September produce tastes different, and science can explain why it happens overnight. Cool nights slow the process that burns sugars in ripening crops, and the fall harvest arriving at markets right now has benefited from exactly that. In many temperate regions, the season also delivers something rare: summer and fall crops available at the same time.
Farmers market tables in September look a lot like August, but what’s underneath the skin has changed. Plant scientists call the daily swing between daytime highs and overnight lows thermoperiod, and it affects photosynthesis, respiration and fruit development in ways that vary by crop. The payoff shows up in flavor, color and texture, and it’s happening right now.
What Cool Nights Actually Do to Produce
When nights cool down, plants slow their respiration rate, the process that burns the sugars produced during the day. Crops nearing harvest hold onto more of those sugars when nights drop, and the effect compounds over weeks. For several of the most popular items at September markets, the timing lines up well.
Apples Are the Most Obvious Example
Most apple varieties produce their highest-quality fruit when overnight temperatures drop below 60 degrees Fahrenheit near harvest. Apples grown with nighttime temperatures between 40 and 60 degrees tend to be firmer, better colored, better for storage and more flavorful than those grown under warmer conditions.
Minnesota orchards are already seeing the difference. Cooler nights this month are improving color development in varieties like Honeycrisp and Gala, and where fruit is approaching peak, the combination of color and quality is one of the clearest examples of thermoperiod at work.
Winter Squash Gets Sweeter on the Vine
Butternut squash sitting on the vine through a run of cool nights is doing something measurable. Low temperatures reduce energy use and increase sugar storage in ripe winter squash, which is why the first butternut of the season often tastes richer than anything available in August. The sweetness summer’s warm nights couldn’t produce shows up in September.
Tomatoes Have a Narrower Window
Tomatoes don’t respond to cool nights the same way apples and squash do. Ripening is generally favored around 60 to 75 degrees Fahrenheit, and nights that fall below 55 degrees can actually slow the process rather than help it. When late-summer heat subsides without nights becoming too cool, tomatoes get more hours in their ideal ripening range and ripen more evenly than they do during peak heat.
September Is the One Month You Get Both
The USDA’s seasonal produce guide lists tomatoes, peppers, corn, eggplant and peaches as summer crops and apples, pears, Brussels sprouts and winter squash as fall crops. In September, both groups show up at many farmers markets at the same time. That overlap opens cooking combinations that are much harder to find in July or October, and the window narrows fast as the month moves on.
Freshness Is Already on Shoppers’ Minds
Produce shoppers are paying more attention to quality than ever. This year, 77% of consumers cited freshness as one of the most influential factors in where they shop for groceries, well ahead of discounts at 58%. The share of consumers who say they try to eat organic whenever possible jumped from 46% in 2024 to 55% this year. September farmers markets are one of the best places to act on all of that, and right now the timing is worth paying attention to.
Jennifer Allen is a retired professional chef and long-time writer. Her work appears in dozens of publications, including MSN, Yahoo, The Washington Post and The Seattle Times. These days, she’s busy in the kitchen developing recipes and traveling the world, and you can find all her best creations at Cook What You Love.