18. Quercus macrocarpa Michx. (bur oak, burr oak, mossycup oak)

Pl. 417 b, c; Map 1856

Plants trees to 35 m tall. Bark medium gray, divided into more or less persistent ridges. Twigs 3–4 mm wide, tan, brown or sometimes grayish, usually pubescent with small, stellate hairs, occasionally with branched spreading hairs or glabrous. Buds (3–)4–6 mm long, brown, pubescent. Petioles 8–30 mm long. Leaf blade 12–22 cm long, 9–13 cm wide, relatively thin and flexible, rounded to obtuse or occasionally acute at the base, divided (40–)60–90% of the width, the lobes 3–5(–7) per side, the median sinuses the deepest, the lower sinuses shallower, the upper sinuses shallow or the blade merely coarsely scalloped above the midpoint; well-developed lobes 20–45 mm wide, oblong to obovate, usually broadened outward, rounded or rounded-obtuse to truncate or shallowly notched apically, often scalloped, sometimes forked (2-lobed); secondary veins 5–11 per side, some reaching the margin at the tips of the lobes, usually others reaching toward sinuses and turning aside before reaching the margin; the upper surface dull, glabrous or with scattered, stellate hairs, the undersurface usually white, sometimes green, usually with 7–14-rayed, stellate hairs, sometimes with 2–7-rayed, spreading hairs, appressed linear hairs also present but usually concealed beneath the stellate hairs, usually felty to the touch,. Acorn stalks 5–33 mm long, the cup 19–25 mm long, 24–46 mm wide, covering 50–70% of the nut, hemispheric, the scales 2–5 mm long, those near the margin long, linear, forming a fringe that projects 6–15 mm beyond the rim of the cup. Nuts 24–32 mm long, 17–33 mm wide, ovoid. 2n=24. April–May.

Scattered to common nearly throughout the state (eastern U.S. west to Montana and Texas; Canada). Bottomland forests, mesic upland forests, banks of streams and rivers, and bottoms and tops of bluffs; also pastures and roadsides.

Quercus macrocarpa is a widely distributed species that is quite variable in plant size and form, acorn size and degree of elongation of the marginal cups scales, and leaf size and division. In Missouri, the middle and/or lower parts of the blade are always lobed on well-developed leaves, but the upper part may be merely scalloped. Specimens that are deeply lobed to the tip have been called f. oliviformis (F. Michx.) Trel. A somewhat smaller, shrubbier variant with relatively small acorns having less-fringed cups that occurs in the northwestern portion of the overall species range has been called var. depressa (Nutt.) Engelm. The acorns are the largest produced by any oak native to the United States, but even within Missouri there is considerable size variation, with some populations from relatively upland sites in northern Missouri producing acorns that are significantly smaller than the enormous ones produced by trees growing in bottomlands in the southern half of the state.

In Missouri, hybrids have been recorded involving Q. macrocarpa and six other oak species.