2018 ASHS Annual Conference

Retailers and consumers generally do not desire or accept potted or fresh cut culinary herbs with flowers as vegetative growth can be negatively impacted. This in turn creates problems for greenhouse growers who often struggle to keep basil (Ocimum) vegetative. Limited photoperiodic studies have been conducted on common herbs such as basil to determine how changes in day length can influence flowering. The objective of this study was to quantify how photoperiod influences growth and development of 4 basil species and 11 cultivars. Seeds of ‘Genovese Basil’, ‘Sweet Thai’, ‘Cinnamon Basil’, ‘Red Ruben’, ‘Sweet Dani Lemon’, ‘Purple Ruffles’, and ‘Nufar OG’ (Ocimum basilicum), ‘Lime Basil’ (Ocimum ×citriodorum), ‘Holy Basil’ (Ocimum tenuiflorum), ‘Mrs. Burns’ Lemon’ (Ocimum basilicum var. citriodora), and ‘Pluto Basil’ (Ocimum minimum) were germinated and grown at 25 °C, under supplemental lighting provided by red:white:blue (R:W:B) light-emitting diodes (LEDs) providing a PPFD of 90 µmol·m^–2·s^–1 and under eight different photoperiods. Photoperiods were a 9-hour day extended with LEDs providing 2 µmol∙m⁻²∙s⁻¹ of R:white:far-red (R:W:FR) light to create 11, 12, 13, 14, 15, or 16 hours; an additional treatment was a 9-hour day with a 4-hour night interruption (NI). Time to first visible bud and flower, node number below the first open flower, and plant height at flowering were recorded for each plant. Photoperiod did not significantly influence days to visible bud or flower for ‘Cinnamon’, ‘Genovese’, and ‘Red Rubin’ (Ocimum basilicum). However, height of ‘Cinnamon’, ‘Nufar’, ‘Sweet Thai’, and ‘Red Rubin’ were significantly influenced by photoperiodic treatments. Generally, plants were more compact under shorter day lengths. Flowering of ‘Mrs. Burns’ Lemon’ and ‘Holy basil’ was hastened under 9-hour photoperiods. For example, ‘Holy basil’ (Ocimum tenuiflorum) under a 9-hour day generally flowered 6, 7, or 10 days earlier than plants under a 15-h, 16-h, or NI treatment, respectively. From this preliminary study, it appears that cultivars investigated of Ocimum basilicum var. citriodora and Ocimum tenuiflorum can be considered facultative short-day plants and Ocimum basilicum and Ocimum ×citriodorum are day-neutral.