Note that the key to the species in Flora of Alberta requires:

rays absent, inconspicuous or short, NOT more than 5 mm long; NOT [rays present, longer than 5 mm)]

This is incorrect.  Note also that the same key requires somewhat contradictory attributes of the leaves.  These are labeled (FOA) below.

Philadelphicus:  Answers to key questions in Budd's Flora (BF) and Flora of Alberta (FOA) leading to this species.  plants usually more than 20 cm high plants NOT producing leafy stolons leaves more or less entire, not dissected, trifid or lobed at the apex; NOT [leaves dissected, or at least some trifid or lobed at the apex] (FOA) basal leaves lobed or toothed, occasionally entire (FOA) stem leaves gradually reduced upward; NOT [stem leaves at middle of stem longer than those above and below] cauline leaves auriculate-clasping heads usually more than one; NOT [head solitary] peduncles straight; NOT arcuate involucre at least 6 mm high; NOT [involucre 4-5 mm high] involucre NOT deep purple involucre NOT glandular involucre NOT densely villous, NOT woolly-villose involucre hairs NOT with purple septa heads with well developed ray florets; NOT [heads discoid or with short inconspicuous ray florets] ray florets colored; NOT white rays NOT yellow

 

Erigeron: Answers to key questions in Budd's Flora and Flora of Alberta leading to this genus. The answers do not match all species of Erigeron found in the Canadian prairies, but they do match this species. juice watery; NOT milky NOT [basal leaves mostly 1-6 cm long, linear to oblanceolate or spatulate, forming a dense rosette; plants with a well-developed taproot; and either densely caespitose, seldom more than 5 cm tall, the heads sessile among the leaves, or with stout, simple stems, 5-15 (25) cm tall, heads solitary, involucres 2-4 cm wide, bracts ciliate to pectinate or fimbriate, rays blue and achenes densely pubescent, obovate to triangular] plants usually with leafy stems; NOT [plants low, stemless, tufted] leaves NOT extending beyond flowers leaves appearing before the flowers leaves alternate or basal; NOT opposite basal leaves NOT cordate, NOT sagittate involucral bracts subequal or more or less imbricate, often green in part, but neither definitely leafy nor with chartaceous base and herbaceous green tip; NOT [involucral bracts either subequal and the outer leafy or more commonly evidently imbricate, with chartaceous base and evident green tip, sometimes chartaceous throughout] bracts in 1 or 2 series; NOT [bracts in several series] flower heads with florets both tubular and ray ray florets pistillate or neuter; NOT perfect ray florets conspicuous ray florets 50 or more; NOT [ray florets 10-50] rays white, to pink, purple or blue; NOT [yellow or orange] stamens united to form a tube around the pistil style-branches 0.5 mm long or less, or obsolete; NOT [style-branches ordinarily more than 0.5 mm long] style-branches lanceolate or broader, acute to obtuse; NOT [style-branches lanceolate or narrower, acute or acuminate] pappus of capillary bristles, at least in part; NOT [pappus of 1 or 2 short awns, a mere crown or none] pappus composed of numerous hairs; NOT [pappus absent or composed of a few hairs]