post rorem, ‘after the dew’, seems nonsense and Shackleton Bailey has not unreasonably proposed post florem, comparing for the idiom Columella RR 2. 11. 10 diebus quadraginta, quibus post florem ad maturitatem devenit. But ros here stands for ros marinus, ‘rosemary’, as in Vergil, Georg. 2. 212–13:

nam ieiuna quidem clivosi glarea ruris

vix humilis apibus casias roremque ministrat.

The poet is not presenting us with a piece of botanical information about the relative seasons of the violet and rosemary; he means rather that all flowers wither and fade, one after the other. Four specific examples illustrate his point. For the collocation of these same four flowers see Ovid, Met. 12. 410–11:

ut modo rore maris, modo se violave rosave

inplicet, interdum canentia lilia gestet