Few things worry new gardeners more than a tomato that refuses to ripen evenly. The shoulders stay stubbornly green while the rest of the fruit turns red, and the immediate assumption is that something has gone wrong—too much sun, too little fertilizer, or perhaps some mysterious disease.
But in many heirloom tomatoes, those green shoulders aren't a defect at all. In fact, they may be one of the reasons those tomatoes taste so good.
In this week's post, we'll take a brief dive into tomato ripening biology to uncover what causes green shoulders, why modern tomatoes seldom have them, and the surprising connection between this so-called flaw and some of the best-tasting tomatoes you'll ever grow.
Perhaps no tomato is more famous for its green shoulders than Cherokee Purple. Its beautiful green striping has inspired everything from paintings and stationery to fabric prints and home décor.
What Causes Green Shoulders on Tomatoes?
The short answer is chlorophyll.
Just as leaves contain chlorophyll to capture sunlight and produce sugars through photosynthesis, immature tomato fruits contain chlorophyll as well. In many heirloom varieties, the tissue near the stem develops especially high concentrations, creating the dark green shoulders that persist until the fruit begins to ripen.
For much of human history, this was perfectly normal. In fact, if you browse old seed catalogs or paintings of tomatoes, you'll notice that many varieties displayed some degree of green shouldering. It wasn't considered a defect—it was simply how tomatoes looked.
Of course, not all green shoulders are created equal. Heat stress and nutrient imbalances can sometimes cause a condition known as yellow shoulder disorder, in which portions of the fruit fail to ripen properly and remain hard or discolored. The green shoulders we're discussing here are different. They are genetically controlled, common among heirloom varieties, and typically disappear as the fruit reaches full maturity.
The question, then, is why some tomatoes possess these dark green shoulders while most modern varieties do not. The answer lies in a single genetic mutation that silently transformed the appearance—and perhaps the flavor—of the modern tomato.
While most famous for its exceptional flavor, Pink Brandywine is almost as well known for its green shoulders, which disappear as it reaches peak ripeness. The difficult part, though, is waiting.
The Mutation That Changed Modern Tomatoes
If you've ever wondered why supermarket tomatoes tend to ripen to a perfectly uniform shade of red while many heirlooms retain dark green shoulders, the answer largely comes down to a single genetic mutation.
The so-called uniform ripening (u) mutation was first discovered in the late 1920s and quickly caught the attention of tomato breeders. From a practical standpoint, the benefits were obvious. In an era when tomatoes were harvested almost entirely by hand, uniformly colored fruit made it easier for workers to identify ripe tomatoes in the field. The mutation also reduced problems like stress-induced cracking and sunscald, resulting in fruit that looked better and suffered fewer losses before reaching the consumer.
It was, by almost every measure, a tremendous success.
Over the following decades, the mutation found its way into most modern tomato varieties and remains extraordinarily common today. Few people questioned the tradeoff because, frankly, there didn't appear to be one. The tomatoes ripened more evenly, packed more efficiently, and looked more appealing on grocery store shelves.
Then, in 2012, researchers uncovered the mechanism behind the trait.
The uniform ripening (u) mutation affects a gene known as GLK2, which regulates chloroplast development within the fruit. In tomatoes lacking the mutation, the shoulders of immature fruits develop large numbers of chloroplasts, giving them their characteristic dark green appearance. Tomatoes carrying the u mutation, by contrast, produce fewer chloroplasts and therefore ripen to a more uniform color.
At first glance, this might seem like little more than a cosmetic difference.
But chloroplasts aren't simply decorative. They're tiny sugar factories, capturing sunlight and storing energy that ultimately contributes to the flavor of the ripe tomato. And as researchers would soon discover, reducing the number of these chloroplasts came with an unexpected consequence.
Some newer "heirloom" types have been bred to improve overall ripening. We've noticed that Carbon tomatoes are faster to ripen their shoulders.
The Sweet Truth About Green Shoulders
The connection between green shoulders and flavor comes down to a surprisingly simple idea: tomatoes can photosynthesize.
Of course, nobody is going to mistake a tomato fruit for a leaf. Leaves do the overwhelming majority of the plant's heavy lifting. But immature tomato fruits contain chloroplasts of their own, allowing them to capture sunlight and manufacture sugars and starches during development.
And the darker the fruit, the more chloroplasts it tends to possess.
In 2012, researchers discovered that tomatoes carrying the ancestral version of the GLK2 gene developed substantially more chloroplasts in their shoulders than tomatoes with the modern uniform ripening mutation. Those extra chloroplasts accumulated greater quantities of starch while the fruit was green. As the tomatoes ripened, that starch was converted into sugars, resulting in fruit that contained roughly 10 to 15 percent more sugar than their uniformly ripening counterparts.
That's a remarkable tradeoff.
The mutation that gave us perfectly uniform red tomatoes also reduced one of the very things gardeners care about most: flavor.
Of course, sugar isn't the whole story. A tomato's flavor also depends on hundreds of volatile compounds which, incidentally, are also reduced in modern lines. But sweetness provides the foundation upon which flavor is built. It's one reason that heirloom varieties like Cherokee Purple, Black Krim, and other dark-shouldered tomatoes have earned such devoted followings among gardeners.
Though it seems counterintuitive, vibrant colors begin with green. The chloroplasts that give immature tomatoes their color eventually transform into chromoplasts—the tiny factories responsible for producing the brilliant reds, oranges, and yellows of ripe fruit. Pictured: Big Elmer
More Than Sugar: Green Shoulders, Lycopene, and Flavor
But the story doesn't end with sugar.
The chloroplasts that give heirloom tomatoes their dark green shoulders don't simply disappear as the fruit ripens. Instead, they transform into chromoplasts—the specialized structures responsible for producing carotenoid pigments like lycopene and beta-carotene.
In other words, the tiny solar panels that helped power the fruit while it was green eventually become the pigment factories that give ripe tomatoes their rich colors.
Researchers demonstrated this connection by overexpressing the ancestral GLK2 allele (the one that causes green shoulders.) The resulting fruits contained 17 percent more lycopene and 38 percent more total carotenoids than their conventional counterparts. The effect wasn't merely cosmetic; it confirmed that the same genetic machinery responsible for dark green shoulders also helps determine how much pigment ultimately accumulates in the ripe fruit.
This may help explain why many grocery store tomatoes possess that familiar pale, orange-red appearance. Harvesting fruit before they fully ripen certainly contributes to the problem, but modern commercial varieties also carry the uniform ripening mutation, which reduces chloroplast development and ultimately limits the number of chromoplasts available to produce carotenoid pigments.
And those implications extend far beyond color.
Carotenoids serve as the raw materials for an important class of flavor compounds known as apocarotenoids. These molecules contribute many of the fruity, floral, and complex aromas that distinguish a truly exceptional tomato from an ordinary one. Fewer carotenoids mean fewer opportunities to produce these flavor compounds, creating a cascade of effects that extends far beyond simple sweetness.
Tomato Colors Explained: How Tomatoes Get Their Unique Colors
As it turns out, the genetic change that solved the problem of green shoulders also diminished many of the traits that gardeners prize most. By almost every measure of eating quality, these uniformly ripening tomatoes contained less sugar, fewer carotenoids, and fewer of the flavor compounds that make a truly exceptional tomato memorable.
Green shoulders aren't a sign of things gone wrong—they're a sign of good things to come. With time, they'll disappear and your patience will be rewarded with a surplus of flavor. Pictured: Hensley's Settlement Tomato
Conclusion
The story of green shoulders is, in many ways, a cautionary tale.
The uniform ripening mutation was a remarkable achievement. It made tomatoes easier to harvest, reduced losses from cracking and sunscald, and produced the beautifully uniform red tomatoes that consumers came to expect. By nearly every agricultural and commercial measure, it was a tremendous success.
And yet, as we now know, that same mutation also produced tomatoes with less sugar, fewer carotenoids, and a diminished capacity for generating many of the flavor compounds that make a truly great tomato memorable.
It's a useful reminder that appearance and quality are not always the same thing.
Modern gardeners have already begun to rediscover this lesson through the popularity of so-called "ugly tomatoes." They may be lumpy, cracked, cat-faced, or unevenly colored, but time and again people are willing to overlook those imperfections because the flavor is simply superior. The same principle applies to green shoulders. What looks like a cosmetic flaw may actually be evidence that the fruit developed exactly as nature—and generations of gardeners—intended.
So the next time you harvest a tomato with dark green shoulders, don't be too quick to judge it. Those extra pigments may represent more than an oddity of appearance. They are a reminder that, sometimes, the sweetest, healthiest, and best-tasting tomatoes are not the prettiest ones in the basket.
Want to dive deeper on tomato flavor? Learn why heirlooms often taste better, how a tomato's color influences its flavor, why green tomatoes—shockingly—are so good, and so much more on our blog.
