How to Wash Hemp Fabric

The Chemistry

Hemp fabric behaves in ways that are genuinely counterintuitive if you treat it like other fabrics — it violates two of the most common assumptions about washing: that fabrics weaken when wet, and that fabrics wear out and get rougher over time. Hemp (Cannabis sativa) is a bast fibre — the same broad category as linen (Linum usitatissimum) and ramie. It is extracted from the stalk of the plant by retting (biological or chemical breakdown of the pectin binding the fibres to the stalk) followed by mechanical processing. The resulting fibre bundles are composed primarily of cellulose (80–90% in high-quality hemp, compared to approximately 70% in linen) with the remainder made up of lignin, pectin, hemicellulose, and wax residues. The first counterintuitive fact about hemp: it is stronger when wet than when dry. Cellulose-based fibres generally increase in tensile strength when wet because water swells the amorphous regions of the cellulose polymer, causing the microfibrils to align more closely and increasing hydrogen bonding between adjacent chains. Hemp's high cellulose content makes this effect more pronounced than in cotton (cotton is approximately 90% cellulose but has shorter chains on average; hemp has longer cellulose chains with higher degree of polymerisation). Cotton is about 10–20% stronger when wet; hemp can be 20–30% stronger when wet. This is in direct contrast to viscose/rayon (which loses 40–50% strength when wet) and bamboo viscose (similar or worse). There is no wet agitation risk specific to hemp — it can handle machine washing without the fragility concerns of regenerated cellulose fabrics. The second counterintuitive fact: hemp gets softer with repeated washing. New hemp fabric often feels noticeably stiff, coarse, or papery compared to cotton. This is because freshly processed hemp retains residual lignin and pectin in the bast fibre matrix. Lignin is a rigid cross-linked polymer that fills the spaces between cellulose microfibrils and provides structural rigidity to the plant stalk — it is this same compound that makes wood stiff. Pectin is the biological adhesive that binds bast fibre bundles to the stalk. Neither lignin nor pectin bonds permanently to the cellulose fibre under normal washing conditions: hot water, agitation, and alkaline detergent progressively dissolve and remove them over repeated washes, leaving behind a progressively softer, more flexible cellulosic fabric. The softening is not a sign of fabric degradation — it is the opposite. The cellulose fibre itself has not changed; the stiff non-cellulosic components are being removed. Hemp also undergoes laundering shrinkage, but differently from wool (which felts irreversibly via scale interlocking) and differently from cotton (which shrinks primarily on the first wash from yarn tension relaxation). Hemp shrinkage is primarily mechanical: the yarn counts relax from the tension introduced during weaving. This is most pronounced on the first wash (3–5% is typical) and substantially smaller on subsequent washes. Pre-washing new hemp garments in hot water before cutting or sewing is standard practice in textile production for this reason. Hemp does not felt. It has no scale structure on the fibre surface (unlike wool, which felts via scale interlocking under heat and agitation). Aggressive machine washing cannot cause hemp to felt — the agitation risk for hemp is the same as for cotton: potential dimensional relaxation if dried at high heat, potential colour fading, or possible abrasion damage to printed or dyed surfaces. Hemp has documented natural antimicrobial properties. The mechanism is not a chemical compound unique to hemp (some marketing claims notwithstanding) but is primarily physical: the bast fibre surface texture, combined with hemp's high moisture wicking and evaporation rate, creates an environment that is drier than cotton after use and mechanically less hospitable to bacterial adhesion. Hemp retains less residual moisture than cotton after washing and drying, which reduces the window for bacterial and mould growth between wears. This has meaningful practical implications: hemp garments and bedding can typically go 1–2 more wears between washes than equivalent cotton items before developing odour.