Growing spearmint, thyme, oregano, and rosemary in Northern Wyoming using plastic tunnels

Abstract

Growing perennial herbs in a 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, an experiment was set up to test the feasibility of season extension methods on quantitative and qualitative production of spearmint (Mentha spicata L.), thyme (Thymus vulgaris L.), Greek oregano (Origanum vulgare subsp. hirtum), and rosemary (Rosmarinus officinalis L.). 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 this study did not vary significantly between the season extension methods. Gamma-terpinene concentration of thyme essential oil was significantly higher in LtHt than Lt and Ht in the second harvest. In oregano, the major essential oil constituent carvacrol was modestly higher in LtHt than Lt and Ht in the both harvests, while another major constituent p-cymene was significantly lower in LtHt than Lt in the first 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 value for oregano in LtHt were significantly higher than the same herb grown in Ht and Lt in the second harvest. Similarly, thyme had significantly higher ORAC value in LtHt than Lt in the second harvest. This study demonstrated that LtHt can provide optimal conditions for spearmint, thyme, and oregano 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.