The objective of this study was to find the influence of different humic acid concentration and foliar application times on aboveground biomass and essential oil production of hyssop (Hyssopus officinalis L.) in 2008 and 2009 at experimental field of Ferdowsi University of Mashhad in Iran. The field experiment was a factorial based on randomized complete block design with three replications. Two times of foliar applications of humic acid (one time in first cut, two times in both cuts) were applied along with four humic acid concentrations (0, 100, 200 and 300 mg/l) in this experiment. Aboveground and leaf biomass and essential oil production, harvested at flowering were measured as annual production of hyssop (H. officinalis L.). Essential oil percentage did not response to different treatments in both years of experiment. Different application times did not change harvested aboveground biomass and essential oil production. Humic acid concentration showed great influence on aboveground and leaf biomass and essential oil production. Highest concentration of humic acid (300 ml/L) resulted in highest amounts of aboveground (+88%) and leaf (+77%) biomass and essential oil production (+71%) compared to control treatment. Hyssop produced higher biomass (+4%) and oil (+3%) in highest concentration of humic acid (300 ml/L) in the second year of the experiment compared to the first year.

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