Submitted to: Industrial Crops and Products
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/17/2016
Publication Date: 9/1/2016
Publication URL: http://handle.nal.usda.gov/10113/63127
Citation: Shiwakoti, S., Zheljazkov, V.D., Schlegel, V., Cantrell, C.L. 2016. Growing spearmint, thyme, oregano, and rosemary in Northern Wyoming using plastic tunnels. Industrial Crops and Products. 94:251-258.

Interpretive Summary: Essential oils are important bioactive substances of aromatic plants and herbs with diverse applications in various industries such as pharmaceutical, food and cosmetic industries. However, there are many places in the world where growing perennial herbs is dubious because of unfavorable growing conditions. Growing herbs in greenhouses could mitigate the adverse growing conditions beyond the short summer season. However, constructing and maintaining greenhouse is costly for most of the farmers. A solution to this barrier is construction of plastic tunnels (high tunnels, and low tunnels) which are relatively inexpensive and simple to construct and maintain compared to greenhouses. High tunnels (HT) and low tunnels (Lt) have become a significant feature in intensive horticultural production systems around the world as means for extension of the growing season of crops. The objectives of this study were to: 1) compare herbage yield, essential oil yield and composition, and antioxidant capacity of four herbs grown under three different season extension methods, and 2) determine whether growing herbs in late fall in Northern Wyoming is feasible. The four herbs were Greek oregano (Origanum vulgare subsp. hirtum L.), native spearmint (Mentha spicata L.), common thyme (Thymus vulgaris L.), and rosemary (Rosmarinus officinalis L.). These four herbs were selected based on their high popularity with customers for their extensive use as culinary herbs, and due to their relatively high resiliency to adverse conditions. This study demonstrated that low tunnel within high tunnel can provide optimal conditions for spearmint, thyme, oregano, and rosemary fresh herbage and essential oil production in northern climates, even when the temperature falls below the freezing point in late fall.

Technical Abstract: Growing perennial herbs in northern climate such as Northern Wyoming is a challenge. Due to short frost-free period, high wind, and inclement weather it is impossible to harvest any herbs twice a year (summer and late fall) without using any form of season extension methods. Hence, we set up an experiment to test the feasibility of season extension methods on quantitative and qualitative production of native spearmint (Mentha spicata), thyme (Thymus vulgaris), Greek oregano (Origanum vulgare subsp. hirtum), and rosemary (Rosmarinus officinalis). The following season extension methods were tested: high tunnel (Ht), low tunnel (Lt), and low tunnel within high tunnel (LtHt). Except for rosemary, the herbage production of the studied herbs in LtHt was significantly higher than the herbage production in Lt and in Ht in the second harvest (late fall); rosemary died before the second harvest. The essential oil content of the herbs from our study did not vary significantly between the season extension methods. In thyme essential oil, '- terpinene concentration was significantly higher in LtHt than Lt and Ht, but significantly lower in LtHt than Ht and Lt were in the second harvest. In oregano, the major essential oil constituent carvacrol was modestly higher in LtHt than Lt and Ht, while another major constituent p-cymene was significantly lower in LtHt than Ht and Lt, in the second harvest. The chemical composition of spearmint essential oil was not affected by the different season extension methods in this study. Oxygen Radical Absorbance Capacity (ORAC) determined the antioxidant capacity of herbs. The ORAC values for thyme and oregano in LtHt were significantly higher than the ORAC values for the same herbs grown in Ht and Lt, in the second harvest. This study demonstrated that LtHt can provide optimal conditions for spearmint, thyme, oregano, and rosemary fresh herbage and essential oil production in northern climates, even when the temperature falls below the freezing point in late fall. LtHt can also improve the antioxidant capacity of thyme and oregano.