Three sets of species pairs keyed out at the beginning, Rubus odoratus and R. parviflorus, R. acaulis and R. pubescens, and R. occidentalis and R. strigosus are normally all diploid and are fairly clear-cut, although hybrids exist between the members of each of the pairs. The remainder of our taxa (R. phoenicolasius excepted) are in subgenus Rubus. These are the dewberries and blackberries, a challenge to the botanist in more than one way. Rubus subgenus Rubus includes diploids and polyploids. Some taxa are sexually reproduced and others are apomicts. Over 400 so-called species have been described from eastern North America, most of these now regarded as apomicticly reproducing clonal lines. The special problem with Rubus is the biennial habit and the presence of characters, thought to be important in distinguishing taxa, in the vegetative first-year stems, which are rarely collected by casual botanists. Good specimens, to show all characters thought to be helpful in identification, should include portions of the first-year unbranched vegetative stem, or primocane (generous sections from the tip as well as the middle), and of the second-year stem, or floricane, with inflorescences or infructescences, which are on short leafy lateral shoots branching from the old primocane—now the floricane. Leaves of primocanes in some taxa usually have 5 leaflets; floricane leaves have 3 leaflets (or some of them are simple). The shape of primocane and floricane leaflets may also differ. Notes on the habit of the plant are important: whether the stems are erect, ascending, arched, immediately prostrate, or trailing (starting upward but soon back on the ground), and whether they root at the tips.
Some Rubus such as R. flagellaris, are strong trailers, and others, especially R. occidentalis, may develop long, arching, tip-rooting primocanes. But in neither case do these climb and they are here all considered shrubby.
The fruit in Rubus is an aggregate of numerous small juicy druplets (each from a separate carpel of the flower); the receptacle on which these are inserted is fleshy and falls with the fruit in the dewberries and blackberries, but is firm and remains on the plant in the thimbleberries and raspberries.
The treatment that follows is a practical one, recognizing species complexes, that should allow the user to make a preliminary sorting of most specimens, even if they lack the primocane with its characters of habit, armature, and leaflet shape so often used in defining segregate species. For a list of all segregate entities noted from Michigan see Michigan Flora.
1. Stems without prickles (new stems may be glandular-bristly if leaves are simple).
2. Leaves all simple (palmately lobed), mostly (7–) 9–22 cm or more broad; petals 18–25 mm long; fruit readily separating from receptacle; plant with erect woody stems and shreddy bark.
3. Petals pink to purple (rarely white); calyx lobes with ± dense purple elongate gland-tipped hairs or bristles; from the southeastern Lower Peninsula north as far as southeastern Presque Isle Co. as a native; elsewhere a rare escape.
3. Petals white; calyx lobes with yellow-orange very short gland-tipped hairs; from the Upper Peninsula south to Alpena Co.
2. Leaves trifoliolate, the leaflets less than 5 (–7) cm broad; petals 4–14 (–18) mm long; fruit not separating from receptacle; with trailing woody stems at or below ground level and with tight bark, the erect shoots herbaceous.
4. Petals deep rose-pink, ca. 10–14 (–18) mm long; flowers solitary on glandless pedicels; leaflets nearly or quite obtuse to rounded at apex.
4. Petals white (sometimes drying pinkish or turning with age), 4–7 (–8) mm long; flowers 1–3 (–4), the pedicels usually with a few stalked glands; leaflets sharply acute to acuminate.
1. Stems with prickles, or at least bristles (rarely completely smooth in the erect R. canadensis); leaves all or mostly compound.
5. Floricane leaves densely white-pubescent beneath; fruit red or black at maturity, readily separating from the receptacle (except in R. bifrons).
6. Petals pale pink; fruit not separating from the receptacle; primocane leaves palmate.
6. Petals white; fruit readily separating from the receptacle; primocane leaves pinnate.
7. Pedicels and peduncles with glandless mostly broad-based slightly recurved prickles; mature fruit purple-black with narrow belts of white tomentum between the druplets.
7. Pedicels and peduncles with gland-tipped ± straight bristles; mature fruit red, without tomentum between the druplets.
8. Pedicels and calyx with stipitate glands inconspicuous, yellowish, mostly 0.2–1.5 mm long, mostly shorter than the bristles.
8. Pedicels and calyx with conspicuous, abundant purplish stipitate glands, the longer ca. 2–5 (–10) mm long, bristles absent or shorter than the glandular hairs.
5. Floricane leaves green on both sides (pubescence if any not both dense and conspicuously white); fruit black at full maturity, separating from the floral tube with the fleshy receptacle included.
9. Leaflets (all or many of them) pinnately lobed or ± incised nearly or quite to the midrib, besides the usual teeth; calyx lobes armed with firm prickles.
9. Leaflets merely toothed, at most shallowly lobed; calyx lobes unarmed (may be glandular-bristly).
10. Floricane prostrate or trailing, slender, the flowering shoots arising ± perpendicularly from the essentially horizontal cane; primocanes ± ascending or trailing, rooting at the tips.
11. Floricanes (flowering shoots and/or trailing old canes) armed only (or almost entirely) with slender prickles or bristles, these slightly if at all expanded at the base; leaflets usually ± obovate, rounded or obtuse at apex (especially lower on flowering shoots), firm (even persistent or winter-green); flowers small, the petals 3.5–6 (–9) mm long.
R. hispidus complex
11. Floricanes armed mostly or entirely with strong broad-based prickles; leaflets mostly acute to acuminate, thin; flowers larger, the petals 8–15 (–20) mm long.
12. Primocanes glaucous with a whitish bloom; mature fruits irregular and asymmetrical with only 1–few large druplets maturing.
12. Primocanes greenish, not glaucous; mature fruits with numerous druplets.
R. flagellaris complex
10. Floricane as well as primocane erect or arching, usually not rooting at tip (plants depressed by snow or their own weight might be confusing if they appear prostrate; R. setosus is often trailing, see text).
13. Canes armed with bristles or slender narrow-based prickles; plants relatively low, even well-developed ones scarcely 1 m tall.
R. setosus complex
13. Canes armed with stout broad-based prickles (these sparse and small in R. canadensis); plants when well-developed stout and at least 1 m tall.
14. Pedicels and axis of inflorescence (also petioles and young growth) with conspicuous stalked glands; leaflets pubescent beneath.
R. allegheniensis complex
14. Pedicels and other parts glandless or nearly so; leaflets glabrous or pubescent beneath.
15. Leaflets glabrous beneath (or a few hairs only on principal veins); old canes with few or no prickles (though new growth usually has a few).
R. canadensis complex
15. Leaflets pubescent beneath (between the principal veins) or canes strongly armed (or both conditions present).
R. pensilvanicus complex