Farmers in Yuma, Ariz., like to tell visitors that they produce 90% of the country's winter greens.

So if you're eating a salad in Buffalo, Boston, or Cincinnati, there's a pretty good chance the lettuce was grown near the U.S.-Mexico border with water from the Colorado River. In San Francisco, 650 miles northwest of Yuma, an agricultural startup called Plenty is doing just that.

Leafy greens like bok choy, lettuce, and baby kale grow inside a warehouse on towers that rise five meters off the floor. A rainbow spectrum of LED lights shines on the plants for most of the day, offering them ideal growing conditions.

"We bring the plant in, and we just say, 'how do we get the best results, period,'" says Plenty co-founder Nate Storey. "We keep the growth rate at maximum the whole way through. Their lives are very short, but they are very good."