The magnolia tree on the north side of Rittenhouse Square looks as if it were plucked from a Dr. Seuss book. Its split trunk is wrapped in a whimsical sweater of pinks, blues, purples, and oranges.

The tree cozy is the work of Jessie Hemmons, 23, a graduate student in psychology at Chestnut Hill College and census worker - and a graffiti artist with a soft side.

Hemmons is part of a growing trend of rogue knitters who have taken their "yarnbombing" to the street to brighten the cityscape. She ties crocheted flowers to lampposts, wraps bike racks with rainbow-colored covers, and gave the Rocky statue a scarf.

Her motivation is simple.

"Times are tough," Hemmons said. "People want to see something bright and pretty."

Yesterday morning she put up her largest installation yet. Passersby stopped to watch and snap pictures as Hemmons began stitching about 15 feet of knitting - a 30-hour project - to a tree near 19th and Walnut Streets.

The yarnbombing trend made headlines this month when three women in West Cape May, known only by their tag name, Salty Knits, began putting up knitting under the cover of night in the borough's Wilbraham Park and outside private businesses.

Both they and Hemmons, a Montgomery County native, were traditional knitters - Hemmons sells her wearable creations at Bambi Gallery in the Piazza and on her Web site, ishknits.com - before stumbling upon a book published last fall called Yarn Bombing: The Art of Crochet and Knit Graffiti.

Authors Leanne Prain and Mandy Moore of Vancouver popularized the term for the street art. They say yarnbombing can be political. Anti- and pro-Olympic yarnbombing popped up around the city during the Winter Games, for example. But the goal most often is to make people happy.

"I think it just tickles them," Moore said. "It captures people's fancy, and it's so easy to do."

At yarnbombing.com, Moore and Prain document projects that go beyond a basic parking-meter wrap: covers for rickshaw handlebars in Nepal; crocheted butterflies pinned to the trees of a public park in British Columbia; a giant crocheted American Spirit cigarette on a streetlamp in Laguna Beach, Calif., replicated down to the golden filter.

The book includes patterns for street-art projects and accessories for graffiti knitters, and gives advice, such as what to tell people if you are approached mid-installment.

"Um, it looks nice," Hemmons said, a bit tentatively, when Karl Mullen, who lives near Rittenhouse Square, asked her why she was wrapping the tree.

"It's wonderful," Mullen said. He had spotted her work before.

While kids passed by musing about whether the tree was cold, Mullen had a more philosophical interpretation.

"When you wrap something up," he said, "it makes you imagine further or think about what's underneath."

The evolution of knitting as street art is largely credited to the work of Magda Sayeg, 36, an artist who began yarnbombing five years ago.

Sayeg owned a struggling clothing shop in Houston. One gloomy day, she knit a colorful cover for the shop's door handle.

When she realized it made other people as happy as it made her, she started covering stop-sign poles and watched with glee as drivers paused to take photos.

"I was just responding to my environment, which was a lot of steel, a lot of parking lots, a lot that was not pretty," she said.

The idea took off. Sayeg founded a knitting collective, called Knitta Please, that yarnbombed New York City and was mentioned on Saturday Night Live. In 2008, she was commissioned as part of a street-art exhibit to cover an entire Mexico City bus.

Sayeg, who now lives in Austin, has traveled the world with her knitting, covering a rock on the Great Wall of China, bike racks in Paris, a gondolier's oar in Venice, and the handset of a London pay phone.

Now she's knitting full time - about 12 hours a day - working on a book, and preparing for her first solo exhibit, at a Rome gallery in May.

Though Sayeg said she generally keeps her knitting off trees and other public art, she is encouraged that other knitters are taking up her mission to cover "eye pollution" in cities with something beautiful.

"It's really just a simple, sweet, loving message," she said.

Hemmons perched on the shoulders of her boyfriend, Jerry Kaba, to finish the highest bit of seam work, then sewed a final section around the base of the tree. The couple stood back to admire the work.

"What's your favorite part?" she asked him.

They agreed on an eye-catching cable-knit block of purple and orange.

"I want to continue to add to it," Hemmons said.

'I personally love it'

Hemmons, who is hoping other knitters will join her, has done about 15 projects, most in Center City and some in the Fairmount area, where she lives. Many have been taken down within a day or two. A few, like a wrap for a bike rack outside Monk's Cafe on 16th Street, have lasted.

"I personally love it," said Monk's waitress Stephanie Eikamp, who hooks her bike to the rack most mornings when she arrives at work. She thinks more should get Hemmons' treatment. The racks "are pretty boring otherwise."

Some yarnbombing has drawn the ire of public officials concerned, as with other forms of graffiti, about defacing property. But knitters say their work is temporary.

Hemmons sees the knitting as "a very harmless, public form of me."

"If it's a nuisance, if it's dirty, cut it off," she said.

In a good way

Wendy Rosen, 16-year president of the Friends of Rittenhouse Square, said she noticed knitting in the square for the first time last week when her daughter pointed out another of Hemmons' tree wraps that has since been taken down. They were tickled.

"It's bright, it's cheerful, it's minimal," Rosen said. "People stop and say, 'What's that?' But not in a bad way."

Rosen said the knitting hadn't raised an alarm with the city's anti-graffiti officials she talks to regularly.

West Cape May has largely embraced the Salty Knits, who have posted about 20 projects around the borough.

One member of the group, who wouldn't give her name, said the trio started yarnbombing to entertain themselves.

"It's been good for our town in ways that we didn't ever expect it to be," she said.

Mayor Pam Kaithern said the Salty Knits have brightened the dull landscape of early spring. And the unknown identity of the knitters adds to the fun. In a town "bursting with artists," she said, "it could be just about anybody. I think it's better not to know."

Contact staff writer Chelsea Conaboy at 856-779-3893 or cconaboy@phillynews.com.