Quick note for people who are using Nesom’s chapter in FNA as their primary source for detailed descriptions of Oxalis species in the northeastern US. When he writes that O. dillenii and O. stricta have “petals yellow, without red lines” he must not mean that they have no red on the flowers at all – because here are multiple examples, in western NY alone, of both those species with distinct red marks in the center:
https://www.inaturalist.org/observations/47666253
https://www.inaturalist.org/observations/54111501
https://www.inaturalist.org/observations/85728193
https://www.inaturalist.org/observations/80126511
https://www.inaturalist.org/observations/123989290
“Lines” would extend much further along the petal, I suppose.