Determining the Optimum Supplemental LED Intensity for Winter Production of Cut Snapdragons 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 as well as spectral composition 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 snapdragons (Antirrhinum majus ‘Opus Apple’), using four levels of LED supplemental lighting (measured at pot level): 55, 95, 135, 175 µmol·m^-2·s^-1provided daily on a 14-h photoperiod.  Each lighting treatment was concurrently replicated four times, for a total of 16 experimental plots.  Plugs at the 2–4 true leaf stage were transplanted into 8” pots, three per pot, on Dec. 5, 2015.  Pots were arranged in a 3x3 square under each lighting treatment (27 plants per plot).  The greenhouse environment was set at 14 °C day and 12 °C night with a constant 65% relative humidity. The plants were grown over a full crop cycle and harvested after the bottom three florets were fully open.  Growth metrics such as plant height, stem diameter, number of leaves and branches, chlorophyll content, and date to first floret opening were measured regularly.  Harvest metrics including: stem and inflorescence length, number of leaves and lateral branches, number of florets, and fresh weights of inflorescence, stem, leaves and lateral branches were taken on all flowers immediately after harvest.  Four plants on each plot were further measured for chlorophyll content, leaf area, and dry weights. Early vegetative growth metrics showed that stem length decreased and stem diameter increased as supplemental light intensity increased.  Mean leaf chlorophyll content index (i.e. SPAD readings) measured at solar noon on Feb. 23, 2015 were 94.6, 114.1, 122.2 and 1370.2 for the 55, 95, 135, 175 µmol·m^-2·s^-1 treatments respectively.  Relative to the 55 µmol·m^-2·s^-1 treatment, the 95, 135 and 175 µmol·m^-2·s^-1 treatments reduced the time to first floret opening by 4.7, 5.8 and 13.2 days respectively.  The percentage of plants that produced marketable flowers were 85%, 98%, 98%, and 100% for the 55, 95, 135, 175 µmol·m^-2·s^-1 treatments, respectively. Days between first floret opening and harvest was not substantially different between the light treatments.  Other growth and harvest metrics will be also reported in this poster.