Starbucks tries out a no-cash cafe in Seattle

Starbucks is experimenting with refusing cash at a store in downtown Seattle.

Since Tuesday, money is no good at the cafe inside the Russell Investments Center unless it’s in the form of plastic.

There is no sign announcing the policy, but a barista Wednesday declined to take a $20 bill in payment for a short latte and a piece of lemon cake, and said the store was not accepting cash.

A Starbucks spokeswoman said the store is the only one to test accepting “only cashless forms of payment.” The company isn’t saying how long the experiment will last or whether it will be expanded to other locations.

Starbucks says its mobile payment and ordering app is a fast-growing success, to the point that last year it blamed slow sales growth at stores on crowding by people who had ordered from their phones.

The second-floor Russell Center cafe, dominated by armchairs, couches and at least one chaise lounge, is reached from the lobby of the 42-story building, which is the corporate home of Zillow, an online real-estate company, and Russell, an international financial firm.

Jonathan Zhang, a professor of marketing at the University of Washington’s Foster School of Business, said it’s likely the Russell Center store sees few cash transactions anyway.

“I would expect in that location that cash represents only a tiny percentage of business,” said Zhang, who studies and consults with retailers on e-commerce and mobile payments. “They are not giving up much,” maybe 5 percent or less, because the clientele is predominantly white-collar workers who tend to use plastic.

The Los Angeles-based vegetarian chain Sweetgrass, with 75 locations in California and the Northeast, is reportedly the largest restaurant company to go cashless. Co-CEO Jonathan Neman told an industry conference in September that the decision has worked out well and has not dented its bottom line.

The no-cash test may open Star-