Phenolic compounds are natural chemicals found in plants that have strong antioxidant activity that can be harnessed to benefit human health. With the rise of hemp as a skin care ingredient, it is especially of interest to optimize hemp cultivation to obtain higher phenolic content. A 2020 research paper provided insight into how hemp cultivation methods can alter phenolic content and antioxidant activity. 
Eight Swiss fiber-type hemp varieties were grown at different sowing densities and their total phenolic content and antioxidant activity levels were quantified, including hemp flavonoids and terpenes. Varieties studied included Felina 32, Futura 75, Fedora 17, Fibror 79, Santhica 27, and Santhica 70, and two dioicous cultivars, KC Virtus and Finola.
The researchers found that harvest period and plant maturity at harvest had the biggest impacts on total phenolic content and development of terpenes and flavonoids. Polyphenol and flavonoid content decreased during the flowering growth stage for all plants. However, plants developed a higher terpene content as they matured. More sesquiterpenes were present in early flowering but monoterpenes dominated in late flowering. The total phenolic content of hemp plants was greatest when plants were harvested in July during their early flowering growth stage. Plants harvested at this time also had the greatest antioxidant activity.
Sowing density of hemp plants influences the morphology of plants. Hemp plants produced larger flowers at a low sowing density such as 30 seeds/m2. Consequently, these plants yielded higher phenolic concentrations. The research team suggests that sowing densities of 150 seeds/m2 generally favor oil and seed production, and 300 seeds/m2 tends to favor fiber production. Specific cultivars can very though, so while this pattern in high density sowing can predict morphological changes for most varieties, it will not apply to all plants.
While many would expect fully mature hemp plants to have the best phenol content and antioxidant ability, the researchers explain that in many fruits, there is a phenomenon of better phenol content and antioxidant ability prior to maturation. This is the first time the phenomenon has been reported for hemp flowers, but it has previously been observed in peaches and medlar fruits.
This study demonstrates that there may be value to harvesting hemp crops early when it comes to cultivating hemp for applications where strong antioxidant activity is desired. However, if you are cultivating hemp for cannabinoid or terpene production, early harvesting would be undesirable.
- André A, Leupin M, Kneubühl M, Pedan V, Chetschik I. Evolution of the polyphenol and terpene content, antioxidant activity and plant morphology of eight different fiber-type cultivars of Cannabis sativa cultivated at three sowing densities. Plants. 2020;9(12):1740. [journal impact factor = 3.935; times cited = 5]
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