The effects of light quantity and vernalization on growth and flowering of Arabidopsis, Achillea, Gaura, Isotoma, Lavandula and Veronica

Herbaceous perennials often have a photoperiod or vernalization requirement for flowering. During greenhouse production, reduced flowering of herbaceous perennials has been observed following inadequate cold or when grown under low light conditions even when provided with appropriate photoperiodic conditions for flower induction. Our knowledge of light quantity effects on flowering is limited, and the bulk of our knowledge on vernalization is based upon studies with annual and biennial crops. Therefore, experiments were performed to (1) characterize the influence of daily light integral (DLI), the average amount of light received per day, and (2) quantify the effects of vernalization on growth and flower induction for diverse herbaceous species.; Achillea millifolium 'Red Velvet', Gaura lindheimeri 'Whirling Butterflies' and Lavandula angustifolia 'Hidcote Blue' were grown at 20°C with a 16-h photoperiod with varied DLI from 5-20 mol·m-2·d-1. Low DLI did not limit flowering and had little effect on time to visible bud or flower of these species, hence the minimum DLI required for flowering was <5 mol·m-2·d-1, the lowest light level tested. All species exhibited prostrate growth with weakened stems when grown with <10 mol·m-2·d-1. Visual quality and shoot dry mass increased linearly with DLI from 5-20 mol·m -2·d-1 with no evidence of decline. In the past, 10 mol·m-2·d-1 was suggested as an adequate DLI for production of herbaceous perennials, yet these results suggest that 15-20 mol·m-2·d-1 should be considered a minimum for production of Achillea, Gaura and Lavandula.; The influence of vernalization temperature (-2.5 to 20°C) and duration on subsequent flower induction was investigated in Achillea 'Moonshine', Isotoma axillaris, Veronica spicata 'Red Fox' and three Arabidopsis thaliana accessions, Columbia, Rodasen from Sweden, and Bolsena from Italy. Veronica, Isotoma and Rodasen exhibited obligate, Bolsena exhibited near obligate, and Achillea and Columbia exhibited weak vernalization responses. Veronica was most effectively vernalized at -2.5°C, Isotoma was equally and most effectively vernalized at 5 to 12.5°C while -2.5 to 15°C were effective in vernalizing Rodasen. Veronica and Isotoma were not vernalized above 10 and 12.5°C, respectively. Vernalization at 5 to 10 or 12.5°C was not required for but appeared to hasten flowering and increase inflorescence number of Columbia and Achillea, respectively. (Abstract shortened by UMI.)...