impractical to store acorns for more than 6 months, however storage in sealed containers or in sand at 32-36dF is probably best.  In general, seed of the white oaks lose viability more rapidly in storage over winter than seed from the black oaks.

One study of fall-planted acorns in experimental seedbeds found germination rates of 0-84% across different US provenances (Santamour & Schreiner, 1961).


General rules for collecting and storing acorns:

1. Collect acorns before they lose much water.

2. Ensure acorns are fully hydrated, soak in clean tap water overnight before placing them in storage.

3. Surface dry the acorns just before depositing them in storage to reduce mold growth.

4. Place acorns into cold storage as soon after collection as is possible.(Connor, 2004)


In one study (Q. phellos and Q. laevis), germination experiments revealed equal or greater germination frequencies for partially consumed acorns than for intact acorns.  (Steele, et al., 1993).


For a propagation protocol for growing bareroot oaks see Hoss, 2004.