Let's call it the Banana Problem: the trade off between eating locally and eating what you want. Even if you live in California, many foods, like bananas, just aren't local; venture elsewhere, like the Northeast, and locavores languish in a purgatory of last season's apples and potatoes for a third of the year. Savoring a banana, or even a winter tomato, means eating something that was picked unripe by poorly paid laborers, or even slaves, and shipped hundreds or thousands of miles, emitting greenhouse gases each step of the way.

So, how to enjoy that banana without a side of guilt? I found an answer last December, on a rocky ridge 7200 feet above sea level in the aptly named town of Basalt, Colorado. The ground was crusted with snow, the temperature had recently hit 10 below zero, and Jerome Osentowski was preparing to plant bananas.

Osentowski, director of the Central Rocky Mountain Permaculture Institute, has built what must be one of the world's most sophisticated greenhouses, which he is filling with papayas, passion fruit, citrus, and other tropical and Mediterranean crops. In winter, it stays between 30 and 50 degrees warmer than the air outside--and unlike conventional greenhouses, it uses hardly any fossil fuels. Thermal mass, in the form of heavy rock walls, absorbs and stores heat, and an intricate system of underground pipes sucks in hot, moist air during the day and releases it as the interior cools at night.

Osentowski has grown tropical crops in Basalt for years--and he likes to say that if he can do it there, it can be done anywhere: imagine mangoes from Pennsylvania, or pineapples from the urban farms springing up in Brooklyn or Detroit.

To be sure, there would be serious barriers, namely knowledge and cost. Osentowski practices intensive "forest gardening," densely packing perennial crops and growing them in tiers that resemble the vertical layers of a rainforest. Imitators would have to learn these techniques and invest tens of thousands of dollars in greenhouses--perhaps even more if people pursued scaled-up versions of Osentowski's cheap but small 2200-square-foot prototype.

Even if raising tropical fruit far from the Equator never takes off, Osentowski's greenhouse offers many lessons. As organic gardening pioneer Eliot Coleman has shown for decades, uncomplicated "hoop houses," plastic or metal frames covered with plastic sheeting, can extend growing seasons for weeks and months. Perhaps simpler versions of the Basalt greenhouse could do even better, growing seasonal crops like tomatoes and strawberries year-round in places where they can now be found only in late summer and spring.

Above all, this innovation forces us to rethink the possibilities of what truly sustainable agriculture might look like. Would cherishing traditional crops in season still be best? Probably. Might die-hards pooh-pooh such newfangled--or "unnatural"--greenhouses? Yes. But if given the choice, I'd take bananas from Connecticut over those from Costa Rica.