Canada’s wheat farmers still have more than a month before they start the bulk of this season’s grain harvest, but they’re already worried about where to store a bumper crop.

Even though growers planted less after prices fell last year, beneficial weather boosted the outlook for yields across Canada’s Prairies. The potential for near-record output means farmers are stocking up on grain bags to store wheat on the ground amid expectations that bins and silos will be overflowing.

“There’s such a demand because the crop looks so big this year,” said Bruce Nordick, a sales manager at Grain Bags Can...