About 2,200 growers and more than 700 markets have paid to participate in the program after growing frustrated that some of the produce sold at markets was actually trucked in from hundreds of miles away.
"One of the common misconceptions consumers have is that `local' means it's coming from the nearest farmland to their city," said Rick Jensen, chief of inspection and compliance with the California Department of Food and Agriculture. "But you can buy stuff in Los Angeles from Kings County or from the Central Coast or from anywhere in the state."
California is taking the lead in certifying where food is grown, but organizers of the effort said the issue is prevalent throughout the country. It may be even more common in cold-weather spots without California's long growing seasons, they said.
The number of farmers markets in the country increased by 16 percent in the last year, according to data released Tuesday by the U.S. Department of Agriculture. The current USDA Farmers Market Directory lists more than 6,100 farmers markets.
In California, vendors who want to sell at certified farmers markets must register with their county's Department of Agriculture and display certificates at their stands.
The state began laying out guidelines for markets in the 1970s but the current certification program was enacted in 2000. Counties charge up to $60 per hour to certify and inspect farms, while the state collects fees from market operators depending on number of vendors. The state food and agriculture department brought in $229,000 last year from registered farmers markets.
The vendors must list the produce they grow and sell, and the area where it's grown. Most counties honor each other's certification, letting growers travel throughout the state to sell their products.
County officials do spot checks and vendors at certified markets who fail to display certification face fines.
"Farmers markets are a growing segment of agriculture and so is the local food movement," said Jay Van Rein, spokesman for the California Department of Food and Agriculture. "People like knowing where their food came from."
Mai Thao, a 19-year-old whose family runs Mao Farms near Easton in California's Central Valley, said it's good for everyone when people know where and how their food is grown.
"People ask us all kinds of questions: Do we use pesticides? Is this a family business? What are we harvesting next?" she said Saturday at the Vineyard Farmers Market in Fresno, Calif.
But not all consumers are savvy, Jensen said.
"We've been spoiled by large grocery stores that offer everything year-round," he said. "If you go to a market, you have to think, `Is this a product that is growing in this area at this time of year?' If not, then it's probably being brought in from elsewhere."
The Mao Farms stand on Saturday had bok choy, lettuce, spinach, cilantro, green onions, green beans and other veggies grown at the family's organic farm just eight miles away. The fall means a lot of greens, and as it gets colder, more root vegetables, Thao said.
That insider knowledge is the benefit to farmers markets, Thao said.
Though officials try to keep track of farmers markets, it's impossible to track everything, said Fred Rinder, deputy agriculture commissioner for Fresno County. California has separate rules for individual roadside stands.
Promoters of local food worry that lax regulation at venues like flea markets allow sellers to hawk fruits and vegetables bought from wholesalers who ship in from other states and countries under the guise of California produce.
But some unofficial markets like a small, six-vendor cluster of organic producers who gather in a Fresno parking lot can offer customers great food, said 23-year-old Eddie Dowe who makes a 20-minute trip into the city every weekend to shop for local, organic produce at a non-sanctioned spot.
"Most people don't realize that the fruits and vegetables in supermarkets travel hundreds of miles and are picked days or even a week in advance," said Dowe, who has been a vegetarian for three years.
On Saturday, he loaded up on lettuce, chard and apples from Smith's Family Farms, a more than 100-year-old operation in Fresno.
"Buying local is one of the most important things you can do," he said. "It's a way to take control of your food."