Are Light-emitting Diodes a Viable Supplemental Lighting Alternative to Grow Greenhouse Tomatoes in a Northern Climate?

Overhead (OH) high-pressure sodium (HPS) lamps are the preferred type of supplemental lighting (SL) for greenhouse vegetable production because their high-intensity capability allows them to deliver significant supplemental photosynthetically active radiation (PAR). Furthermore, OH-HPS lamps currently are the most economically viable mass-produced light source available for greenhouse SL that provide adequate PAR. Light-emitting diodes (LEDs) are a promising SL technology for greenhouse crop production because they offer the opportunity to improve energy efficiency for greenhouse lighting. We conducted four 5-month experiments in a glass-glazed greenhouse during 2012 and 2013. The objective was to quantify growth and yield of winter-to-summer [increasing natural daily light integral (DLI)] and summer-to-winter (decreasing natural DLI) high-wire tomato production in a temperate climate (lat. 40°N, long. 86°W) with and without (control) SL. We evaluated two different SL positions + sources [OH-HPS vs. intracanopy (IC; lighting from within the canopy)-LED] and compared those to a control for several production and energy-consumption parameters. Even though minor changes were made between experiments, general productivity and yield trends remained consistent. Neither the number nor total mass of fruit were different between OH-HPS and IC-LED treatments, and both SL treatments yielded more fruit and had more fruit mass than did controls. The electrical conversion efficiency of IC-LED energy into fruit biomass was higher than that of OH-HPS, and energy savings ranged from 28% to 50% for IC-LED relative to OH-HPS SL. These results suggest that with ongoing, anticipated energy-efficiency improvements, as well as ever-improving light-distribution architectures, LEDs could become the dominant future SL technology for greenhouse crop production, eventually replacing OH-HPS.  Significant opportunities remain to optimize spectral-quality effects on plant growth and development. Extensive trials are needed to establish economically viable ‘best practices’ for how to use LED lighting in greenhouse production and to further promote its wide-spread adoption.