Every gardening site gives the same one word answer to why are tomatoes red. That word is lycopene, and it is correct as far as it goes.
It is not the whole story, though. Temperature can shut lycopene production down completely. Some tomato varieties are genetically wired to skip red altogether.
This guide covers the actual biology behind the color switch. It also covers the heat ceiling that stops it. It ends with the gene that decides red, orange, or yellow in the first place.
The short version
Why are tomatoes red? A ripe tomato turns red because chlorophyll breaks down and lycopene takes over. That pigment shift happens inside the fruit’s own cells as they mature.
Heat can block it entirely. Above roughly 85 degrees Fahrenheit, tomato plants stop making lycopene. Fruit ripens yellow or orange instead of red.
Some varieties never turn red no matter the weather. A single enzyme called lycopene beta cyclase decides the outcome. That trait is genetic, not a ripeness problem.
What is actually happening inside the fruit
A green tomato is green because of chlorophyll, the same pigment that colors leaves. As the fruit matures, its chloroplasts convert into chromoplasts. That is a different kind of cell structure built to hold pigment rather than run photosynthesis.
Research published in Frontiers in Plant Science describes what happens next. Lycopene accumulates three to four times faster than chlorophyll disappears. That speed difference is why the shift from green to red looks so sudden.
Ethylene is the hormone that sets the whole cascade in motion. Tomatoes are a climacteric fruit, meaning they ripen in response to a burst of that hormone. Without an ethylene signal, the color change stalls even on a fully grown fruit.
That is a separate issue from heat. It explains why a picked green tomato can still ripen indoors once ethylene starts flowing.
Why Are Tomatoes Red Only Below a Certain Temperature?
Temperature controls lycopene production directly, and there is a real ceiling. Purdue Extension and University of Illinois Extension both put that ceiling at 85 degrees Fahrenheit.
South Dakota State University Extension lists the threshold slightly higher, at 86 degrees. That one degree gap is minor. It still matters, since it confirms this is a documented biological limit, not a rounded folk number.
Above that ceiling, a tomato plant cannot make lycopene or the related pigment carotene. Fruit still ripens in the sense that it softens and develops a separation layer. It just comes out yellowish green to yellowish orange instead of red.
The ideal range for genuine red color sits much lower. Purdue Extension puts it at 68 to 77 degrees Fahrenheit. Ripening slows well before the fruit ever reaches that upper heat ceiling.
This explains a pattern that shows up constantly in gardening forums. Growers on Houzz and GardenWeb have asked some version of why their tomatoes will not turn red across many separate growing seasons. The ones who mention weather almost always describe daytime highs in the 80s and 90s.
Why some tomatoes never turn red at all
Heat explains a stalled ripe tomato. It does not explain an orange or yellow variety that ripens right on schedule and simply never turns red.
That comes down to one enzyme. Research published in Horticulture Research identifies lycopene beta cyclase as the switch that decides a tomato’s final color.
This enzyme converts lycopene into beta carotene and alpha carotene. A variety with reduced enzyme activity keeps its lycopene intact and ripens deep red.
A variety with increased enzyme activity converts most of that lycopene away instead. That variety ripens orange rather than red.
Yellow varieties take a different genetic path. They carry a separate mutation that reduces overall carotenoid production, rather than just redirecting it.
That distinction matters for anyone growing orange or yellow heirlooms, and our guide to tomato colors maps the whole rainbow. A yellow tomato is not an orange tomato that stalled partway through ripening. It is following its own genetic program from the start.
Does red mulch actually make tomatoes redder?
Red plastic mulch has a real, documented yield benefit, and it gets misunderstood constantly. Kansas State University Research and Extension reports an average 12 percent increase in tomato yield with red mulch. Difficult growing conditions can push that gain up to 20 percent.
USDA Agricultural Research Service physiologist Michael Kasperbauer explained the actual mechanism behind that gain. Red mulch reflects specific light wavelengths that the plant reads as a growth cue. That cue pushes more energy into growth above ground.
That mechanism has nothing to do with fruit pigment directly. In one USDA field trial under heavy nematode pressure, tomato plants on red mulch produced 17 pounds of fruit. Plants on black mulch produced only 8 pounds, a gain the researchers traced to overall plant health.
Red mulch is a real tool for yield. It is not a way to turn individual fruit a deeper shade of red, and no research reviewed for this guide claims otherwise.
Frequently asked questions
Why are tomatoes red instead of some other color?
Ripe tomatoes are red because lycopene accumulates as chlorophyll breaks down during ripening. Most cultivated varieties are bred to keep that lycopene intact, which produces the deep red color shoppers expect.
At what temperature do tomatoes stop turning red?
Extension sources put the ceiling at 85 to 86 degrees Fahrenheit. Above that range, plants stop producing lycopene and carotene, and fruit ripens yellowish orange instead of red.
Why are some heirloom tomatoes orange or yellow instead of red?
An enzyme called lycopene beta cyclase converts lycopene into other carotenoids in those varieties. Higher enzyme activity means less lycopene stays intact, which produces orange or yellow fruit instead of red.
Does red plastic mulch make tomatoes turn redder?
No. It reflects light wavelengths that boost overall plant growth and yield, documented at up to a 20 percent increase in some trials. It does not add pigment to individual fruit.
Can a tomato ripen and still not turn red?
Yes, in extreme heat. The fruit can soften and develop normally in every other way above about 85 degrees Fahrenheit, but the red color simply will not form.
The bottom line: why are tomatoes red comes down to lycopene replacing chlorophyll as fruit matures. Heat above 85 degrees can shut that process down, and a single gene can redirect it toward orange or yellow instead. The color on a ripe tomato is a genuine biological signal, not just a ripeness checkbox.
For the heat thresholds that affect the whole plant, not just color, see our how much sun do tomatoes need guide. For the full ripening timeline from flower to harvest, see our how long do tomatoes take to grow guide.
SOURCES
University of Illinois Extension, Good Growing: Stalled Green Tomatoes, Why Won’t They Turn Red? https://extension.illinois.edu/blogs/good-growing/2025-08-29-stalled-green-tomatoes-why-wont-they-turn-red
Purdue University, Yard and Garden: Tomatoes Not Ripening? https://www.purdue.edu/hla/sites/yardandgarden/tomatoes-not-ripening/
South Dakota State University Extension: Will Heat Make My Tomatoes Ripen Faster? https://extension.sdstate.edu/will-heat-make-my-tomatoes-ripen-faster
Frontiers in Plant Science: Genetic Regulation and Structural Changes During Tomato Fruit Development and Ripening. https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2014.00124/full
Oxford Academic, Horticulture Research: Metabolism and Regulation of Chlorophyll and Carotenoids in Tomato Fruits. https://academic.oup.com/hr/article/13/7/uhag084/8505573
Kansas State University Research and Extension: What Color Mulch Do Tomatoes Prefer? Red, of Course. https://www.ksre.k-state.edu/news/stories/2021/04/tomatoes-prefer-red-colored-plastic-mulch.html
United States Department of Agriculture, Agricultural Research Service, ARS ARonline Magazine, October 1997. https://agresearchmag.ars.usda.gov/1997/oct/mulch