Flowering control in Watsonia: Effects of corm size, temperature, photoperiod and irradiance

The role of corm size, light and temperature in flowering of Watsonia species was evaluated to facilitate their commercial production. In addition to exhibiting desirable ornamental attributes, the species selected represented the major climatic regions in South Africa. A day/night temperature regime of 12/7 °C released vegetative dormancy in all species and thereafter elicited vernalization in Watsonia pillansii – highlighting an obligate cold requirement for this species. Flowering of Watsonia borbonica and Watsonia tabularis was not enhanced by additional chilling, but rather by long (16 h) or day-neutral (12 h) photoperiods. Microscopic examination of the shoot apical meristem revealed that extension of the 2nd leaf was a critical stage developmentally, and signified the anatomical transition to flowering. Late-development temperatures to a maximum of 25 °C ensured healthy vegetative growth and supported the maturation of the inflorescence and the opening of floret buds. Irradiance did not affect flower induction, but a minimum light intensity of 150 μmol m−2 s−1 proved essential in sustaining the energetic demands of the competitive growth and reproductive processes. Excessively high irradiance (950 μmol m−2 s−1) impacted negatively on attractiveness through increased bud blasting. Flowering success was not correlated to corm mass, but rather to the environment under which the corm was stored, or the conditions under which the plant was grown. Understanding the phenology of these species in situ and the link between flowering and season provide a useful tool for predicting the artificial requirements necessary to elicit optimal flowering under industry conditions.

► Watsonia species exhibit vast ornamental potential that warrants domestication.
► Cool temperatures (12/7 °C) released vegetative dormancy in all species.
► Flowering was initiated by cold or lengthening photoperiod, depending on species.
► Moderate irradiance sustains the simultaneous growth and reproductive processes.
► In situ phenology drivers predict optimal cultivation conditions for Watsonia.