Flax is not cut at harvest. It is pulled from the ground, root and all, because the fibre runs the full length of the stem and every centimetre lost to a blade is a centimetre lost from the thread. The pulled stalks are then laid back down on the same field in long, flat rows, and left there. That stage, which can last from two weeks to well over a month, is called retting, and it is the answer to the question of what linen retting is: the slow, natural process that frees the fibre from the plant.

Inside a flax stem, the fibres sit in bundles between the outer skin and a woody core. They are held in place by pectins, natural substances that act like a glue. Spinning is impossible while that glue holds. Retting lets the bacteria and fungi that live in soil and air break the pectins down, gently enough to loosen the fibre without weakening it. The word itself comes from an old Dutch and Flemish root linked to rotting, which is close to what happens, although a well-retted field smells of hay rather than decay.

Today almost all fibre flax in Western Europe is dew retted. The coastal band that runs from Normandy through Picardy and Flanders into the Netherlands has the climate for it: mild summers with alternating rain, dew and warm sun, which keep the stems damp enough for the microbes to work and dry enough to stop them before they go too far. Farmers turn the rows once or twice so that both sides ret evenly, and watch the colour change from green-gold to the silver and grey-brown tones that signal readiness. The full journey, from sowing in spring to the finished cloth, is traced in how linen is made, from the flax field.

Judging the moment to lift the flax is a matter of experience. Under-retted stems hold on to their woody core, and the fibre comes out harsh, uneven and full of fragments. Over-retted stems have been attacked too long, and the fibre loses strength. Growers test a few stems by hand, snapping and rubbing them to see how cleanly the fibre separates, and a dry or very wet summer can change the timing by weeks. It is one reason flax harvests vary from year to year, and why the same field does not give quite the same fibre twice.

Flanders has a particular place in this history. For centuries the River Lys, the Leie in Dutch, was famous for water retting: flax was bundled into crates and sunk in the river around Kortrijk, whose soft water was said to give an especially pale, fine fibre. Flax came from far afield to be retted there, and the river was known as the golden river. Water retting in the Lys ended in the twentieth century, as dew retting in the fields took over, but the region's skill with flax remains, and the Belgian linen certificate today follows the flax from the field to the fabric.

After retting, the dried stems go to the scutching mill, where they are broken and beaten to separate the fibre from the woody fragments, called shives. Hackling then combs the fibre into long, parallel strands, known as line, and shorter tangled fibres, known as tow. The long line fibre is the material for fine shirting, usually spun wet to give a smooth, even yarn, a process explained in wet-spun linen. The length of that fibre, which retting must preserve, is the subject of long-staple linen fibre.

Retting leaves its mark on the finished cloth. The natural colour of undyed linen, the soft grey-beige often called flax or oatmeal, comes partly from the retting, and well-retted fibre spins into yarn that takes dye evenly and softens with wear. A shirt made from carefully retted, long-line flax has a clean surface and a supple hand from the first day, and keeps improving with washing; the men's linen collection shows what that cloth looks like once cut and sewn.

Retting is also one of the reasons linen is considered a natural fibre in the fullest sense. Flax in Western Europe grows largely on rain, the retting is done by the weather and the soil's own microbes, and the plant leaves little to waste: the shives become board and bedding, the seed becomes oil. When a linen garment eventually returns to the soil, it is going back to the same kind of field that began the work.

Read more: What Is Belgian Linen?  ·  The Belgian Linen Certificate of Origin