Vernalization required to induce flowering in rosettes of garlic mustard (Alliaria petiolata)

Abstract Garlic mustard [Alliaria petiolata (M. Bieb.) Cavara & Grande] is an invasive Brassicaceae species native to Europe. As obligate biennials, A. petiolata rosettes require a sufficient length of cold during the winter to flower the following spring. As such, mild winter temperatures could limit the species' potential western and southern distribution in North America. The goal of this research was to characterize the vernalization period required for A. petiolata rosettes to develop the capacity to flower. The objectives of this research were: (1) confirm whether A. petiolata rosettes required a vernalization period to flower; (2) define base and upper temperatures that satisfy the vernalization requirement of A. petiolata rosettes under field conditions; (3) determine the vernalization requirement for flowering and calculate chilling degree days (CDD) accumulated during the cold period; and (4) in a common garden, determine the vernalization requirement of A. petiolata rosettes collected from the southern and northern ranges of its distribution in North America and Europe. The probability of flowering increases as A. petiolata rosettes accumulate CDD. This relationship is defined by a binary logistic (logit) function, with base and maximum temperatures of –3 and 4 C, respectively. The regression equation model predicted that 68 and 120 CDD would result in 50% and 99% probability of flowering, respectively, across all locations. Rosettes from five different seed sources varied in the CDD required for flowering when grown in a common garden. Rosettes originating from a Croatia seed source flowered after exposure to fewer CDD than those from Scotland. In North America, rosettes originating from seeds from Arkansas were more likely to flower after exposure to fewer CDD than those from Ohio or Minnesota. Our results may be used to predict the potential distribution of A. petiolata in North America.