The leaves of Cornus make the genus easily recognizable. They are opposite in all but one Michigan species, entire, and with parallel lateral veins arching strongly as they approach the margins. Combinations of size, shape, texture, venation, and pubescence are rather distinctive for most species, but they do vary depending on conditions, and blades tend to be larger and less papillose beneath in the shade. Almost any character in the keys is open to exception, but identification is easier than the apparently overlapping statements might suggest. A diverse genus, sometimes split into several.
Cornus mas L., the European Cornelian Cherry, was collected in Ann Arbor (Washtenaw Co.) in 1949 as an “escape from cultivation,” but from a street running beside a park, and perhaps it was merely planted. It has bright yellow flowers in compact umbels appearing in early to mid April before the leaves expand, and conspicuous red fruit.
1. Leaves alternate or so crowded as to appear whorled, but not all distinctly opposite.
2. Plant a shrub or small tree; leaves all alternate (or crowded at ends of branches); inflorescence without subtending bracts; fruit dark blue.
2. Plant herbaceous; leaves all or mostly in one whorl; inflorescence subtended by large white bracts; fruit bright red.
1. Leaves clearly all opposite (or in pairs at ends of shoots).
3. Flowers in a dense head-like cluster subtended by 4 large showy white (or pinkish) bracts; fruit dark red, in tight heads.
3. Flowers in a compound cyme without subtending bracts; fruit white to blue, in an open infructescence.
4. Leaf blades rough to the touch above, owing to tiny projecting stiff hairs.
5. Lateral veins of leaves 3–5 on a side; pith usually brown in old twigs; rare, only in southeastern Lower Peninsula.
5. Lateral veins of leaves 6–8 on a side; pith white; frequent throughout most of Michigan.
C. rugosa (in part)
4. Leaf blades smooth to the touch above, glabrous or with soft closely appressed hairs.
6. Pith of 2-year-old twigs white (paler than surrounding wood); lateral veins of leaves 4–9 on a side.
7. Branchlets green, flecked with purplish streaks; leaf blades often less than 1.5 times as long as wide, with 6–8 (–9) lateral veins per side.
C. rugosa (in part)
7. Branchlets unspotted except for paler lenticels, the older ones usually red (rarely green); leaf blades ca. 1.5 times as long as wide or longer, with 4–5 (–6) lateral veins per side.
6. Pith of 2-year-old twigs light brown (darker than surrounding wood) or often white in C. foemina, lateral veins of leaves 3–5 on a side.
8. Calyx lobes (0.6–) 0.8–1.3 (–2) mm long; styles abruptly swollen for ca. 0.5–1 mm below the stigma; inflorescence flat or slightly convex; ripe fruit dark blue (with pale patches) on yellow-brown to maroon pedicels; bark of older branchlets reddish.
8. Calyx lobes less than 0.8 mm long; styles at most slightly expanded (no broader than the stigma); inflorescence strongly convex or pyramidal; ripe fruit pale blue to white, on bright red pedicels; bark of older branchlets gray.