Evaluation of Light-emitting Diode and Metal Halide Supplemental Lighting for Greenhouse Bibb Lettuce Production in the Midwest United States

Controlled environment vegetable production is an expanding sector in horticulture in the US due both to consumer demand for locally produced crops as well as environmental concerns. However, many regions experience winter light levels that reduce growth rate and salable produce. Leafy crops are attractive for supplemental lighting in greenhouse operations due to lower light requirements than fruiting crops, allowing growers to enhance production with lower capital investments and operation costs. A key current grower question is the potential for light emitting diode (LED) technology to augment or replace high intensity discharge lighting (HID) in greenhouse vegetable production. This work is intended to evaluate LED and HID lighting compared with natural lighting in bibb lettuce crops in Ohio winter and spring production. All crops were produced in a double layer polyethylene greenhouse in Lodi, OH, and two runs of the experiment were carried out from January through March, 2014. Two commercial bibb lettuce (Lactuca sativa) cultivars (‘Flandria’ and ‘Teodore’) were grown using the nutrient film technique with solutions maintained at 1.8- 2.0 mS cm^-1 electrical conductivity (depending on ambient conditions) and 5.9 pH. Following a 96-hr germination period, seedlings were produced under lighting treatments for approximately two weeks prior to transplanting, and then harvested after an additional four weeks in the NFT channels. Three lighting treatments were compared including: 1) a naturally lighted control, 2) supplemental HID lighting (400W metal halide; PARSource, Petaluma, CA), and 3) LED lighting (Philips GreenPower deep red/blue, Eindhoven, The Netherlands). Supplemental lighting was installed to provide similar average light intensity of 100 µmol/m²/sec. Lights were operated for 16 hours per day in run one and 12 hours per day in run two to provide an additional 5.8 and 4.3 mol/m²/day, respectively, for both lighting treatments. Fresh shoot weight was measured on harvested plants along with a chlorophyll content index (CCM-200, Opti-Sciences, Hudson, NJ) and tipburn rating. Supplemental lighting showed biomass increases over controls of 193% and 104% in metal halide and 252% and 164% in LED in runs one and two, respectively. However, the potential for quality impacts, including increases in the chlorophyll content index and tipburn, were also observed in both lighting treatments. These experiments indicate opportunities for alternative lighting to impact crop yield during winter and spring greenhouse production, but illustrates that growers implementing supplemental lighting should be aware of the potential for crop quality impacts.