WELLSTON, Oklahoma—One day in the early fall of 2018, while scrutinizing the finances of his thriving Colorado garden supply business, Chip Baker noticed a curious development: transportation costs had spiked fivefold. The surge, he quickly determined, was due to huge shipments of cultivation supplies—potting soil, grow lights, dehumidifiers, fertilizer, water filters—to Oklahoma. Baker, who has been growing weed since he was 13 in Georgia, has cultivated crops in some of the world’s most notorious marijuana hotspots, from the forests of Northern California’s Emerald Triangle to the lake region of Switzerland to the mountains of Colorado. Oklahoma was not exactly on...