Polygonum

1. Flowers in the axils of short bracts, all but the lowermost bracts less than twice as long as the flowers, the inflorescence thus appearing to be a remotely flowered slender spike; plants stiffly erect with leaves mostly linear (rarely all nearly elliptic) and very sharply acute.

2. Pedicels with a swollen joint near the middle (but not far above the sheathing ocreolae); leaves not over 1 (–1.1) mm wide.

P. articulatum

2. Pedicels usually jointed at or near the summit; leaves at least (1.5–) 2 mm wide.

3. Flowers and fruit becoming strongly reflexed; leaves flat or with margins revolute but glabrous; fruiting perianth ca. 3.5–4.5 mm long.

P. douglasii

3. Flowers and fruit erect; leaves longitudinally folded or grooved (W-shaped in cross-section) with minutely ciliate margins; fruiting perianth ca. 3–3.5 mm long.

P. tenue

1. Flowers in the axils of foliage leaves, these mostly at least twice as long as the flowers; plants erect to ascending or prostrate, with leaves acute to obtuse or blunt.

4. Mature achene with the minutely-dotted background not disrupting the very shiny surface aspect; plant erect with ascending branches and narrowly elliptic acute leaves.

P. ramosissimum

4. Mature achene dull or sparkling with minutely-dotted surface breaking up the smooth shiny aspect, rarely glossy; plant erect to spreading or low and prostrate with leaves various (narrowly elliptic and acute almost only when plant is prostrate).

5. Outer tepals scarcely if at all keeled (rarely slightly keeled), not exceeding the inner ones; plants with at least the lower branches spreading, the whole plant often prostrate and mat-forming; leaves (1.5–) 2–8 (–15) mm broad, often narrowly elliptic, acute.

P. aviculare

5. Outer tepals becoming prominently keeled (hence boat-shaped) and mostly exceeding the inner ones, often forming a definite beak-like projection beyond the achene; plants ± erect; leaves (at least the widest) (6–) 8–18 (–24) mm broad, elliptic to obovate, usually obtuse to rounded at apex.

6. Tepals fused at least half their length (slightly beyond widest portion of fruit), their margins whitish.

P. achoreum

6. Tepals fused less than half their length (not as far as widest portion of fruit), their margins yellowish (or a few rosy).

P. erectum

All species found in Polygonum

Polygonum achoreumSMARTWEED 
Polygonum articulatumJOINTWEED 
Polygonum aviculareKNOTWEED 
Polygonum douglasiiWESTERN SMARTWEED 
Polygonum erectumERECT KNOTWEED 
Polygonum ramosissimumBUSHY KNOTWEED 
Polygonum tenueSLENDER KNOTWEED 

Citation:

MICHIGAN FLORA ONLINE. A. A. Reznicek, E. G. Voss, & B. S. Walters. February 2011. University of Michigan. Web. August 19, 2017. http://michiganflora.net/genus.aspx?id=Polygonum.