| | 1. Largest head 6–14 mm; longest chaff scale generally 2.8–4 mm, or if shorter then hidden by dense shaggy hairs |
| |
| | 2. Largest head 9–14 mm, ± ovoid, receptacle deeply lobed; chaff scales narrowly cylindric, length > 3 × width, wing ± 1/2 distance from base to tip of scale |
| P. brevissimus var. multiflorus |
| | 2' Largest head 6–9 mm, spheric, receptacle entire or shallowly lobed; chaff scales obovoid, < 3 × width, wing ± 2/3 distance from base to tip of scale |
| |
| | 3. Stems generally spreading, branched from base, central stem 0 or not dominant; uppermost leaves generally lanceolate to ovate, longest generally 8–15 mm, generally 1.5–4 × > wide; wool dense, loose; fruit ± obovoid |
| P. brevissimus var. brevissimus |
| | 3' Stems ± erect, generally branched above, central stem dominant; uppermost leaves generally linear to oblanceolate, longest generally 17–35 mm, generally 4.5–9 × > wide; wool thin, ± appressed; fruit ± cylindric |
| P. elatior |
| | 1' Largest head generally < 6 mm; longest chaff scale generally < 2.8 mm, visible through thin, ± appressed, cobwebby or silky hairs |
| |
| | 4. Leaves linear to narrowly oblanceolate, generally > 5 × width, generally > 3 × heads; fruit ± cylindric |
| P. oregonus |
| | 4' Leaves generally ovate to obovate, generally < 5 × width, < 3 × heads; fruit ± obovoid |
| P. tenellus |
| | 5. Uppermost leaves ovate to widely elliptic, generally < 2 × width, ± appressed to heads; disk flowers generally 4-lobed |
| var. globiferus |
| | 5' Uppermost leaves oblanceolate to obovate, generally > 2 × width, spreading below heads; disk flowers generally 5-lobed |
| var. tenellus
|