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CommentsLate Horse Gentian is easily distinguished from other species of Horse Gentian (Triosteum spp.) by its stem-surrounding opposite leaves (they are connate-perfoliate). Other species in this genus have opposite leaves that taper to narrow bases that are little wider than the central stem. Except for this difference in their leaves, Late Horse Gentian is very similar in appearance to Early Horse Gentian (Triosteum aurantiacum) and they prefer similar habitats. Another species, Triosteum angustifolium (Yellow-Flowered Horse Gentian), has yellow flowers and more narrow leaves; its state distribution is restricted to southern Illinois.