MABTON, Wash.—A portable floodlight replaced the sun as workers at Alvarez Farms threw boxes of potatoes and green beans onto their shoulders and loaded a delivery truck.

The day’s 94-degree heat had receded, and laborers took advantage, loading the shipment in the relative cool of the first hours of a Yakima Valley night.

Inside a large refrigerator, a dusty and sweaty Eddie Alvarez, 28, stacked boxes of produce to be hauled to the truck.

“There’s no hour to clock out until we get done,” he said.

Loading the truck was the last task of the day in...