ESCALON, CALIF.—The shrink-wrapped boxes of fresh California walnuts stacked almost to the ceiling in Don Barton’s California packing facility should be headed to Europe for holiday baking and to Asia for New Year celebrations. Instead, newly cleaned and shelled nuts—about $10 million worth—are stuck at his processing plant near Sacramento, thousands of miles from their destinations, as the global supply chain crisis squeezes ports. Transportation and supply-chain problems are hurting farmers across the U.S. West Coast, a major global supplier of specialty crops like fruits and nuts popular at year-end celebrations. Ships that would normally pick up walnuts from Barton’s company, Gold River Orchards, are skipping the Port of Oakland where the nuts are usually exported, or showing up at odd or unexpected times that make it difficult to get the product to the docks. “We are shipping right now less than half of what we should be shipping and …