Americans are used to buying whatever fruits and vegetables they want whenever they want them, thanks largely to imports from Mexico. For avocados, particularly the ubiquitous Hass variety, those are an absolute necessity. “Avocado trees are real fickle,” explained Steve Barnard, chief executive of Mission Produce, a major distributor. “They don’t like it too hot or too cold. Southern California is the only place you can grow Hass in the U.S. because of climate, temperature, soils, and whatnot