Peppermint is a therapeutic and aromatic herb. In order to determine the effect of plant density and harvesting time on growth and oil production of peppermint a field experiment was carried out at Sari Agricultural Science and Natural Resources University, in Iran, during 2010-2011. A split plot experiment was performed in a randomized complete block design with four replications. The main plots were four levels of density (8, 12, 16 and 20 plat 1 m-2) and the sub plots were two levels of harvesting time (25 th may and10 th September. Fresh biomass yield, plant height, dry biomass yield and fresh leaf weight, increased significantly with the decreasing plant density. The value of fresh biomass and essential oil yield were significantly (p< 0.01) increased by increasing plant density from 8 (plant m-2) to 20 (plant m-2). Maximum oil yield and dry matter was obtained in the first harvesting.

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