Multi-season Evaluation of Substrates for Optimized Arugula and Lettuce Production in Hydroponics

Rockwool and peatmoss are commonly used substrates in the greenhouse industry due to their quality, stable pH, exceptional water retention properties and air porosity that is important for plant root development. Although rockwool is commonly used in deep water culture (DWC) hydroponic systems as the base support, there is a lack of studies comparing different types of substrates that could be used in DWC systems, especially considering the increasing market value and awareness of sustainable production in controlled environment agriculture. We identified 13 commercial substrate mixes with different compositions and conducted a series of studies in a DWC system in a greenhouse for three seasons to evaluate their effects on arugula ‘Slow Bolt’ (Eruca sativa L.) and lettuce ‘Summer Crisp’ (Lactuca sativa L.) growth, yield, and quality. The substrates tested significantly influenced the growth, yield, and quality of both arugula and lettuce. The average leaf fresh weight per plant could range from 44 to 190 g for arugula and 89 to 265 g for lettuce. The peat-based products outperformed the coir and other inorganic substrates (phenolic foam, rockwool). The substrate with 75% peat + 25% fine coir produced the greatest plant height, width, and biomass for arugula and lettuce over three growing seasons. Examining arugula and lettuce growth, the fall season produced plants with higher water and nutrient use efficiency, while plants grown during the winter had lower resource use efficiency. Further research is needed to engineer hydroponic substrates suitable for various seasons of leafy green production that results in comparable yield and quality.