Yarn-bombing – a sort of grafitti with wool - has Glasgow residents in stitches

A storm clamps Scotland in its jaws and shakes the country into limp submission. Ferries cancelled.

• Yarn bombers on Glasgow Underground

Roads flooded. Trees toppled. From the headquarters of BBC Scotland and STV, by the Clyde in Glasgow, they are broadcasting severe weather warnings.

The river that flows past those offices is a roiling black mass. To walk along its banks, or to try to cross the Millennium Bridge, is to feel the stinging bite of freezing gale and rain. It is, in short, a night during which anyone practised in knitting would do well to stay at home and finish their scarves and jumpers.

Yet, for two young women, their identities obscured by hoodies and parkas, a night like this, with no one around, is an opportunity to use their knitting skills in a rather unorthodox way.

"This is yarn-bombing in Scotland," declares the one who uses the alias Purlesque, yelling to be heard over the wind. "You can see why it didn't start here."

Her "partner in knitting crime", Night Knit Pixie, just laughs and adjusts the drawstring on a red mohair cover, one of eight – in various bright colours – which they are placing over boat moorings tonight, transforming the black metal stumps into something akin to fairy-tale toadstools. Some of the covers have labels on them, which read: "Made you look? Made you smile? Yarnbombing: making things pretty with yarn" – and the address of a Twitter account.

The idea is to come back at first light the following morning and photograph the work for their blogs. As the rain begins to die down, Purlesque and the Pixie walk off, glowing with satisfaction at a job well done, discussing what they should yarn-bomb next. "How about," wonders Purlesque, "the Finnieston Crane?"

Yarn-bombing, also known as knit-tagging or guerrilla knitting, is a growing international movement in which objects are knitted and then placed in public spaces as a form of graffiti, often witty or appropriate to its context.

Night Knit Pixie, for example, who is based in Kilmarnock, once knitted a tiny version of the three pyramids of Giza and left them outside the entrance to the Dick Institute as a tribute to the museum's temporary display of an Egyptian mummy. Any attempt to define what this is all about is inevitably woolly. Public art? Direct action? A prank? Perhaps all three.

The scale can vary from colourful crochet sewn around the poles of street signs to entire buildings or vehicles being wrapped. Yarn-bombing began – need it be said? – in America, and has since spread to Australia and Europe. Within the UK, groups include London's Knit The City, the Welsh Yarnarchists, and in Cornwall the Graffiti Grannys who laid knitted mice along the harbour at Mousehole.

The movement has now arrived in Scotland, a country known for its wool and tradition of weaving, but not – until now – for posses of young women taking their knitting into the streets.

"It allows people to be an exhibitor without having to explain themselves," says Einav Leshetz, a 27-year-old Australian who has moved to Glasgow to make a documentary about the phenomenon. She has filmed yarn-bombers at work in Germany, Israel, France and the Netherlands as well as in her native country. "It's the just-because. It's bringing back Dada. Art for the sake of art.

"But people who practise this aren't necessarily talented knitters. They don't have to be. It can be somebody, like me, who just knows how to stitch rows, and no one judges you because it's not really about that."

So what is it about? Why do it? "The risk element is exciting," says Purlesque, a Glaswegian in her early thirties.

"Staking out the building and seeing where the cameras are. There's the fear of being caught and knowing you are doing something a little bit naughty."

The legality of yarn-bombing is ambiguous. It isn't destructive like regular graffiti, and so far it has avoided obscenity or inflammatory language, though it could be argued that it's a form of littering.

In any case, no one ever seems to have been arrested or prosecuted for covering something with knitting; that came closest when members of Knit The City were questioned by police and served with a stop-and-search notice for decorating a telephone box in Parliament Square.

"You'd have to be a very bored cop," laughs Magda Sayeg, the 36-year-old American credited with inventing yarn-bombing.

It happened in Houston in 2005. That's Houston, Texas, not Renfrew-shire.

Sayeg owned a fashion boutique and one day – bored and frustrated by the steel and glass architecture that surrounded her – decided to cover the external door handle with knitting. "It wasn't this 'Eureka!' moment," she recalls.

"I had selfish intentions to make myself happy. To my surprise it caught the attention of people passing by my store. So I decided to go out and tag things in the urban environment and that's when I really saw the reaction. People were getting out of their cars and taking pictures, which was really seductive for me."

Working anonymously as PolyCotN, Sayeg and a group of friends founded the world's first yarn-bombing crew, its name – Knitta, Please – inspired by the hip hop scene in which graffiti (spray-paint variety) is such a key part. Going out at night, they decorated fire hydrants, stop signs, cars and the like.

Though she still keeps her hand in with this sort of unsanctioned work, Sayeg has turned yarn-bombing into a career, and her trajectory is an exemplar of how far the scene's best practitioners can go. Commissioned by galleries, design museums and arts festivals, she travels the world with her needles and wool.

She has covered a bus in Mexico, the gun of a huge military statue in Bali, and has explained yarn-bombing to our own cultural ambassadors, Richard and Judy.

As a result of her international profile, Sayeg is very much a recruiting sergeant for yarn-bombing, a role she relishes. "You're inspiring knitters to do something that goes beyond function, and that's really exciting," she says.

"People have always looked at this craft as something that you do only for function. You make a jumper, you make socks, you don't make something just to make yourself happy, to feed your own heart and soul and others around you."

Sayeg, who is keen to visit Scotland, would be pleased to see the yarn-bombers of the Glasgow PicKnitters group, a dozen of whom have assembled in the Kibble Palace, the flying saucer-like Victorian greenhouse in the Botanic Gardens.

They range in age from their twenties to their sixties, and even include in their number one man – 24-year-old Joe Giblin from Stevenston – who is knitting pom-poms with which he intends to decorate a tree by Ayr College, where he is studying fashion.

"People think knitting is a bit twee and old-fashioned," he says. "You might go to your granny's and see 300 shawls hung up in a cupboard. That serves no purpose. But yarn-bombing does. It makes people smile and laugh."

That is obvious from the near-hysterical reaction of members of the public in Kibble Palace when confronted with a marble statue of King Robert of Sicily, naked except for a monkey perched on his left thigh, which has been adapted by the PicKnitters; the wee animal now wears a small woollen bunnet.

"The monkey?" says Linda McQueen, a 62-year-old PicKnitter in a pink crocheted beret. "We can't tell you who did it. A guerrilla."

McQueen lives in Paisley. She has six grandchildren. "Yarn-bombing is about letting people see that it's not just old people like me that knit. It's fabulous. You're not knitting for you or your people. You are putting it out there for the public. I love being part of that."

While she is talking, her blue plastic bag falls open and a round, pink knitted object rolls on to the ground. "This is a bit awkward," she says. "My boob's fallen out."

She has knitted a breast, at the request of the local health board, to help in breastfeeding classes.

"They want every shape and size. I made that last night. My partner helped me pick the colour for the nipple."

Glasgow's PicKnitters have so far yarn-bombed the Botanic Gardens with knitted pumpkins and Scotland's First Peoples gallery of Kelvingrove Museum with knitted icicles and penguins ("like David Lynch does Pingu," a PicKnitter recalls).

On the eve of Remembrance Sunday, one of their number placed knitted poppies at the Cenotaph on George Square. There is even talk of yarn-bombing the iconic statue of the Duke of Wellington on Queen Street, replacing with a woolly hat the traffic cone that Glaswegian tradition dictates adorns his head.

The group was founded by Johanna Flanagan, a 32-year-old with a masters in knitting from the Royal College of Art. Having worked as a fashion designer in London and New York, and grown sick of the commercial world, she returned home to Glasgow and now teaches knitting and fashion.

"There is a feeling when you are yarn-bombing that you're being really cheeky and mischievous," she says. "Since I've stopped going out clubbing and getting hammered and coming home at dawn, maybe this is a release for that.

"But I think the important thing is you feel like you're involved. When I moved back here, I was so stressed out, and I really wanted to reconnect. I hadn't lived here since I was 17. Decorating your city, you feel like you're having a one-to-one with the place."

She is, literally, weaving herself back into the fabric of Glasgow life.

One important aspect of yarn-bombing is that the work is given away. You take no money for what you have created; and if it is done anonymously, or under one of the favoured punning pseudonyms such as Deadly Knitshade, no credit either.

For yarn-bombers, this is part of the appeal. It gives them a furtive buzz to see the public take pleasure in something they have made. There's a real generosity to this, especially as some of the pieces are highly accomplished and required a lot of work and time to make; for example, the creations of the French yarn-bombers Collectif France Tricot which have included a splendid Edenic snake and apple in a Parisian tree.

"It's similar to someone like Banksy," says Flanagan, "who's doing pieces of work worth millions on random walls."

Yarn-bombing isn't quite in that league. An early and rather basic piece by Magda Sayeg was sold on eBay for just $15. The principle is the same, though. Flanagan lived in Notting Hill during a period when Banksy was working there, and one day saw him at work, which has been an inspiration for what she is doing now.

"It happened so fast," she recalls. "He knew exactly what he was doing and in seconds he was gone. Because people just saw him as some kid with a spray-can, he was allowed to develop. The same thing is happening with yarn-bombing. It isn't taken seriously, so there is no pressure, which allows it to flourish. But the people who do it are starting to realise that they are artists, they're not just kids playing about."

Yarn-bombing is inherently transient. It is rare for a piece to remain in place for long. Often, it is taken by a member of the public, or removed by officials.

This is why the internet is vital. Blogs allow bombers to maintain a record of their work and to alert each other as new pieces appear. The covered moorings by the Clyde were, before long, being blogged about in Iowa.

This global reach is one reason why some yarn-bombers believe their work has the potential to make political statements. It's an intriguing idea. Fragile and cosy, what could be less threatening to than knitting? Yet, looked at another way, knitting has great symbolic value.

It says something about the importance of home and family, nurturing and protecting, creativity rather than destruction.

Veronika Tudhope, a 46-year-old mother of two from Kilmarnock, has been involved in the green and peace movements for many years and is presently busy knitting a miniature aircraft carrier and Chinook helicopters to be used in Edinburgh next month in an anti-military campaign.

She has previously made Trident missiles and has plans for at least one nuclear submarine. She is interested in moving into yarn-bombing and believes it may be possible to use it as a form of protest at Faslane.

"Another thing I've been thinking about is the student riots of a few days ago. What would be the yarn-bombing alternative to breaking the windows of the Conservatives' building? Using knitting, you wouldn't have to put people in danger of broken glass."

Perhaps yarn-bombing will develop in that direction in future. For the moment, however, Glasgow PicKnitters have a more straightforward project in mind.

Making the short walk from the Botanic Gardens to the subway at Hillhead, they troop into one of the carriages. Several members have knitted colourful four-inch wide strips, which they begin to sew around the handrails of the carriage while the other passengers look on rather bemused.

The atmosphere is jolly and industrious. The click of needles seems to echo the clank of wheels on the track.

"This is a little bit subversive without actually doing any harm to anybody," says Marion McLarty, a 59-year-old from Troon, her glittery nail-polish sparkling as she sews a purple strip round a pole. "And it adds a little to the general amusement. Although, having said that, the driver doesn't look very amused."

Right enough. The train has stopped in Govan and the driver, visibly angry, has come out of his cab to complain. "You need permission to do that," he tells the yarn-bombers.

"But this is a public place," says Flanagan. "And we've all bought Discovery tickets."

Her argument does not prevail. A supervisor is called and the knitting is cut down. Everyone tuts and sighs, and some comments are made about typical men who feel threatened by women, but then a decision is made to get off at Kelvingrove and get a nice cup of tea.

This has been a setback for Scotland's fledgling yarn-bombers, but it is unlikely these women will be put off for long. When the rain was soaking her mohair covers, I had asked Purlesque how wool would cope with the Glasgow weather. Her reply – "It's actually surprisingly resilient" – could serve as a decent description for the movement as a whole.

Yarn-bombing, after all, has already spun out in several interesting directions since its beginnings in Texas and who knows where it may yet wind up?

This article was first published in Scotland On Sunday, 19 December, 2010