GRAND RAPIDS, MI - Gardeners have noticed green, fully formed tomatoes stubbornly hanging onto their vines, refusing to progress toward the ripe, vibrant vegetables of summer eagerly awaited by many.
It turns out the recent extended high temperatures caused by climate change are dramatically slowing down the ripening process of tomatoes, according to recent studies including a 2022 study in the Agronomy Journal.
"It slows the ripening and the turning red," said Deanna Hedlund, a consumer horticulture educator with Michigan State University (MSU) Extension based in the Kent County office. "They might just develop into full-size green tomatoes, but then the final bit of ripening is just slowed."
Hedlund said it's not only high daytime temperatures that are causing problems, but also warm nights.
The optimum ripening temperature for tomatoes is between 68 and 77 degrees Fahrenheit, she said. "If it's over 85 and then you're staying over 70 at night, that's when you're going to have a problem."
The long-term forecast predicts the heating trend to continue.
10 warmest years on record
According to the World Meteorological Organization (WMO), 2024 was the hottest year of the 175-year weather record. 2015-2024 have all been among the 10 warmest on record.
So far, the first six months of 2025 were only slightly behind 2024's temperatures, according to the National Oceanic and Atmospheric Administration, and it is likely 2025 will end up in the two to four hottest years.
"The plant basically stops working when it gets too hot. It just kind of shuts down all processes," said Ben Phillips, a vegetable crops educator with MSU Extension based in Benton Harbor.
Excessive dryness compounds the problem, he said.
It has been a dry summer in much of West Michigan. According to the U.S. Department of Agriculture's Midwest Climate Hub, several areas are classified as either abnormally dry or in a moderate drought as of Aug. 5.
"One of the best things to do is keep them well watered or it becomes sort of a downward spiral," Phillips said. "If you don't supplement the water, they'll just tank on you."
Hedlund said the heat is slowing the production of two natural compounds, carotenoids called lycopene and carotene, that are responsible for the red color.
Those chemicals are some of the same pigments that cause leaves to turn color in the fall, she said.
The ripening process
Hedlund said the slow ripening generally does not hurt tomatoes, but if it takes too long they could split or become susceptible to fungal infections.
The ripening process can be sped up by placing fully formed green tomatoes in a bag and storing them on the kitchen counter.
"The taste is better for the ones that you let completely ripen on the vine," said Hedlund, who said you can also try new recipes. "Like the idea of a fried green tomato."
She cautions that slow ripening can also be caused by other problems, and it's important to rule out other potential issues like overfertilization.
It's not just tomatoes that are feeling the heat.
For example, broccoli, cauliflower and Brussels sprouts are known as cool season crops.
"They don't like it hot," Phillips said. "Broccoli gets misshapen and miscolored and cauliflower will do the same thing."
Another heat related issue can occur in green beans. When the nighttime temperature doesn't drop below 70 degrees Fahrenheit at night the flowers drop off the plant, preventing fruit from developing altogether, he said.
That means green beans will go through periods in the summer when they don't bear fruit, which can be a problem for large producers who harvest with machines.
Pumpkins, however, love the heat and are ripening too quickly, Phillips said.
"Pumpkins are way ahead of schedule for a lot of people," he said. "Everyone's like, 'We've got ripening fruit now. What am I going to do?' It's an October crop. September is usually when people are harvesting pumpkins."
"It's going to be hard to keep them until the market is asking for them," he said.
Besides directly impacting vegetables, climate change is changing insects and diseases that can affect crops.
For example, heat can change the activity of pollinators, the insects that pollinate vegetables' flowers, Hedlund said. That can cause misshapen vegetables.
Crop diseases and insect pests that are common farther south are moving north with the milder winters, causing new problems here.
Advice to gardeners
A May 2025 climate report by the World Meteorological Organization predicts record temperatures to continue for at least the next five years. Experts predict that if the burning of fossil fuels continues at current levels temperatures could increase significantly from 2024 levels by the end of the century.
Phillips advises gardeners to prepare for temperature increases by trying out crop varieties that were developed in warmer areas.
Some seed catalogs provide information about the different varieties that could give an indication of how they might do in Michigan in the future. Then growers can make an educated guess about which new varieties to try.
"For example, a variety that was bred in Georgia may be something that we would do better with up here as it just keeps getting warmer and more humid," he said.
Hedlund encourages people with gardening questions to take advantage of MSU Extension's gardening resources online. Gardeners can also call the gardening hotline at 1-888-678-3464 or submit questions online to their "Ask an Expert" service.
For adults interested in diving deeper into the science behind successful gardening, MSU Extension offers the "Foundations of Gardening" online course twice a year. Registration for the Fall 2025 session closes on Aug. 27.
The course is the first step towards becoming an MSU Extension Master Gardener volunteer, but continuing in that process is optional after taking the course.