Growth and essential oil responses of Nepeta species to potassium humate and harvest time

Nepeta is a perennial herbal plant that belongs to the Lamiaceae mint family. Nepta has different species that are widely used in the agriculture, medicine, and pharmaceutical industries. A field experiment was conducted in 2015 and 2016 to determine the effects of potassium humate foliar application [0 (control) and 400 ppm] on growth, essential oil and constituents of three Nepta species [Nepeta cataria (catnip), Nepeta cataria var. citriodora (lemon catnip), and Nepeta grandiflora (giant catmint)] under Egyptian conditions. In all three species, the application of potassium humate increased growth indicators, essential oil content (%) and yield (mL/plant), and flavonoids in each of the two harvests. However, the height of giant catmint was significantly higher than that of lemon catnip, which was higher than that of catnip. Lemon catnip that received potassium humate and harvested second gave the highest essential oil content and yield. The major constituents were geraniol and nepetalactone in catnip; citronellol and geraniol in lemon catnip; and o-cymene, c-terpinene, p-cymene and carvacrol in giant catmint. The highest p-cymene in giant catmint, citronellol in lemon catnip, and geraniol in catnip were obtained from the application of potassium humate, while the highest o-cymene, c-terpinene and carvacrol in giant catmint, geraniol in lemon catnip, and nepetalactone in catnip were obtained from control plants. This study demonstrated the variations among species of nepeta, and how they respond to the application of potassium humate. The findings of this study can guide the customization of potassium humate applications to the three species for achieving desired growth and essential oil production outcomes.