Hemp Dew Retting
topic
Dew retting (field retting) exposes swathed hemp stems to alternating moisture and drying cycles over 3–6 weeks in temperate climates (optimum temperature 15–25°C, moisture 40–60% RH), allowing epiphytic fungi — principally Cladosporium herbarum, Penicillium and Aspergillus species — to colonise stem surfaces and secrete pectinase and hemicellulase enzymes degrading the middle lamella. Fibre bundle separation is achieved when shive content falls below 5% (ISO 6938 assessment). Dew-retted fibre strength is 500–700 MPa with bundle fineness Nm 4–10; quality variability CV% of 25–35% is higher than water or enzymatic retting due to non-uniform microbial distribution across the swath. Turning swaths once (at 2–3 weeks) improves uniformity by 15–20%. Dew retting requires no water, infrastructure, or chemical inputs, producing 80% of European hemp fibre at total processing cost of €0.10–0.20/kg stem. Over-retting risk increases significantly after 7+ weeks, reducing tensile strength by 20–40% through cellulose degradation.
Role
Dominant commercial retting method for European hemp fibre production due to zero infrastructure cost and minimal environmental impact, despite higher quality variability than water or enzymatic alternatives — accepted for industrial yarn and composite applications where uniformity is less critical.