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Key to Rubus, Key D: American blackberries

Rosaceae

Rubus

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1 Canes armed primarily with bristles or slender-based prickles.
  2 Canes armed with slender-based prickles; leaves soft-pubescent beneath
  2 Canes armed with bristles as well as slender-based prickles; leaves thinly pubescent to glabrous beneath
1 Canes armed with heavy, stout-based, often recurved, prickles (or the canes essentially unarmed, the broad-based prickles few or almost absent).
    3 Leaflets oblanceolate to obovate, definitely wider beyond the middle, generally obtuse or rounded at the tip; leaves densely white- or gray-tomentose beneath; [primarily of the Coastal Plain]
    3 Leaflets lanceolate to ovate, widest below or near the middle, generally acute or acuminate at the tip; leaves glabrous to pubescent beneath, but the pubescence not notably tight and white or gray; [collectively widespread].
      4 Pubescence of the inflorescence rachis and pedicels predominantly gland-tipped, glandular hairs often present also on the young primocanes and the branches of the floricanes.
        5 Inflorescences racemose, flaring widely towards the tip, or leafy and < 2× as long as wide; calyx lobes > 7 mm long, > 3 mm wide.
          6 Primocane leaves with central leaflets ovate to elliptic-oblong, usually about 2/3× as wide as long
          6 Primocane leaves with central leaflets ovate to elliptic-oblong, usually about 3/4× as wide as long (or wider)
        5 Inflorescences narrowly racemose, usually appearing almost leafless (the bracts much smaller than the leaves, at least 2× as long as wide; sepals 5-7 (-8) mm long, 2.0-3.5 mm wide.
             7 Primocane leaves with terminal leaflets 1-1.4× as long as wide
             7 Primocane leaves with terminal leaflets < 1.6× as long as wide.
                 9 Primocane leaves with central leaflets ovate-elliptic to nearly orbicular; floricanes generally arching to low-arching, 5-13 (+) dm tall; inflorescences leafy-racemose.
                   10 Leaflet undersurfaces glabrous to moderately soft-hairy.
                     11 Leaves glabrous (or very nearly so) beneath, sometimes with prickles on the midveins of the leaflets; canes with at most few and weak prickles; leaflets of the primocanes with attenuate to caudate apices
                     11 Leaves softly pubescent beneath, usually with prickles on the midveins of the leaflets; canes with many and strong prickles; leaflets of the primocanes with acute to acuminate apices
                 9 Primocane leaves with central leaflets narrowly elliptic, elliptic, or obovate; floricanes generally erect to arching, 7-40 dm tall; inflorescences various.
                       12 Primocane leaves with central leaflets narrowly elliptic, > 2× as long as wide; inflorescences condensed at the ends of weak, leafy racemes or small, flaring clusters, the bracteal leaflets with jaggedly incised margins
                       12 Primocane leaves with central leaflets elliptic to obovate, < 2× as long as wide; inflorescences with substantial flower stalks and bracteal leaflets with regularly toothed margins.
                          13 Primocane leaves with central leaflets generally widest above the midpoint; leaflet lower surfaces densely velvety, grayish-green; inflorescences with 5-11 flowers, often condensed at the ends of stout, well-armed racemes
                          13 Primocane leaves with central leaflets generally widest at or below the midpoint; leaflets softly pubescent beneath, but not grayish-green; inflorescences with 5-16 (-20) flowers, typically racemose (rarely short and flaring).