Deep winter greenhouses for organic production of leafy greens in the northern United States

Deep winter greenhouses (DWGs) are passively heated structures designed for winter production of high-value cold tolerant leafy greens in cold climates. These greenhouses allow growers to extend the season and help meet consumer demand for locally and organically grown fresh produce year-round. Leafy greens, including arugula (Eruca sativa), lettuce (Lactuca sativa), and mustard (Brassica juncea) crops, are well-adapted to cool growing conditions in deep winter greenhouses; however, these production systems are novel, and productivity is highly variable across locations. This research sought to optimize production of leafy greens in organic soilless substrates in DWGs. In addition, we investigated production within distinct periods, or “sub-seasons”, characterized by irradiance and photoperiod differences within the winter season in these systems. The effects of organic substrate mixes on the growth parameters of cultivated species of arugula (‘Astro’), mustard (‘Red Giant’), and lettuce (‘Five Star Lettuce Mix’), grown in the “diminishment”, “solstice”, and “expansion” sub-seasons, respectively, were evaluated. Substrates were analyzed at the end of each designated sub-season. Planting and harvest dates, harvest weights, and microclimate parameters of temperature, relative humidity, and growing degree days were recorded throughout sub-seasons. First harvest weight yields were influenced by sub-season, location, with sub-season × location interaction; thus, sub-seasonal differences within DWG microclimates and across locations were observed. The standard grower mix with blood meal maximized production of arugula, mustards, and lettuce in DWGs. This research contributes to increased understanding of cultivar, substrate, and microclimate interactions relevant to organic leafy green production in these unique winter growing environments.