Evaluation of Organic Winter Greens Production In Unheated High Tunnels

The winter production of cold tolerant organic crops using unheated high tunnels in Colorado shows promise due to high light intensity and abundant cloud-free days; however, to date, no studies have been performed to identify appropriate crops and management techniques are best suited to Colorado's unique environment. Three treatments of floating row covers over salad were evaluated in terms of temperature ranges experienced, plant development, and nitrate accumulation. Ten suitable crops were planted in a certified organic, unheated high tunnel at Colorado State University's Horticulture Field Research Center. Lactuca sativa (‘Winter Density,' ‘Galisse,' ‘Cherokee,' ‘Focea,' ‘Magenta'), Valerianella locusta ‘Vit,' Diplotaxis tenuifolia ‘Sylvette,' Brassica rapa ‘Mei Qing,' Brassica rapa ‘Early Mizuna,' and Brassica juncea ‘Yukina,' were direct seeded on Oct 30th, 2008. The treatments consisted of 1) “1X” a single layer of floating row cover (FRC)(Agribon19 0.5oz. spun bond polyester) suspended with wire hoops, 2) “2X” two layers of FRC suspended with wire hoops, and 3) “0X” uncovered. The treatments were replicated three times over nine completely randomized beds. Three lettuce varieties, ‘Galisse,' ‘Cherokee,' and ‘Focea,' were analyzed for nitrate accumulation at harvest on Feb. 11th, 2009. Temperature increases over external temperatures of 2.90⁰C, 6.82⁰C, and 7.61⁰C were observed for 0X, 1X, and 2X treatments, respectively, while minimum temperatures were elevated 9.94⁰C, 19.59⁰C, and 21.79⁰C, respectively. Lettuce varieties showed a 436% increase in mean plant mass between the 0X and 1X treatments, but only a 23% increase between the 1X and 2X treatments. Asian greens showed disparity, with ‘Mei Qing' and ‘Yukina' showing an increased mass between 1X and 2X treatments of 90.9% and 89.8%, but with ‘Sylvette' and ‘Early Mizuna' showing decreased mass of 207.0% and 64.8%. Nitrate accumulation increased with the amount of covering, but differed by variety, with mean concentrations for ‘Cherokee,' ‘Focea,' and ‘Galisse' of 190.4, 150.4, and 157.0 mg/kg NO3 for 0X, 415.0, 683.4, and 439.3 mg/kg NO3 for 1X, and 727.3, 1176.3, and 577.1 mg/kg NO3 for 2X treatment, respectively. No sample demonstrated nitrate levels above the European Commission regulated safety level of 4500 mg/kg NO3. Results indicate that organic winter production of salad greens in unheated high tunnels with the use of floating row cover is a viable option for Colorado growers, but differential responses to the thickness of cover necessitates further study to determine best management practices for individual crops.