This is closely related to erectum but concentrated in cool temperate to boreal regions of North America (K). There is an unverified report of possible achoreum from JEFF based on an immature coll. to be accessed at CONN (https://www.inaturalist.org/observations/30791061). P. achoreum differs as follows (FNA 5): perianth tube 40-55% of perianth length (versus 20-40%) ; tepals more or less keeled (versus not so); pedicels 1.3-2 mm (versus 2-7 mm), enclosed in ocreae; leaves usually homophyllous (versus heterophyllous), 8-35 x 3-15 mm (versus 30-80 x 8-25 mm), with petioles 0.3-1.5 mm long (versus 1-5 mm).