The heat wave's impact on Washington's US$2 billion apple crop - the state's most valuable agricultural product - is uncertain, as harvest is at least six weeks away.

Apple growers are used to sleepless nights as they respond to springtime frosts, but have little experience with sustained heat in June.

"We really don't know what the effects are. We just have to ride it out,"  Todd Fryhover, president of the Washington Apple Commission, told Reuters.

Growers have been protecting their orchards with expansive nets that protect fruit against sunburn, and by spraying water vapor above the trees. Apples have stopped growing for the time being, Fryhover said, but it is possible the crop may make up for lost time if weather conditions normalize.

Meanwhile, the region should still produce a roughly average-sized cherry harvest, but not the bumper crop initially expected, said B.J. Thurlby, president of the Northwest Cherry Growers.

The heat peaked in late June, in the thick of the harvest of cherries. Temperatures reached 118 degrees Fahrenheit (48 Celsius) on June 28 at The Dalles, Oregon, along the Washington border, near the heart of cherry country.