Zynga announced today the launch of Zynga Serve Rewards, a new virtual game cash rewards program with a prepaid card that is co-branded with American Express and connected to the Serve digital wallet from the credit card company.

Customers will need to register for the Zynga Serve Rewards program to get a prepaid card in the mail, which they will need to fund via bank account, debit card, or cash. Once the Serve card is activated online or by phone, it can be used anywhere American Express cards are accepted for purchases.

Once registered with the rewards program, Farmville players will be able to plant a special “Money Tree” in their online farms, which will start to produce virtual awards, according to Jeff Karp, chief marketing and revenue officer at Zynga. The money tree will evolve or level up as players progress in the game with virtual items like colors, glow effects, butterflies and toucans. The virtual tree will also serve as a dashboard for the prepaid card service, said Karp, allowing players to check their rewards status in-game. The plan over the next year is to extend the virtual rewards for real life spending to other Zynga games as well, like CastleVille and CityVille.

At launch, only the first five purchases of $25 or more on the Zynga Serve Rewards card will earn Farm Cash rewards. The program is set to eventually provide rewards above the initial first five purchases of $25 or more.

“We’re excited to partner with American Express to invent new ways for people to experience Zynga play in more parts of their day,” said Mark Pincus, chief executive and founder of Zynga. “Together we can add surprise and delight to every day shopping.”