Impact of developmental stage and harvest time of peppermint (Mentha piperita L.) on growth and volatile oil yield

To study the effect of three growth stages of peppermint (Mentha piperita L.) (pre-budding, budding and 50% flowering) and three harvest times during the day (7:00 am, 12:00 pm and 5:00 pm) in the growth and yield of volatile oil, the experiment was conducted in the fields of the agricultural research station (Al-Bandar area) affiliated to the College of Agriculture, Al-Muthanna University, under the environmental conditions of Al-Muthanna Governorate located in southern Iraq, at latitude 31.32139° N and 45.30407° E, for the period from 15/2/2024 to 16/6/2024, and was implemented according to a randomized complete block design with three replications. The results of the study showed the significant effect of the developmental stages of the plant on all the studied traits. Peppermint plants harvested at 50% flowering stage were significantly superior and gave the highest values ​​in plant height (61.33 cm), number of branches (27.38 branch. plant-1), shoot fresh weight (19.07 gm. plant -1), shoot dry weight (4.74 gm. plant-1), fresh weight of leaves (12.80 gm. plant-1), dry weight of leaves (3.10 gm. plant-1), percentage of volatile oil (2.40 %) and volatile oil yield per plant (74.06 µL. plant-1). While harvest times had a significant effect on the traits of fresh weight of the vegetative mass, fresh leaf weight, volatile oil content, and volatile oil yield per plant only, which was excelled by the peppermint plants that harvested at 7.00 am and gave the highest values (17.00 gm. plant-1, 11.50 gm. plant-1, 2.29 % and 65.20 µL. plant-1) respectively.