A heat wave is threatening to take the bloom off one of the world's largest garden festivals, as more than one million colorful tulips in Canada's capital began wilting Tuesday.

More than 100 heat records were broken across the country, according the weather office, while Ottawa posted a near-record 27 degrees Celsius (80.6 Fahrenheit) just as the Canadian Tulip Festival was getting underway.

"You're at the mercy of the weather. Some years there was frost on the ground, and others the tulips bloomed too soon," festival head Allan Wigney told AFP.

"Most of the tulips in the beds look pretty good now, but if it stays 27 degrees for another week, it's not going to be good for any plants."

The festival started in 1953 as a nod to the Dutch royal family, which had sent 100,000 tulip bulbs to Ottawa in gratitude for Canadians having sheltered Princess Juliana and her daughters during the Second World War Nazi occupation of the Netherlands.

Over the past six decades, it grew into a major tourist event, adding music concerts, celebrity talks and more, drawing up to 500,000 tourists a year.

Last decade, however, heavy rains and cold drove away many visitors and the festival to the brink of bankruptcy.

One year, a cold snap left ghostly gardens with only green stems throughout the city -- all the tulip petals had fallen off days after blooming. Festival organizers adapted by moving several events indoors.

So far this year, tens of thousands have attended the event.

Garden tourism is very popular, Wigney noted.

"It's a big international business. A lot of people go see the flora" at similar events all over the world, he said.

But tulips only bloom for a short while and timing it right is very difficult for the 18-day festival that ends May 20.

"The tulips are looking pretty good right now but there's no guarantee with weather and vegetation," Wigney said.

"I hope people don't blame us for the weather. Of course, the tulips are a big part of the festival, but there's a lot to do" there beyond perusing the flowers, he added.