Light emitting diode and metal halide supplemental lighting for greenhouse Bibb lettuce production in the Midwestern United States

Abstract: Lettuce (Lactuca sativa) is one of the most common vegetable crops produced in greenhouses in the United States. Yet, it is difficult to maintain consistent production cycles in many areas due to seasonal variation in ambient light. This presents a challenge to profitability, so, many growers utilize supplemental lighting to provide more consistent production in greenhouse leafy crop operations. Research has frequently been carried out to investigate the relationships between light, temperature, and carbon dioxide (CO₂) in greenhouse lettuce crops to optimize production and profitability. Much of this work has been carried out using high intensity discharge lights (HID), specifically high pressure sodium (HPS) and metal halide (MH). Currently, growers are considering whether light emitting diode (LED) technology can augment or replace HID lighting for greenhouse-grown vegetables. To improve knowledge in this area, this study evaluated LED and MH lighting in Bibb lettuce crop in the Midwestern United States during low light seasons in 2014. Three lighting treatments were compared: 1) a naturally lighted control, 2) supplemental MH lighting, and 3) supplemental LED lighting. Three sequential runs comparing the three lighting treatments were carried out between January and April of 2014. At the conclusion of each run, fresh shoot weight for all plants was measured along with a chlorophyll content index and a subjective tipburn rating. Supplemental lighting increased biomass over naturally-lighted controls by 194%, 104%, and 39% in MH and 253%, 165%, and 55% in LED in runs one, two, and three, respectively across the two cultivars. Differences in the chlorophyll content index and tipburn rating were also observed in lettuce under both supplemental lighting treatments when compared to the naturally lighted control. This study illustrates that supplemental lighting can improve greenhouse lettuce yield during low light seasons, but suggests growers implementing supplemental lighting should carefully monitor and manage crop quality.