A high daily light integral can influence photoperiodic flowering responses in long day herbaceous ornamentals

Commercial producers of herbaceous ornamentals have reported that long-day plants sometimes flower under short-day conditions when the photosynthetic daily light integral (DLI) is high, but little research-based information is available to support the phenomenon. We grew seedlings of tickseed (Coreopsis grandiflora), echinacea (Echinacea × hybrida), lavender (Lavandula angustifolia), lobelia (Lobelia × speciosa), and salvia (Salvia longispicata × farinacea) in a controlled-environment greenhouse. They were then transplanted and grown under photoperiods of 10, 11, 12, 13, 14, or 15 h under an average DLI of 5.3 or 13.4 mol·m−2·d−1 and at an air temperature setpoint of 20 °C. Based on flowering percentage, the DLI influenced the photoperiodic responses in four species. Flowering percentages of tickseed, echinacea, lavender, and lobelia were lower under the low DLI compared with the high DLI under photoperiods <11, <12, <12, or <13 h, respectively. Time to flower for all species generally decreased as photoperiod increased. In addition, most species had more inflorescences and were more compact under the higher DLI. Under the high DLI, all echinacea and lobelia plants flowered under all photoperiods tested. Salvia flowered in all treatments, and formed fewer leaves before flowering under photoperiods ≥14 h compared to other photoperiods. However, under the higher DLI, salvia plants took slightly longer to flower under 15 h than 14 h. To clarify its photoperiodic response, we performed an additional experiment with salvia the following year. At germination, seedlings were placed under photoperiods of 14, 15, or 16 h, or a 9 h day with a 4-h night interruption and received an average DLI of 9.2 mol·m−2·d−1. Flowering was delayed under the 16 h and night-interruption treatments compared to under 14 and 15 h. We conclude that photoperiod and DLI interact to influence flowering of at least some long-day plants and that salvia has an intermediate photoperiodic flowering response.