BOSTON — It's come to this: Maple syrup is being rationed in Vermont.

"For the first time in 19 years, I have to do portion control," said Karen Griffin, who owns Libby's Diner in Colchester. After rising demand and shrinking supply pushed up the price of maple syrup 49 percent in the past year, she no longer allows customers to pour with abandon.

The syrup her guests get with pancakes and waffles is the real stuff, not any maple-flavored imitation. Griffin, 58, serves syrup made from tapping maple trees for their sweet sap. Vermont is the top U.S. source, producing 500,000 gallons last year.

As the state's annual harvest got under way last week, restaurants and shops are paying as much as $70 a gallon for the syrup, said Rick Marsh, president of the Vermont Maple Sugar Makers' Association. That's up from about $40 last year, he said.

Blame Canada. When syrup dispensers on tables in Vermont and elsewhere run low, they're likely to be refilled with syrup imported from the country with a maple leaf on its flag. Quebec produces 75 percent of the world supply, said Gary Gaudette, president of the International Maple Syrup Institute in Spencerville, Ontario.

Quebec's 2008 harvest, hampered by unusual cold and snow, fell 40 percent, he said. Canada had emptied the 65 million pounds of syrup in its so-called strategic reserve a year earlier, so there was no backup supply.

"People in Vermont and other places, when they run out, have nowhere to turn for extra syrup," Gaudette said.

At Libby's Diner, Griffin is trying to decide whether to serve maple syrup in small, disposable packets or limit consumption some other way. The glass dispensers with spouts, filled to the brim, are being banished.

Maple syrup already has been removed from tables at the Penny Cluse Cafe in Burlington. Now, owner Charles Reeves serves syrup only if french toast or pancakes are ordered, and the rising cost is factored into the price of those dishes. Reeves even stopped selling gift jars of syrup when the price hit an "embarrassing" $12 for 10 ounces.

"It could get worse before it gets better," said Reeves, 42. "It's a shame something so much a part of the state is suddenly so expensive that you may not see it anymore."

Maple syrup doesn't rank high in Vermont's agriculture, with sales totaling $13 million in 2007, the latest figure available. But it's prominent in the state's heritage and a hook for tourism.

"It goes back to the very beginning of the state and before that," said Roger Allbee, the state's agriculture secretary, who grew up on a Vermont farm that produced syrup. "It's very important to the image. Maine has lobsters, Massachusetts cranberries and Vermont maple syrup."

The state's long freeze-and-thaw cycles in early spring promote the flow of tree sap, said Tim Wilmot of the University of Vermont's Proctor Maple Research Center.

"Vermont syrup gives you the flavor of the woods," he said. "Nowhere else is it better."

The shortage is occurring as maple syrup's popularity spreads, particularly in Europe and Asia. Sales worldwide totaled 130 million pounds in 2007, up from 40 million pounds 30 years ago, Gaudette said.

The sweet stuff is also moving beyond the breakfast menu, for use in foods such as fish entrees and salad dressings. At New York's PDT restaurant, the $13 bacon-infused, bourbon-and-maple-syrup old fashioned is a popular cocktail, said Don Lee, beverage director.

A maple syrup diet has adherents such as singer Beyoncé Knowles. She told BBC television in 2006 that she lost 14 pounds for her "Dreamgirls" movie role by consuming only lemon-and-maple-syrup flavored water for about two weeks.

U.S. maple-syrup output increased 30 percent last year, including an 11 percent gain in Vermont, according to the U.S. Agriculture Department. Connecticut, Maine, Massachusetts, New Hampshire, Michigan, New York, Ohio, Pennsylvania and Wisconsin also produce syrup.

Some Vermonters see an opportunity. Eric Sorkin, a Columbia Law School graduate, moved to Cambridge with his wife 10 years ago to escape the bustle of Washington and raise organic vegetables. He borrowed money and invested $1.4 million in equipment to install 27,000 taps in maples on his land this year.

"You've probably caught on to the last bull market in America," said Sorkin, 38.