Retting of hemp is a biological process that results in the separation of the fiber bundles of the outer cortical sheath from the woody core and from each other by decomposition of the surrounding less resistant thin-walled cell~. Since the decomposition process is a continuing one, it must be terminated before damage to the fiber has commenced and yet it must be allowed to continue sufficiently long so that the fibers come away readily from the woody stalk and upon hackling separate from each other. The quality of the fiber is largely dependent upon the success of retting and the proper decision as to its end-point.
Fiber from field-retted hemp cannot be expected to approach the uniformity and quality of that retted by controlled methods. Field retting in Iowa is dependent upon dew and rain to supply moisture for the activity of the microorganisms, which are predominantly fungi (8). Dew alone is usually quite insufficient to permit the hemp to ret completely. Hemp rets unevenly in the field because dew is deposited on the exposed surface of the stalks, causing retting to proceed more rapidly in this region than on the under side. This is partly overcome by turning the hemp during the process of retting, but in spite of this practice, retting is by no means uniform. When frequent rains accompanied by warm weather occur early in the fall, there is some danger of damage to the fiber through over-retting (7). Thus, field-retted hemp usually is a mixture of unretted, partly retted, well retted and overretted material.
Fuller, W. H. and Norman, A. G.
"The retting of hemp II. Controlled retting of hemp,"
Research Bulletin (Iowa Agriculture and Home Economics Experiment Station): Vol. 27
, Article 1.
Available at: https://lib.dr.iastate.edu/researchbulletin/vol27/iss343/1