Oenothera biennis and its close relatives form a complex that has been much debated. Currently, it seems that five other species have been split off; Oenothera elata, O. nutans, O. oakesiana, O. parviflora, and O. villosa. There are at least eleven obsolete synonyms. Characters of hairiness, glandularity, sepal shape, and relative size of flower parts have been used to describe proposed species. Now that we can compare genomes, hopefully this is, or will soon all be settled.