A large and predominantly tropical genus, not nearly as difficult in Michigan as the mostly temperate segregate Dichanthelium. The key to the Panicum capillare complex is somewhat intricate but contrasting more species pairs is intended as an aid to gaining familiarity with this group.

 

1. Spikelets all or mostly 3 mm or more in length, strongly nerved.

2. First glume more than half as long as second glume; lower floret staminate; plants over 5 dm tall, essentially glabrous (except for margin and throat of leaf sheath), perennials from strong scaly rhizome, with panicle terminal and over 15 cm tall.

P. virgatum

2. First glume not over half as long as second glume (except in P. miliaceum), lower floret with neither stamens nor pistil; plants shorter, usually ± pubescent, fibrous rooted annuals, with panicles usually shorter or several per stem.

3. Leaves (blades and sheaths) essentially glabrous.

P. dichotomiflorum (in part)

3. Leaves pubescent (at least on sheaths).

P. miliaceum

1. Spikelets less than 3 mm long, strongly nerved or not.

4. Spikelets covered with mealy-looking warts, but nearly nerveless.

P. verrucosum

4. Spikelets not warty, nerved or not.

5. Sheaths of middle and upper leaves glabrous on back (may be ciliate on margins).

P. dichotomiflorum (in part)

5. Sheaths sparsely to heavily pilose on back.

6. Panicle 2-3 times as long as wide, with clearly ascending branches.

P. flexile

6. Panicle less than 2 times as long as wide, branches ± spreading at maturity.

7. Peduncles long-exserted from sheaths, exserted portion half as long as panicle or longer.

8. Spikelets lanceolate in outline, acuminate at apex, the longest 2.4-3.3 mm long.

P. capillare (in part)

8. Spikelets ovate or narrowly ovate in outline, acute at apex, the longest 1.9-2.3 mm long.

P. philadelphicum

7. Peduncles not or only slightly exserted from sheaths, exserted portion less than half as long as panicle.

9. Spikelets 1.5-2.0 mm long.

10. Spikelets 0.8-1.2 mm wide; arranged in loose pairs or solitary at the ends of the panicle branches.

P. gattingeri (in part)

10. Spikelets 0.6-0.8 mm wide; arranged in racemose groups of 2-6 at the ends of the panicle branches.

P. tuckermanii

9. Spikelets 2.1-3.5 mm long.

11. Spikelets lanceolate in outline; second glume and sterile lemma forming an acuminate beak 1/5-2/5 as long as the spikelet and extending 0.4-1.2 mm beyond the tip of the fertile lemma.

P. capillare (in part)

11. Spikelets narrowly ovate in outline; second glume and sterile lemma forming an acute beak 1/10-1/5 as long as the spikelet and extending 0.2-0.5 mm beyond the tip of fertile lemma.

P. gattingeri (in part)