"late," 1590s, from French sérotine, from Latin serotinus "that which comes late; that which happens in the evening," from sero, adverb of serus "late" (see soiree). Also as a noun, a type of small, brown bat, from 1771. Related: serotinous, in botany (1880) "appearing later in the season than usual."
Late in developing, opening, or blooming. For example, serotinous pine cones may persist unopened on the tree for years and only burst open during a forest fire. Serotinous flowers on trees develop only after the tree has produced leaves.