Forest Trees of Illinois (Third Edition)

by Robert H. Mohlenbrock

GROUP A

Leaves needle-like or scale-like, often evergreen.

1. Leaves in clusters of 2 or more. 2. Leaves in clusters of 10 or more, falling off during autumn. 3. Leaves blue-green; cone-scales smooth [Larix laricina] 3. Leaves bright green; cone-scales hairy [Larix decidua] 2. Leaves in clusters of 2-5, evergreen. 4. Leaves in clusters of 5 [Pinus strobus] 4. Leaves in clusters of 2-3. 5. Leaves in clusters of 2. 6. Most or all the leaves less than 3 inches long, usually twisted. 7. Leaves up to 1½ inches long; sheath at base of leaves not longer than ⅛ inch [Pinus banksiana] 7. Leaves 2-3 inches long; sheath at base of leaves at least ⅓ inch long [Pinus sylvestris] 6. Most or all the leaves longer than 3 inches, rarely twisted. 8. Leaves mostly less than 5 inches long; cone-scale with a small spine [Pinus echinata] 8. Leaves mostly more than 5 inches long; cone-scale without a spine [Pinus resinosa] 5. Leaves in clusters of 3. 9. Leaves flexible, mostly less than 5 inches long; sheath at base of leaves about ¼ inch long [Pinus echinata] 9. Leaves stiff, mostly more than 5 inches long; sheath at base of leaves about 1 inch long [Pinus taeda] 1. Leaves solitary. 10. Leaves evergreen, of 2 types, some of them scale-like and about ⅛ inch long, some of them needle-like and up to ⅓ inch long [Juniperus virginiana] 10. Leaves falling off in autumn, all needle-like, usually at least ½ inch long. 11. Leaves 3-angled. 12. Leaves blue-green [Larix laricina] 12. Leaves bright green [Larix decidua] 11. Leaves flat [Taxodium distichum]

GROUP B

Leaves broad and flat, never needle-like or scale-like, compound.

1. Leaves alternate. 2. Leaflets 3 [Ptelea trifoliata] 2. Leaflets 5 or more. 3. Main stalk between leaflets winged on either side [Rhus copallina] 3. Main stalk between leaflets not winged. 4. Leaves with an even number of leaflets, often doubly compound. 5. Leaflets not more than ½ inch wide. 6. Pods usually 12 inches long or longer, with 6 or more seeds [Gleditsia triacanthos] 6. Pods 1-2 inches long, with 1-3 seeds [Gleditsia aquatica] 5. Some or all the leaflets at least one inch broad. 7. Leaves doubly compound; leaflets smooth along the edges; leaves without a foul odor when crushed [Gymnocladus dioicus] 7. Leaves once compound; leaflets usually with a few coarse teeth along the edges; leaves with a foul odor when crushed [Ailanthus altissima] 4. Leaves with an odd number of leaflets, once compound (twice compound in [Aralia spinosa], which has prickly leafstalks). 8. Leaflets without teeth along the edges. 9. Leaflets less than 1 inch broad and 2 inches long; twigs with short spines at base of leaf stalks [Robinia pseudoacacia] 9. Leaflets up to 4 inches long and up to 2 inches broad; twigs without spines. 10. Some or all the leaflets alternate; base of leafstalk conspicuously swollen [Cladrastis lutea] 10. All the leaflets opposite; base of leafstalk not swollen [Rhus vernix] 8. Leaflets toothed along the edges. 11. Leaves doubly compound, often with small prickles on the leafstalks and on some of the veins [Aralia spinosa] 11. Leaves once compound, without prickles. 12. All leaves with 11 or more leaflets. 13. Leafstalks and lower surface of leaves completely smooth [Rhus glabra] 13. Leafstalks and lower surface of leaves (at least on the veins) hairy. 14. Pith of twigs chambered when cut lengthwise. 15. Pith pale brown; bark dark brown, with roughened ridges [Juglans nigra] 15. Pith dark brown; bark gray, with smooth ridges [Juglans cinerea] 14. Pith of twigs solid. 16. Leaves strongly aromatic when crushed; twigs not covered with velvety hairs. 17. Bud scales with yellow hairs; nut 1½ to 2 times longer than broad [Carya illinoensis] 17. Bud scales without yellow hairs; nut about as broad as long [Carya aquatica] 16. Leaves not aromatic when crushed; twigs covered with velvety hairs [Rhus typhina] 12. Some or all leaves with less than 11 leaflets. 18. Buds with 4-6 scales arranged in pairs; most of the leaves with at least 9 leaflets. 19. Terminal leaflet without a stalk or with a very short stalk; buds mustard-yellow [Carya cordiformis] 19. Terminal leaflet with a well-developed stalk; bud reddish-brown [Carya aquatica] 18. Buds with 6 or more overlapping scales; most of the leaves with fewer than 9 leaflets (except sometimes in [Carya tomentosa] and [Carya laciniosa]), never with 11 leaflets. 20. Teeth along the leaf margin with small tufts of hairs [Carya ovata] 20. Teeth along the leaf margin without small tufts of hairs. 21. Lower surface of leaflets densely and uniformly hairy. 22. Stalk to which leaflets are attached densely hairy; bark tight, not shaggy [Carya tomentosa] 22. Stalk to which leaflets are attached smooth or sparsely hairy; bark shaggy at maturity [Carya laciniosa] 21. Lower leaf surface smooth, or with tufts of hairs in the vein axils, or sometimes sparsely hairy along the veins. 23. Scales of buds, stalk of leaves, and young twigs rusty-hairy or -scaly [Carya texana] 23. Rusty hairs and scales not present. 24. Leaflets mostly 5; bud scales without hairs at the tip; bark tight, not peeling [Carya glabra] 24. Leaflets mostly 7; bud scales with hairs at the tip; bark peeling at maturity [Carya ovalis] 1. Leaves opposite. 25. Leaves palmately compound. 26. Leaflets mostly 7; buds very sticky [Aesculus hippocastanum] 26. Leaflets mostly 5; buds not sticky. 27. Fruit prickly; flowers yellow [Aesculus glabra] 27. Fruit smooth; flowers red [Aesculus discolor] 25. Leaves pinnately compound, or sometimes with only 3 leaflets. 28. Leaflets usually 3, or sometimes 5; twigs green or glaucous [Acer negundo] 28. Leaflets usually 7 or more, sometimes 5; twigs not green. 29. Some or all of the twigs square [Fraxinus quadrangulata] 29. Stems not square. 30. Leaflets without stalks [Fraxinus nigra] 30. Leaflets stalked. 31. Stalks of the leaflets winged. [Fraxinus pennsylvanica var. subintegerrima] 31. Twigs and leafstalks smooth 32. Leaflets whitened on the lower surface [Fraxinus americana] 32. Leaflets green or yellow-brown on the lower surface [Fraxinus tomentosa]

GROUP C

Leaves broad and flat, never needle-like or scale-like, simple, opposite or whorled.

1. Leaves (or some of them) in whorls of 3. 2. Leaves with a bad odor when crushed; flowers with conspicuous purple spots [Catalpa bignonioides] 2. Leaves without a bad odor when crushed; flowers merely lined with purple [Catalpa speciosa] 1. Leaves opposite. 3. Leaves at least 6 inches long and 6 inches broad, heart-shaped at the base, never palmately lobed [Paulownia tomentosa] 3. Leaves usually less than 6 inches long and 6 inches broad, not heart-shaped at the base or, if slightly heart-shaped, then also palmately lobed. 4. Leaves palmately lobed. 5. Leafstalk with milky sap when broken [Acer platanoides] 5. Leafstalk without milky sap. 6. Areas between leaf lobes mostly V-shaped; leaves usually silvery or white on the lower surface. 7. Leaves deeply divided, the areas between the lobes extending more than half-way to the midvein [Acer saccharinum] 7. Leaves shallowly divided, the areas between the lobes extending less than half-way to the midvein. 8. Leaves completely covered with a white felt on the lower surface [Acer rubrum var. drummondii] 8. Leaves smooth or sparsely hairy on the lower leaf surface [Acer rubrum] 6. Areas between leaf lobes mostly U-shaped; leaves green or paler on the lower surface, not white or silvery. 9. Leaves with the edges drooping. 10. Leaves green on the lower surface; small leafy outgrowths sometimes present at the base of the leafstalk [Acer nigrum] 10. Leaves grayish on the lower surface; small leafy outgrowths at the base of the leafstalk absent [Acer barbatum] 9. Leaves flat, the edges not drooping [Acer saccharum]

4. Leaves not lobed. 11. Leaves toothed along the edges. 12. Edges of leaves regularly toothed from tip of leaf to base; leaves usually short-pointed or rounded at the tip. 13. Buds, leafstalks, and veins on the lower surface of the leaf covered with rusty hairs [Viburnum rufidulum] 13. Buds, leafstalks, and veins on the lower surface of the leaf without rusty hairs. 14. Leaf stalks wavy along the edges [Viburnum lentago] 14. Leaf stalks not wavy along the edges. 15. Branches spine-tipped [Rhamnus cathartica] 15. Branches not spine-tipped [Viburnum prunifolium] 12. Edges of leaves toothed only above the middle of the leaf; leaves tapering to a long point [Forestiera acuminata] 11. Leaves not toothed along the edges. 16. Upper surface of leaves rough to the touch [Cornus drummondii] 16. Upper surface of leaves smooth. 17. Leaves with deeply impressed veins on the upper surface; lower surface of leaves gray or whitish. 18. Leaves often nearly as broad as long; flowers surrounded by 4 large, white bracts; berries red; twigs green [Cornus florida] 18. Leaves longer than broad; flowers not surrounded by 4 large, white bracts; berries white; twigs gray [Cornus racemosa] 17. Leaves without deeply impressed veins; leaves often paler on the lower surface, but not gray or whitish [Forestiera acuminata]