Searching for an Optimum Supplemental LED Intensity for Winter Production of Cut Gerbera at Northern Latitudes

Cut flower production at northern latitudes typically requires supplemental lighting during the darker months. With LED technologies poised to overtake traditional HID for greenhouse assimilation lighting, it is important to determine the optimum supplemental LED light levels for the production of various commodities and ambient lighting conditions. Lighting manufacturers, suppliers and growers need this information to guide design and operational protocols for greenhouse production.

This project investigated the production of cut gerbera (Gerbera jamesonii ‘Ultima’), using five levels of LED supplemental lighting (measured at pot level): 41, 76, 133, 171 and 179 µmol·m^-2·s^-1 provided daily on a 12 hour photoperiod (DLI ranging from 1.76 to 7.72 mol·m^-2·d^-1). Each lighting treatment was concurrently replicated four times, with two plants per plot (eight plants per treatment total). Plugs were received on October 8, 2015 and transplanted into pots with coco coir media. The greenhouse environment was set at 22 °C day and 16 °C night with a constant 75% RH. Flower initiation began 45 days after transplanting (Dec. 3, 2015) and flowers were harvested until Feb. 26, 2016.

Vegetative growth metrics such as number of leaves and chlorophyll content index were measured biweekly. Flowering metrics including visible bud formation and flower harvest dates, stem length, flower diameter and fresh weight were measured on each flower harvested. Vegetative growth metrics showed that number of leaves and chlorophyll content index increased with increasing supplemental light intensity while time between flower initiation and harvest decreased. Flower fresh weight and total number of flowers harvested increased while stem length decreased with increasing supplemental light intensity. For most metrics investigated, the 133 µmol·m^-2·s^-1 (5.76 mol·m^-2·d^-1) treatment was substantially better than the lower intensities while generally similar to the higher intensities.