These are mostly very aromatic plants, a pleasure to crush in the field and scenting even a herbarium case. The corollas are white, generally dotted or tinged with a pink or purple shade.

1. Main stem (except sometimes at nodes) pubescent only on the angles or glabrous; leaves all or mostly not over 7 mm wide (rarely to 13 mm); outermost bracts of heads pubescent (if at all) only on lower surface and margins.

2. Calyx lobes with distinct subulate tip glabrate at least on upper side; stem glabrous.

P. tenuifolium

2. Calyx lobes with tip not subulate but densely white-pubescent; stem pubescent on angles with spreading and/or incurved hairs.

P. virginianum

1. Main stem pubescent on all sides; leaves often over 7 mm wide; outermost bracts of heads finely pubescent on both surfaces (only on lower surface in P. muticum).

3. Leaf blades on distinct petioles at least 2.5 mm long; inflorescence somewhat open, the branches evident.

P. incanum

3. Leaf blades sessile or essentially so; inflorescence dense and head-like.

4. Principal leaves less than 3 times as long as broad; outermost bracts of heads glabrous on upper surface.

P. muticum

4. Principal leaves over 3 times as long as broad; outermost bracts of heads pubescent on both surfaces.

5. Leaves pubescent beneath on surface as well as veins; involucral bracts merely acute.

P. pilosum

5. Leaves pubescent beneath only on main veins; involucral bracts shortly acuminate.

P. verticillatum