The effects of photoperiod on phenological development and yields of industrial hemp

Shorter photoperiod cause early flowering, restricting yields in industrial hemp ( Cannabis sativa L.). Trials were conducted under different light regimes under the protected structure in Bundaberg,QLD, Australia. Little difference was observed in plant maturity when photoperiod was at or below 13 h 40 min but was significantly delayed when photoperiods exceeded 14 h 40 min. Plant heights at harvest increased significantly with extended light. Plant heights at harvest were 1,188 mm and 1,161 mm for natural light treatments, 1,286 mm in treatment 3 (13 h and 40 min lightperiod) and 1,395 mm in treatment 4 (14 h and 40 min). Stem thickness increased, whereas stem and root yields at final harvest were greater with extended photoperiod. A parallel field trial as separate experiment was also planted at the same location over five consecutive dates (PD1- September 15, October 25, November 25,December 16, and January 23), which showed that November 25 as an optimum timing as evidenced by highest plant height and dry matter yield compared to other planting dates. Results from these trials clearly indicated that day length must exceed at least 13 hand 40 min during the growing season to produce high yielding hemp for the tested variety.