Saskatchewan fans have been known to do some elaborate carving for the perfect melon helmet. (CBC)

With thousands of Roughrider fans descending on Calgary this week, there's suddenly a huge demand for watermelons.

For years, Saskatchewan fans have been wearing hollowed-out melons on their own melons. Now, with the Grey Cup game Sunday pitting Saskatchewan against the Montreal Alouettes, demand for the summery treat has skyrocketed.

"Calgary has taken all the watermelons available in Western Canada," said David Karwacki, CEO of Saskatoon-based fruit and vegetable wholesaler Star Produce. "You just can't get them there quick enough."

Rider fan Kris Moen calls the cool green headwear — sometimes worn with a cloth liner, sometimes without — an essential part of Roughrider culture.

'It's very special. You hold it with thousands of other people that have also worn the watermelon bikini or helmet or whatever way you want to wear your watermelon.'—Rider fan Kris Moen

"It's very special. You hold it with thousands of other people that have also worn the watermelon bikini or helmet or whatever way you want to wear your watermelon," Moen said.

A informal survey of Calgary grocery stores Wednesday showed many have small melons in stock.

But it's not easy to find the full-sized varieties, Canada Safeway spokesperson Betty Kellsey said.

"We have actually reallocated all of our watermelon supply for Western Canada to bring them into our Calgary stores to make sure we're well in stock for the Riders fans as they start to arrive," she said.

Some melonheads are already all set, however.

Anita Richards, a diehard Rider fan who now lives in Calgary, was preparing to carve four helmets Wednesday.

"Yeah, they were really easy to find," she said. "But that's only because we bought them on Monday."

It seems no one is sure when or why the first Rider fan donned watermelon headgear. However, people with the Roughriders organization say the craze took off a few years ago after a Riders TV commercial showed a man carving up a melon helmet.