Our species produce two kinds of flowers: showy (chasmogamous) ones that open in sunshine but shed their petals, which are (5–) 8–17 mm long, late in the day; and tiny apetalous cleistogamous ones crowded and sessile or nearly so on the branches. The two outer sepals are rudimentary or linear, much narrower than the three inner sepals, and look like bractlets. It seems clear that our North American Frostweeds are a distinct lineage from the Eurasian Rockroses, Helianthemum (Kubitzki & Bayer, 2003).
Late in the fall, our Frostweeds produce "frost flowers;" ice crystals formed into intricate patterns along the stems just above ground level. These are visible in the morning after hard night frosts. See the pictures under Crocanthemum canadense.
1. Petaliferous flowers in clusters of 2 or more, not overtopped by leafy branches; upper surface of cauline leaves with only stellate hairs; capsules of cleistogamous flowers rarely over 2 mm in diameter, 1–2 (–3)-seeded; outer sepals of cleistogamous flowers with linear free portion ca. 0.6–1.6 mm long; ripe seeds with reticulate surface [20×].
1. Petaliferous flowers solitary, soon overtopped by leafy branches; upper surface of cauline leaves (at least when young) with scattered simple hairs much overtopping the stellate ones; capsules of cleistogamous flowers ca. 2.5 mm or more in diameter, with at least 5 seeds; outer sepals of cleistogamous flowers rudimentary, usually less than 0.5 mm long; ripe seeds with papillate surface.
All species found in Crocanthemum
MICHIGAN FLORA ONLINE. A. A. Reznicek, E. G. Voss, & B. S. Walters. February 2011. University of Michigan. Web. July 19, 2019. https://www.michiganflora.net/genus.aspx?id=Crocanthemum.