NAPA, Calif. — In a borrowed van, Brad Gates of Wild Boar Farms sped south on Interstate 680 with hundreds of fuzzy tomato seedlings bumping around in the back, their trembling leaves, warmed by the sun, filling the cab with the smell of summer. It was one of a half-dozen deliveries on his to-do list.

Born and raised in Northern California, Mr. Gates has been organically farming tomatoes in the region for 25 years, working on small leased plots and introducing new varieties with cult followings, like the dark, meaty Black Beauty and the striped, rosy-pink Dragon’s Eye.

For most of that time, he sold his tomatoes to top restaurants, including Chez Panisse in Berkeley. But a few years ago he completely rethought his work. Galvanized by climate change, he joined a growing number of farmers who are trying to find a future for their threatened crops — in his case, the queen of the farmers’ market.