Hemp matures in 90-120 days, depending on a number of factors such as variety, seeding date and summer temperatures. Growing conditions, as with other crops, can affect maturity date.
Industrial hemp seed is harvested when the seed begins to shatter. Standing hemp is harvested when approximately 70% to 80% of the seeds are ripe and the grain sample has a seed moisture range of 10% to 20%. Hemp is indeterminate in growth so the seeds at the top of the head will be less mature than ones on the bottom. Hemp is ready to combine when the bracts around the seed have dried up and the seed is exposed.
Hemp head ready for harvest
When hemp is combined, the more immature seeds from the top of the head will retain some of the bracts that surround them. The bracts and the broken plant material in the combine hopper mean the sample is high in moisture. In a dryer, the plant material does dry down fairly quickly.
Later combining can result in increased grain losses due to shattering, bird damage and lower quality grain. Mature fibres tend to wrap more tenaciously around moving parts on the combine as the crop dries down.
As the hemp matures, the bracts surrounding the seed will dry and expose the seed for natural dry down. Fall frosts will assist the dry down process and the harvest can generally commence three to five days after a frost. After a killing frost, the crop will dry down rapidly therefore combining must take place with days.
The probability of shattering will increase as the plant dries. High winds can accelerate the incidence of shattering. Bird damage can be a problem as the seeds mature.