Pop-up shops are popping up everywhere.

No, we're not talking about the Christmas Village at LOVE Park in Center City, or those Halloween marts that seasonally fill empty storefronts - although pop-up retail can stretch to include those.

We're talking about edgy, ephemeral, hole-in-the-wall shops where buying and selling often merge with art, education, performance, and partying.

Places such as the Green on Greene building at Carpenter Lane and Greene Street in Mount Airy. On Dec. 15, the front studio became a shop filled with vintage jewelry, clothing, ephemera, and, whimsically enough, large terrariums. By Saturday, it will all be gone.

"This Christmas season has been huge," said Michele Longo, who organized the nine participating online vendors - and an opening-night party Friday for the neighborhood. (Mount Airy's beloved Weavers Way food coop is across the street.)

Longo's antique jewelry (thecollectedcollage.com) was clearly a draw, but customers also oohed over Deidre Wengen's vintage dresses, and Kristin Swoszowski-Tran's handmade soaps (etsy.com/shop/helixandhive).

Green on Greene owner Pam Rogow, who has had nine pop-ups in her building over the last year, said, "It's nice to do something that's fleeting and permissive."

Pop-up retailing has been catching on around the world since at least 2004, especially during the holiday season. That year, trendwatching.com cited examples including Song Airline's eight-week SoHo outlet, Target's Christmas "galleries," and a "guerrilla store" set up for a year in Berlin by Comme des Garcons, the splashy Japanese fashion label.

This holiday season, one of the more gaga examples is Lady Gaga's pop-up at Barney's flagship store on Madison Avenue in New York through Jan 2. She's selling such wares as studded leather motorcycle jackets for infants.

Part of the allure of pop-ups, trendwatching.com explained on its website, is "about surprising consumers with temporary 'performances,' guaranteeing exclusivity because of the limited time span."

In Philadelphia, where pop-ups seem to be a more recent phenomenon, the trend is also about finding economical nooks and crannies. Landlords can make better use of vacant properties, while tenants can test the consumer waters without making a big investment.

On Saturday, Nigel Richards showed off his 611 Lifestyle clothing line of T-shirts, flannel shirts, bags, and accessories at his second annual holiday pop-up in pricey Rittenhouse Square.

The catch: The four-day sale, which ends Sunday night, was in a chilly, concrete-floor, unfinished storefront that is part of Parc Rittenhouse.

"In the long run, I will probably need a permanent store," said Richards, who owned a record store that built a loyal following during 14 years on South Street. "But this creates a bit of hoopla. We have DJs all weekend, and we had a big party Thursday night."

For Rogow, a former museum executive, helping young start-ups and talented artisans is a priority; that's why she charges a nominal rental of $150 to $250.

She also likes pop-ups that are "culturally and socially meaningful."

Last year, for example, she arranged for Edward Maeder, a former curator of costumes and textiles at the Los Angeles Museum of Art, to offer a week of workshops on vintage silks, hat decorating, and making old-fashioned Valentines.

Pop-ups could be an effective economic and tourism tool in Philadelphia, Rogow says, because the city has affordable real estate, ample commercial vacancies, and lots of mom-and-pop scale sites.

She even bought the Internet domain PopUpPhilly.com and made a formal pitch to some city officials.

So far, she said, there's no funding for such a venture. But you never know what will pop up.