There is slow fashion – the movement to buy better, fewer things, make do and mend – and then there is textile artist Allan Brown’s dress, made from stinging nettles. From start to finish it took him seven years: foraging plants, spinning the yarn, weaving the nettle fabric on a loom, before finally sewing his design. This gentle, reflective documentary quietly observes the process, becoming painfully poignant when Brown’s wife, Alex, mum to their four kids, is diagnosed with stage 4 cancer at the age of 45. The slow process of spinning and weaving becomes his “medicine”.

As Brown points out, stinging nettles are one of the few plants that everyone can identify: childhood tormentors that grow with malicious intent in places where kids are likely to stumble. “They’ve got a fuck you attitude,” Brown says with a smile while out foraging with his golden retriever Bonnie in Sussex woodland.

Brown is a bit of a hippy, and his mellow observations provide a lovely commentary. “I know every inch of this yarn, it’s like an old friend,” he murmurs, stroking a section of the 14,000 feet (4,267 metres) of thread he has spun by hand. After Alex dies, weaving the nettle fabric takes on new meaning.

The finished dress is in the Viking style, simple and elegant, and modelled by Brown’s daughter Oonagh. (You could imagine it worn by a bride from one of the lesser houses in Game of Thrones on her wedding day.) “It was never going to be an haute couture fashion item,” is Brown’s verdict. No, but his dress is sustainable in every respect, with scant environmental cost, a genuine piece of slow fashion. The film too has a meditative effect, with its soothing, gentle rhythms, watching the seasons changing, and sense of time passing.

• The Nettle Dress is in UK cinemas from 15 September.