Trey Willison, a cannabis farmer in Eugene, first started worrying last May about there being too much marijuana in Oregon. He had sold all his “clone” plants to other growers, who were using them to cultivate yet more marijuana. “You start doing the math on that and it just didn’t make sense how people could be growing that many plants,” Willison said. Fast-forward nearly a year and Oregon does indeed have a glut of marijuana; there are over 1m lb of usable but unsold marijuana, according to the state tracking system. That’s more than 128m “eighths” of weed, and almost...