Everyone knows an oak at sight, at least if it is bearing acorns, a distinctive type of fruit borne by no other genus in this region. Yet, distinguishing the species of oak is often difficult in the field and even harder in the herbarium. Herbarium specimens should (but too rarely do) include typical mature foliage, winter buds, and fully mature acorns, as well as notes on bark and stature of the plant.

Oak leaves are extremely variable (in size, shape, and pubescence) from one part of the same tree to another, and from one tree to another of the same species. Leaves growing in the shade, as deep in the canopy or on lower branches, are often less deeply lobed than those receiving full sun, as on upper branches. Leaves on vigorous sprouts may appear quite unrecognizable as to species. Oaks hybridize with other species of the same subgenus, producing first generation progeny with ± intermediate characters and apparently with some resultant introgression of characters following backcrossing. Immature (e.g., flowering) specimens are usually impossible to name, at least without full information on the plant from which the juvenile material came. Young or abortive acorns should be avoided for identification.

Our oaks fall into two subgenera, the white oaks (Quercus subg. Quercus, long called subg. Lepidobalanus) and the black or red oaks (Quercus subg. Erythrobalanus). The latter group is readily recognized by the prominent bristle-tips of the lobes of the leaves (or, in the entire-leaved shingle oak, only at the tip of the leaf). Additional characters are included in the key.

1. Leaf blades entire, unlobed, elliptic to oblong-lanceolate or slightly obovate.

Q. imbricaria

1. Leaf blades toothed or lobed.

2. Lobes and teeth of leaf prolonged into a distinct bristle; acorns ripening their second year (hence located on year-old twigs, below the leaves), densely pubescent on inside of the shell; bark dark and tight, often shiny, becoming furrowed (but not scaly) with age.

3. Leaves with sinuses cut half or less the distance to the midrib (the longest lobes, measured along upper side, thus at most about equaling the uncut part of the blade).

4. Terminal winter buds shining reddish, glabrous or nearly so, terete, usually less than 6 mm long; acorn about a third or less covered by the cup, which has the upper scales appressed; leaves with blades dull above, the petiole and midrib above soon glabrous; throughout Michigan.

Q. rubra

4. Terminal winter buds dull whitish hairy, angled, usually at least 6 mm long by September, longer at maturity; acorn about half covered by the cup, which has the upper scales ± loose and spreading; leaves glossy above, the petiole and/or lower portion of midrib above usually retaining some pubescence well into the summer; Lower Peninsula, especially southward.

Q. velutina (in part)

3. Leaves with sinuses cut about two-thirds or more of the distance to the midrib (the longest lobes thus at least twice as long as the uncut middle part of the blade).

5. Cups of acorns usually gray-pubescent throughout, with uppermost scales loose and spreading, forming a fringe around the acorn; terminal winter buds angled, hairy, usually 6 mm long by September, longer when mature.

Q. velutina (in part)

5. Cups of acorns glabrous or pubescent but with uppermost scales ± appressed, not forming a fringe; terminal winter buds terete or nearly so, glabrous at least on lower half, less than 6 mm long.

6. Acorns not over 12 mm long, cup very shallow and saucer-like, not over 12 mm across; usually of wetland habitats (but often planted on drier sites).

Q. palustris

6. Acorns longer and/or cup broader, usually 12–26 mm broad (and deeper); plants of upland (even dry and sandy) or wetland habitats.

7. Largest leaves 6–10 (–15) cm wide at the widest point; terminal buds 2.5–4.2 mm long, the scales usually ± appressed-pubescent on their backs; larger acorns with cups 12–17 mm broad; upland (often dry and sandy) habitats.

Q. ellipsoidalis

7. Largest leaves (9–) 11–17 (–21) cm wide at the widest point; terminal buds 5–8.5 mm long, the scales essentially glabrous on the back (margins may be ciliate); larger acorns with cups 19–26 mm broad; swamps, often on clay soils.

Q. shumardii

2. Lobes or teeth of leaf rounded or blunt, not bristle-tipped; acorns ripening their first year (hence located among the leaves), glabrous on inside of the shell; bark usually gray and flaky.

8. Leaf blades completely glabrous and usually ± glaucous beneath at maturity.

Q. alba

8. Leaf blades thinly to densely stellate-pubescent over some or all of their surface beneath at maturity.

9. Leaves with the sinuses much deeper on lower half of blade (sometimes nearly to midrib); terminal buds pubescent; branchlets densely pubescent when very young, soon glabrate; older branchlets frequently with corky wings; acorn half or more covered by the deep and strongly fringed cup; throughout Michigan except northwestern Upper Peninsula.

Q. macrocarpa

9. Leaves uniformly lobed or toothed (Q. bicolor often with a few larger or more irregular lobes near middle of blade); buds glabrous or nearly so; branchlets glabrous or nearly so, without corky wings; acorn about a third or less covered by the cup, which lacks fringing awns on upper scales; southern half of Lower Peninsula.

10. All lateral veins of some leaves not ending in a tooth or lobe (some of them fading out or ending at a sinus); leaves often ± irregularly lobed (more shallowly than in Q. macrocarpa) and not bilaterally symmetrical; acorns (usually paired) on peduncles ca. (1–) 3–7 cm long (peduncles longer than petioles).

Q. bicolor

10. All lateral veins (except sometimes the lowermost) of all leaves ending in a large tooth; leaves unlobed and essentially bilaterally symmetrical; acorns usually sessile or nearly so (peduncles if any shorter than petioles).

11. Plant a low colonial shrub of mostly open areas, ca. 1–2 m tall; leaf blades with (4–) 6–8 (–9) teeth per side.

Q. prinoides

11. Plant an erect tree of forests; leaf blades with (7–) 9–13 (–15) teeth per side.

12. Teeth of leaf acute, usually with distinct (even slightly prolonged) callus tip; acorn less than 2 cm long.

Q. muehlenbergii

12. Teeth of leaf rounded, with at most an obscure callus tip; acorn at least 2 cm long.

Q. montana