yarn bombing
n. The surreptitious or unauthorized placement of knitted objects on statues, posts, and other public structures.
Other Forms
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.
—Adriana Barton, “Yarn bombers cozy up to the urban landscape,” The Globe and Mail, January 30, 2009
Grannies of the world are ditching Ovaltine and slippers to take part in a daring new art project: yarn bombing.

The guerilla campaign involves women leaving knitted reminders on objects as varied as trees, lampposts and buses.
—Bella Battle, “Granny graffiti is knitty but nice,” The Sun (London, England), January 21, 2009
2006 (earliest)
There's a subversive group of knitters in Montrose, Texas, for example, calling themselves Knitta, that say they "yarn bomb" neighbourhoods with knitting graffiti — colourful knitted objects they leave in unexpected places — to help bring a warm, fuzzy feeling to their communities.
—Kerry MacGregor, “Not your grandma's knitting bee,” The Ottawa Citizen, March 11, 2006