By Lauren Lewis
Even in a place like California, with year-round food production and farmers markets, farmers depend on bustling summer markets to see them through the leaner winter months. Some of the difference in market attendance is attributable to weather that keeps people away in the winter, but a lot of the difference is certainly due to the produce selection at a summer market. Apricots, plums, peaches, tomatoes, beans, cucumbers, zucchini, corn, peppers, eggplants… I think I’m not alone in feeling a giddiness when the locally-grown versions of all those guys become available. If you look at that list you’ll notice that everything is a fruit. Even the “vegetables” are fruits, in a botanical sense. Summer = fruit.
Why is this? A fruit is the result of one seed dispersal strategy that plants use: endozoochory, or dispersal by vertebrate animals. Plants surround their seed or seeds with a tasty and energy-rich substance (fruit), animals eat the fruit and then deposit the seeds somewhere else after eating them, encased in fertilizer.
For maximum reproductive success, plants need to not only attract fruit-eating animals, but also do so at a time when the seeds that get consumed have the highest chance of germinating and creating a new plant. Soil temperature is a huge determinant of seed germination, and the optimal temperature range for germination of all the common summer vegetables is high compared to fall and winter vegetables. Evolutionary logic says: to disperse seeds at the best time for germination, produce fruit at that same time. And so we get our vegetables-that-are-technically-fruits in the summer, when the soil is warmest.
San Francisco is a notoriously challenging place to grow fruiting plants (fruits and vegetables alike) specifically because of our cool summers. As I write this in Noe Valley in late June, it is completely overcast, misting a bit, and maybe 60 degrees outside. SF gardeners tend to have more success with a small subset of summer veg varieties, like cherry tomatoes as opposed to full-sized, and optimal varieties are highly dependent on the particular location, since our city has some ridiculously small microclimates.
Climate change, however, might start changing those calculations. One recent study estimated that by the end of this century, San Francisco’s climate will be more like San Diego’s, where fruiting crops are currently much more suited. Tree crop producers in the state are already seeing some damage to their yields from fewer chill hours in the year. (“Chill hours” are hours during which the temperature is 45 degrees F or below; all fruit and nut trees except citrus require a certain range of chill hours for proper leaf and bloom production.) And while San Francisco soil might be warmer in 50 years, weather, and crucially rainfall, will also be more unpredictable. For trees, whose fruit output comes years after planting, that’s a serious challenge. But the odd rainy year followed by a San Diego-style summer could greatly expand our options for San Francisco-grown cukes and tomatoes. A silver lining.