Hemp grown for the fiber will be harvested when ~10-20% of plants are flowering (releasing pollen). Depending on cultivar, this should occur 60-70 DAP. Hemp plants grow very little after flowering and male plants senesce, there is no advantage waiting to harvest if you are producing just for fiber.
Growers are using sickle bar mowers, disc mowers, and hay bines to harvest. If using a hay bine, open the conditioner as wide as possible to prevent or reduce crimping the stalk.
After harvest, retting occurs. This is a 2-6 week process of decomposition and drying. Field ret times will vary based on the amount of rainfall. The outer bast fiber separates from the inner hurd, which enables easier and cleaner processing of the stalks.
Baled material is shipped to a processing facility. Currently, there are very few of these facilities in the US. Decorticators are machines that strip the outer fiber from the inner fiber. There are many other uses for hemp grown for fiber though and emerging markets in Indiana and the Midwest are looking at hemp as a building material, a source of fuel, and even as a bioplastic.