Retting is a process in which pectineus substances that bind together elemental bast fibres become degraded. This progression is completed either by microorganisms present on the stems or in soil, acid/bases, or by special enzymes. Some call it a controlled rotting, which indicates that a farmer needs to oversee and properly manage the process.
While three major types of retting can be distinguished including field (or dew), water and chemical retting, Canadian hemp growers should be concerned only about the first type that can be done on farm. Two other types require special facilities (tanks, dryers, specialty chemicals) and therefore are not included in this crop production guide.
Field retting is accomplished by leaving harvested hemp stalks on the ground for several weeks and relying on the weather to facilitate the process. Length of the retting process depends on availability of moisture and air temperature. Warm weather with intermittent precipitation maintaining moisture within the hemp stalks supports microbial activity and accelerates the process of pectin degradation.
To assure even and complete retting, the stems should be turned over when they start changing in color from green to pale yellow (Figure xix - PDF Below). Under optimal weather conditions, this stage can be reached two to three weeks after cutting. Turning over is recommended when hemp is meant for textile applications which demand high quality and uniformity of fibres and if the layer of hemp stalks are thick. Without turning stems over, the stems at the bottom, closer to the ground, could be easily over-retted and hence quality of both hurd and bast fibre would be compromised. While in Canada there is no specialized equipment available to do this task, rotary rakes could be used to avoid loss of fibre yield and quality.
Retting is completed when microbes decompose pectins binding fibres and stems turn color to medium–dark beige with small dark, moldy spots (stage 6-8 in Figure xix – PDF Below). Hurd and bast fibres in properly retted stems separate easily at breaking and bast fibres form characteristic bands.
In some Prairie regions frequently receiving miniscule amounts of rain in August and September, retting could take a long time and sometimes may not be completed before winter. In such cases, the stems could be left on the ground until the following spring for baling in May. Fibre from winter-retted stems is suitable for biocomposite applications; however, for high end textile applications, leaving the stems on the ground may lead to over-retting which weakens fibre strength and reduces its quality. To assure optimal retting conditions, hemp stalks meant for the textile applications may be wetted with a few inches of water to support microbial activity. Such management practice could be implemented in the irrigation districts.