A vine with plump, glossy-looking green tomatoes that are inexplicably refusing to turn a shade of red is very frustrating in the Summer months and yet a common perplexing problem!
You have watered them diligently and probably applied fertiliser, and yet the green tomatoes remain green despite it being the height of, or perhaps late summer.
Here I have done the research and had many summers of practical hands-on expertise growing tomatoes myself, and I can tell you sometimes the reason is counterintuitive. I’ll tell you what’s actually happening and what you can do about it…
The Heatwave Paradox: When More Sun Is the Enemy
Several summers ago, there was a punishing heatwave in July, and my tomatoes’ vines were weighed down with handsome beefsteak tomatoes, and I thought the incredible sunshine would turn them a lovely shade of red within a few days.
However, they remained green and underdeveloped.
To understand why this was, I decided to do a little experiment on two identical clusters of monkeymaker tomatoes that were on the same vine…
I left one of the clusters of tomatoes in full sun, even during the scorching afternoon, whereas for the second, I made a small improvised shade using cardboard, which I propped up in a way that shaded the developing fruit during the hot afternoon hours.
The shaded tomatoes started to turn red within four days, to my excitement! The sun-soaked ones, in contrast, stayed green! I was very surprised because I associate tomato growing with the hot Mediterranean region!
I did the research and spoke to some commercial growers…Here is why this happens…
Tomatoes don’t need lots of extra sunlight to turn red, but crucially, they need an optimum temperature. When the surface of the fruit is hotter than 85°F (29°C), the tomato plants shut down the production of the two vital pigments responsible for the red colour…lycopene and carotene. It doesn’t just slow down there prodcution but grinds to a definitive halt. The repining just switches off at that temperature.
So my makeshift little cardboard shade blocked the intense sunlight during the afternoon, which was enough to lower the temperature to a level where the tomatoes could turn red.
What to take from this… During the heatwave, more sun exposure isn’t a good thing for the fruit. What you can do is use cloth or cardboard to shade the developing green tomatoes from the sun, but not the leaves, as they need sunlight for photosynthesis. You don’t necessarily need to shade them all day, but just during the baking hot afternoon. This can get your green tomatoes to turn red.
The Mature Green Test: Can Your Tomato Actually Ripen Yet?
At the start of my gardening journey, I used to get panicky when Fall (autumn) approached, as there were lots of green tomatoes still left, and I’d urge them to turn red.
From my research, what I discovered is that you cannot force a tomato to turn red if it hasn’t reached what was described to me as the “mature green stage“. The difference between a mature green tomato and an immature one is not immediately obvious.
To test this, my friend ran a simple kitchen counter experiment, where we picked two green tomatoes that looked similar in size and colour but felt different to the touch. The first tomato was rock hard with a dull, matte green finish on the skin. The second tomato, however, had a slightly yellow, green sheen and felt more malleable to gentle pressure when you squeezed it.
They then put both tomatoes into paper bags with a brown, ripe banana, which gives off ethylene gas, which is a catalyst for ripening, for 5 days.
Tomato number 2 turned a gorgeous pink, whereas tomato number one shrank and stayed dull green and didn’t ripen at all.
The botanical explanation is as follows…As the tomato reaches its maturity, a gel forms inside the seed cavities, which you’ll see when you slice a ripe tomato. The gel formation is the stimulant that allows the fruit to respond to the ethylene to initiate the ripening process.
The tomato cannot ripen red regardless of temperature, ethylene or patience, before the gel forms.
The Key to Telling the Difference…A mature green tomato has a bit of give when squeezed, and can have a faint yellow or even white blush at the beginning of the blossom end, and the skin is glossy, not matte.
This is opposed to a green tomato that feels like a firm ball and is a dull green. If your tomatoes are still hard, then the gel hasn’t developed yet, and the tomato is still maturing, so it can’t turn red. The only thing you can do is wait!
The Late-Season Triage: Stop the Plant Trying to Do Everything at Once
During the latter part of August, I had some tomatoes that were sprawling jungles of plants still developing new flowers and tiny new fruit, which unfortunately weren’t going to ripen in time before a frost came.
The full-sized, green tomatoes that were still on the vine can stall its reipening processbecause the tomato plant is dividing its resources and energy between ripening the existing fruit and growing new flowers and tiny fruits that are unlikely to come to fruition.
In the last year, I purposely tested an intervention on two equally overloaded tomato plants growing side by side.
Plant A was left on its own it its chaos of flowering and jungle like vines.
Plant B I pruned off every main growing tip and pinched off any new flowers, and even pruned the small tomatoes that weren’t going to develop before winter.
It was about 10 days, and the mature green tomatoes on plant B had started to turn red whilst plant A was exhibiting the same chaotic state.
The pruning stopped the vegetative growing, and by removing the small tomatoes, the plant could direct its resources to ripening the larger, more mature fruit.
The time to do this…
Do this in late August or early September, and be ruthless, as any smaller tomatoes are not going to grow at this point, and they are just going to take resources away from ones that have a realistic shot at ripening.
Bringing Green Tomatoes In: The Paper Bag Method
If frost is incoming and you have a vine with mature green tomatoes, what I do is bring them indoors rather than leave them to the frost.
Mature green tomatoes ripen very well indoors at room temperature in around 1 to 3 weeks, as long as….
You do not put them in the fridge: Cold temperatures of the fridge stop ripening just as heatwaves do, so below 55°F (10°C) the ripening stops.
Do not put them on a sunny windowsill, as this can stall ripening. It’s the surface temperature of the skin and not light that causes ripening, so keep them at an even temperature that is neither too hot nor too cold.
A paper bag with a ripe banana or apple can create a nicely concentrated ethylene environment for the tomato to ripen.
The Quick Diagnostic
| Situation | Why It’s Happening | What To Do |
|---|---|---|
| Late season, the vine is still flowering and setting | Lycopene production halted above 85°F | Shade the fruit during afternoon heat |
| Tomatoes feel rock hard with no give | Immature green stage — gel not yet formed | Wait — picking now achieves nothing |
| Late season, vine still flowering and setting | Plant splitting resources between new and existing fruit | Triage: top the plant, remove flowers and tiny fruit |
| Frost coming, vine full of green fruit | Season ending | Bring mature green tomatoes inside at room temperature |
Staring at a vine of green tomatoes and wondering if they’ll ever turn? Tell me the time of year, current temperatures, and how firm the fruit feels, I’ll help you work out whether to wait, shade, triage, or bring them in.