Stealthy knitters are leaving their mark on the Great Wall of China, the Golden Gate Bridge and a street near you

Vancouver - Beware the friend who suddenly takes up crochet. She (or he) may be a yarn bomber.

Working under code names such as Incogknito and the Microfiber Militia, a global network of "craftivists" is stitching makeshift sweaters, oversized tea cozies and giant pompoms around public property. Street poles, trees, bike racks, bridges - nothing is safe in target cities such as London, Paris, Chicago and Stockholm.

The craftivists' manifesto, according to Vancouver knitters Mandy Moore and Leanne Prain, is "world yarn domination."

Paris was hit in 2007 when hundreds of knitted socks were slid onto street posts in the Marais district.

In Vancouver's Strathcona neighbourhood, a yarn bomber with the alias KnitGirl is wrapping fences, car antennae and street poles in "Strathcozies."

knitted bus cover
© Magda SayegThe cozy covering this bus in Mexico was created by Magda Sayeg, a Texas-based yarn bomber who is the ringleader of the group Knitta.
And last November in Mexico City, yarn bombers covered an entire bus in woolly graffiti.

The street-knitting scene has gotten so knotty that Ms. Moore and Ms. Prain are writing a tell-all book about their hobby. Published by Arsenal Pulp Press, Yarn Bombing: The Art of Knit Graffiti is set for release this fall.

The duo, who met four years ago at a stitch 'n' bitch, say yarn bombing has gone viral on the Internet.

After "tagging" a park bench or street lamp, guerrilla knitters document their exploits on blogs and websites such as Knittups.se and Knittaplease.com.

Perpetrators tend to be tech-savvy and under age 35. For this generation, Ms. Prain says, "knitting and technology go so closely together."

Yarn bombing is subversive because it rejects the tradition of knitting or crocheting for altruistic causes, Ms. Moore says. One of the most common critiques is that knitters should be making afghans for the homeless instead of adding to the urban fabric, she says.

But, she adds, "I don't think there's any other pastime where people are as expected to give up their time for others."

By creating for their own pleasure, Ms. Prain says, yarn bombers are reclaiming a hobby that was left in the dustbin by feminists who spurned the domestic arts.

Women are not alone in the stealth knitting movement, however.

"A lot of guys are drawn to it, too," says Ms. Moore, adding that men are part of notorious knit-graffiti crews such as the Netherlands-based Knitted Landscape, which leave fuzzy mushrooms and tulips in city parks.

Yarn bombing is similar to the guerrilla gardens that have sprung up in public places recently, according to Matthew Blackett, publisher and creative director of Spacing, a magazine about urban landscapes. "It enriches the experience of being in a city," he says.

The first known act of knit graffiti was committed in 2005 by the Houston, Tex.-based group Knitta, which wrapped a stop-sign pole in a pink-and-purple sweater.

Knitta has since left its mark on the Great Wall of China, the Notre Dame Cathedral in Paris and San Francisco's Golden Gate Bridge, inspiring legions of youth to rush out and buy knitting needles.

Magda Sayeg - a shopkeeper by day - is Knitta's 35-year-old ringleader. The beauty of yarn bombing, she says, is you don't have to be great at knitting or spend hours on a piece. "You can make a stop sign cozy in the time it takes to watch a movie."

Covering a bus in Mexico City is a different ball of wool. Ms. Sayeg says it took her and several other artists four days to sew it up in knitted and crocheted pieces, which were donated and found in thrift stores.

The bus cozy, funded in part by Absolut vodka, still covers the vehicle, which is parked and in use as a ceramic studio.

Now that she's doing large-scale pieces with corporate sponsorship, Ms. Sayeg says, she no longer sneaks around using the cover name PolyCotN. But Knitta's early work was pulled off graffiti-style. "It was done at night," she says, "with people on the lookout to make sure no one was around."

Knitting renegades usually scope out a site with a tape measure and then go away to create pieces to fit. Returning to the scene, often under cover of darkness, they pull out their colourful pieces and furtively stitch them in place.

In Vancouver, no one has been arrested for yarn bombing, Ms. Prain says, but "there are some people who don't want to reveal their identity," including a guerrilla knitter with a union job.

It's one form of graffiti that's hard to hate.

"The key thing about knit graffiti is that it doesn't damage property," Ms. Moore says. "If you object, just cut it down."