The U.S. Department of Agriculture defines the boundaries of each plant hardiness zone, using climate data to allow people to know which plants are able to grow at their location. But there is no API, and the bulk downloads are not useful to most people. Maps are provided as graphics, but data files are sold via Climate Source, a private company, for $300 per region, with the U.S. comprised of 19 regions.
It's possible that the USDA has a contract with Climate Source that make this data impractical or legally impossible to be freed. Ideally, these data would be available as GeoJSON, and there'd be a public API to allow lat/lon pairs to be provided in exchange for a hardiness zone.
See @kgjenkins' Open Plant Hardiness Zones repository for a clever effort to turn USDA's raster images into geodata.