In the early days of Greensgrow, cofounder and chef Mary Seton Corboy would get plenty of puzzled looks as she toiled behind a chain-link fence surrounding an abandoned city block in Kensington.

"Lady, what are you doing in there?" passersby would ask the slender woman, whose frame belied her mighty firebrand nature as a pioneer in urban agriculture.

"It's a farm," she'd holler back.

Inevitably, the next question would be: "Where are the cows?"

Two decades later, the neighbors have no doubt what's happening beyond that fence at 2501 E. Cumberland St. The original three-fourths of an acre urban farm has blossomed into a funky garden center, farm stand, and greenhouse (plus a West Philly satellite) that annually take in $1.8 million in revenue and get 10,000 visitors.

Greensgrow still has no cows. It does boast Milkshake, a hefty pig acquired through Craigslist (it was supposed to be a compact potbelly; oh, well), a rescue duck saved from the cook pot, and lots of hardy Kensington bees, on vacation at the moment in the warmer climes of Georgia.

"Putting a farm in the middle of the city is a very provocative act," says Ryan Kuck, Greensgrow's executive director.

This weekend, the harvest of that chutzpah will be celebrated as the urban farm and more marks its unusual longevity at 20 years - and its impact on this once-blighted block and beyond. Events include a mural unveiling and honoring of Corboy, who died in August at 58 after battling cancer.

From its roots till now, Greensgrow has educated the community about sustainable practices. It has reached out to neighborhood associations and partnered with the Pennsylvania Horticultural Society for beautification projects. More than a few hydroponic home gardens can trace their existence to the farm. Kuck even built a compost toilet on the site.

But its mission also has evolved.

It serves as a critical conduit for rural family farmers to sell produce to city folk through CSAs - city supported agriculture, as Corboy liked to say - and thereby "keeps farming viable throughout the region," according to Kuck. It also has created a community kitchen, subsidized CSAs for low-income families, and developed mobile markets for underserved neighborhoods. Three years ago, Greensgrow launched the West Philly garden center, which last summer moved to its permanent location at 5123 Baltimore Ave. Turns out that sometimes flowers are more popular than food.

All quite remarkable, given Greensgrow's start.

In 1997, Corboy and fellow chef Tom Sereduk were growing mesclun and other produce in a Jersey field to supply city restaurant kitchens. Bountiful experiments with hydroponics led to the idea of an urban farm - a crazy concept back then.

On a trash-strewn lot in Kensington, a Superfund brownfield that had been capped with concrete, the pair repurposed old rain gutters as troughs to hydroponically grow lettuce and other leafy greens for wholesale.

"It was pretty impressive," says Kuck, who has seen the pictures from the late '90s. "It was row after row for the whole block. They tell stories of neighbors coming by and thinking it was some NASA research lab. They were viewed as the local crackpots."

By 1999, revenue exceeded $50,000, and the not-so-crazy-anymore farm registered as the nonprofit Greensgrow Philadelphia Project.

Over the next decade, it prospered. But the work was no breeze. Sereduk describes backbreaking days. "We were constantly vandalized, robbed, harassed," he adds. "It was not a fun place." Even the birds pecked at the baby lettuce. His relationship with Corboy suffered, and Sereduk left to start in 2008 the cut-flower farm Longview Flowers in Lumberton. Later, the two reconciled. "Mary's vision saw [Greensgrow] through those dark days."

In fact, her foresight transformed the urban farm into an innovator and incubator. "Mary liked to call it an idea farm," says Kuck, a Corboy protege who has worked at Greensgrow for 11 years, doing everything from making biodiesel to helping with beekeeping.

Along the way, hydroponics (down to a few trays) has given way to raised beds made of 10 inches of trucked-in soil over the concrete ground and a vibrant garden center. But perhaps Greensgrow's wisest move has involved the mundane task of bookkeeping. Kuck argues that its hybrid social entrepreneurship/nonprofit model, in which earned income provides 91 percent of the budget, ensures the project's stability and the livelihoods of its 90 partner farmers and many employees, who number as high as 40 in the summer season.

"We don't have to chase the money," he says. Instead, they think up ideas and seek grants to "try new things and be bold."

One gamble that paid off was the urban-styled CSA geared to couples and small families. It offers a more diverse mix of stuff, with fewer vegetables and more local products such as roasted coffee, potted plants, even pierogies.

Not every idea succeeds. A truck that offered fresh, local, in-season produce to Camden was abandoned after three years, Kuck says.

That willingness to take risks and reimagine itself has made Greensgrow the grand dame of urban farms locally, and one of the few oldies nationally.

"Greensgrow has to continue to be a dreamer and a schemer," says Jerry F. Naples Jr., a board member and environmental professional. "It will survive because it's resilient."

On a recent day, workers at the Kensington site shoveled snow and prepared for the season's start. Shelves were full of pansies and the oversize blooms of ranunculus. Colorful ceramic planters dotted the grounds.

"It's a nice oasis in a tough neighborhood," says Carolynn Angle, who used to buy greens when she worked as a chef at Northern Liberties' Standard Tap.

After Corboy's death, Angle traded the kitchen for another labor of love. "I wanted to help the farm continue in her honor," says the farmhand. "This was her life. It's turned into something that the community needs to feel alive."

As Greensgrow looks ahead to the next 20 years, Kuck says Corboy is never far from his mind, especially her insistence, as she once said, that Greensgrow "reinvent itself every few years to stay ahead of the curve."

A poster of the gray-haired personality, frowning, of course, is tacked to the wall across from Kuck's desk.

"So," he says, "we have a good dialogue going still."

Lkadaba@gmail.com @exinkygal