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Key to Setaria

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1 None of the spikelets subtended by a stiff bristle
1 Terminal spikelet on each branch subtended by a single bristle (other spikelets also sometimes so subtended)
  2 Most spikelets other than the terminal lacking a subtending bristle; leaves flat or plicate.
    3 Blades flat, 2-5 mm wide; only terminal spikelet of each branch with a subtending bristle
    3 Blades plicate, 10-80 mm wide; a single bristle sometimes present below non-terminal as well as terminal spikelets.
      4 Annual; leaves 10-25 mm wide
      4 Perennial; leaves 20-80 mm wide
  2 All spikelets subtended by 1 or more bristles; leaves flat.
        5 Bristles 4-12 below each spikelet.
          6 Annual, with fibrous roots.
             7 Spikelets 2.0-2.5 mm long; bristles reddish
             7 Spikelets 3.0-3.4 mm long; bristles yellow
          6 Perennial, noticeably rhizomatous.
               8 Panicle 3-8 (10) cm long; plant from knotty rhizomes; [native, common (sometimes weedy)]
               8 Panicle 5-25 cm long; plant from thick rhizomes; [exotic, rare]
        5 Bristles 1-3 (rarely 6) below each spikelet.
                 9 Bristles retrorsely scabrous.
                   10 Leaves strigose on the lower surface; sheath margins glabrous; panicles 2-6 cm long
                   10 Leaves scabrous on the lower surface; sheath margins ciliate; panicles 5-15 cm long
                 9 Bristles antrorsely scabrous
                     11 Perennial
                     11 Annual.
                       12 Upper lemmas smooth and shiny (occasionally with obscure rugosity)
                          13 Culms to 1 m tall; spikelets ca. 3 mm long; [exotic, of ruderal sites]
                          13 Culms to 6 m tall; spikelets ca. 2 mm long; [native, of marshes]
                       12 Upper lemmas distinctly transversely rugose, dull.
                            14 Upper lemmas coarsely rugose; leaves 4-7 mm wide; [native]
                            14 Upper lemmas finely rugose; leaves 4-25 mm wide; [exotic, generally of ruderal sites].
                              15 Panicles verticillate; rachises visible, scabrous
                              15 Panicles densely spiciform; rachises not visible, villous.
                                16 Leaves softly pilose on the upper surface; panicles arching and drooping from the base; spikelets 2.5-3.0 mm long
                                16 Leaves scabrous on the upper surface; panicles nodding only at the tip; spikelets 1.8-2.2 mm long.
                                  17 Panicles 10-20 cm long; culms 10-25 dm tall; leaves 10-25 mm wide
                                  17 Panicles 3-8 cm long; culms 2-10 dm tall; leaves 4-12 mm wide