Identifying trees by leaf and bark is a practical skill that connects you to the landscape, improves navigation, and supports plant conservation. In Illinois, a relatively small set of species dominates woodlands, streets, and parks. By focusing on a few reliable leaf and bark characters–arrangement, margin, lobing, venation, texture, and bark pattern–you can recognize most common trees with confidence.
Leaves and bark provide two complementary sets of characters. Leaves are often most diagnostic in spring and summer, while bark becomes indispensable in winter. Use both whenever possible.
Use this key as a rapid triage–then confirm with species-specific characters below.
Leaf: Simple, alternate, 5-9 rounded lobes with deep sinuses. Upper surface light green; underside paler and sometimes slightly fuzzy.
Bark: Mature bark breaks into light gray, flaky plates; ridges are rounded rather than sharp. Younger trunks smoother with a silvery tone.
Where common: Upland oak-hickory forests and dry ridges across Illinois.
Field takeaways: Rounded lobes and light gray, scaly bark make white oak unmistakable. Acorns are typically relatively large with shallow caps.
Leaf: Simple, alternate, 7-11 pointed lobes with bristle tips; sinuses shallower than white oak. Leaves are glossy dark green above.
Bark: Mature bark has narrow, shallow furrows with flattened, plate-like ridges that can appear almost blocky. Color is dark gray to blackish.
Where common: Urban plantings, bottomlands, upland woods.
Field takeaways: Pointed lobes with bristle tips separate red oaks from white oak. Bark tends to be darker and more furrowed.
Leaf: Opposite, simple, palmate with 5 lobes and smooth sinuses between lobes. Margins mostly smooth with some small teeth near tips. Leaves turn brilliant orange-red in fall.
Bark: Young bark is smooth; mature bark develops narrow, interlacing furrows that create irregular plates. Color is gray-brown.
Where common: Upland woods and older urban park plantings.
Field takeaways: Opposite, palmate leaves and splendid fall color identify sugar maple. The bark’s interlacing pattern helps in older trees.
Leaf: Opposite, palmate, usually 5 deep lobes with very irregular, coarse teeth. Leaves are noticeably paler underneath; lobes are deeply cut.
Bark: Gray and scaly on mature trunks; younger bark is smooth and silver-gray. Often becomes blocky on large specimens.
Where common: Floodplains, stream banks, and many planted landscapes.
Field takeaways: Very deeply cut leaves with a silvery underside and a tendency to crack or split in storms are typical.
Leaf: Opposite, pinnately compound with 3-7 leaflets (commonly 3). Leaflets are irregularly toothed and often asymmetrical.
Bark: Smooth and greenish-gray on young stems, becoming ridged and furrowed with age.
Where common: Disturbed sites, stream edges, and urban lots.
Field takeaways: A maple with compound leaves is almost always boxelder in Illinois. Also look for samaras in clusters.
Leaf: Alternate, pinnately compound with typically 5 leaflets that are lanceolate and coarsely serrated.
Bark: Distinctive shaggy plates peeling in long vertical strips–very diagnostic on mature trees.
Where common: Upland forests, often mixed with oaks.
Field takeaways: The dramatic peeling bark is a quick identifier even when leaves are absent.
Leaf: Alternate, pinnately compound with 7-11 slender leaflets that are serrated.
Bark: Smooth on young trees; becomes tight and interlaced with shallow ridges, not as shaggy as shagbark.
Where common: Moist bottomlands and floodplain forests.
Field takeaways: Count leaflets and note habitat; the bitter-tasting nuts and sulfur-yellow terminal buds can confirm ID.
Leaf: Alternate, pinnately compound with 15-23 leaflets. Leaflets are lanceolate with serrated margins and a slight oily sheen.
Bark: Dark brown to black with deep interlacing ridges that form diamond-shaped furrows. Strongly aromatic when cut.
Where common: Rich bottomlands and older farm windbreaks.
Field takeaways: Big compound leaves with many leaflets and deeply ridged dark bark are the hallmarks. Nuts and the strong scent when wood is cut are confirmatory.
Leaf: Alternate, simple, palmate with 3-5 lobes; often large (up to 8-10 inches). Margins coarsely toothed.
Bark: Very diagnostic–patchy, exfoliating bark reveals a mottled trunk of white, cream, and greenish-gray patches.
Where common: Floodplains, riverbanks, and wet bottomlands.
Field takeaways: If you see a white patchy trunk on a very large tree, you are almost certainly looking at a sycamore.
Leaf: Alternate, triangular (deltoid) with coarse, irregular teeth. Leaves are glossy and can be quite large.
Bark: Young bark is smooth and greenish; mature bark becomes deeply furrowed with thick ridges, often gray to dark brown.
Where common: River bottoms, floodplains, and alluvial soils.
Field takeaways: Triangular leaves that tremble in the wind and deeply furrowed bark on older trunks identify cottonwood. Look for cottony seed fluff in late spring.
Leaf: Alternate, simple, ovate with an asymmetrical base and sharply serrated margins. Surface rough with a prominent midrib.
Bark: Warty, corky ridges forming irregular knobby plates–very distinctive texture.
Where common: Bottomlands, streets, and upland sites tolerant of drought.
Field takeaways: The corky, warty bark and uneven leaf base are diagnostic; fruit are small drupes that persist into winter.
Leaf: Alternate, simple, oblong to lanceolate with finely serrated margins. Leaves smell of bitter almond when crushed.
Bark: Young bark smooth and reddish, with horizontal lenticels. Mature bark is plated and scaly–often described as burnt potato chips.
Where common: Upland woods and forest edges.
Field takeaways: Smell crushed leaves and examine bark texture; small clusters of dark cherries are a definitive sign in summer.
With focused observation and systematic use of the leaf-and-bark workflow above, you will quickly become competent at identifying the common trees of Illinois.