Effects of Supplemental LED Spectral Composition and Irrigation Electrical Conductivity on Vegetative Biomass, Fruit Yield and Quality, and Lighting Electrical Cost in Greenhouse Strawberry Production

Greenhouse strawberry production has rapidly increased around the world, since greenhouse production allows for protection from adverse weather events and optimized climate control. During the winter months, supplemental electric lighting is essential to maintain production. However, the use of supplemental lighting can account for up to 30% of operating expenses, impacting the profitability of production. In addition, strawberries are prized for their taste, so maintaining fruit quality during production is essential. This study examined the impact four different supplemental overhead LED spectral lighting treatments (red–blue (95% red, red-rich), pink, warm white, and white) and two different irrigation electrical conductivity (EC) levels (low = 2 mS cm−1 and high = 3 mS cm−1) on morphology, physiology, fruit quality and electricity costs in two strawberry cultivars—cv. ‘Albion’ and cv. ‘Favori’—grown in rockwool during the 2023–2024 winter greenhouse season in Harrow, Ontario, Canada. The results showed that plants exposed to a board spectrum generally had higher photosynthetic rates and vegetative biomass. However, a threshold was observed when the green light fraction was above 24% (i.e., plants grown under the white-light treatment), resulting in reductions in photosynthesis and vegetative biomass. Although differences in photosynthetic capacity and biomass were observed, there was no impact of the light spectrum or irrigation EC on yield in either cultivar. Compared to the red–blue spectrum, plants grown under pink and warm white were observed to have lower titratable acidity (TA) and higher ratios of total soluble solids (TSSs) relative to TA. Importantly, it was determined that due to the higher efficacy of the red–blue supplemental lighting fixtures, the red-rich spectrum resulted in the lowest estimated lighting electricity cost per kilogram under the tested conditions. This study suggests that the use of supplemental high-efficacy, narrow wavelength, red-rich supplemental lighting can maintain yield and improve economic feasibility for greenhouse-grown strawberries.