This post is my best explanation of “tomatoes don’t ripen if it’s too hot out”.
Why does that need an explanation? Because I’m still getting a few ripening tomatoes, despite the heat.
After some close observation, I think the correct statement is “tomatoes won’t start the ripening process if it’s too hot out”. Once they’ve started the ripening process — passed beyond the “mature green” stage to “breakers” (see chart below) –I think they continue to ripen, heat or no heat.
The upshot is that you don’t get a nice, sharp cutoff of tomato production, contemporaneous with a string of hot days and nights. You get a dip in tomato production some days later, as the ones that have already started ripening continue. But the ones that haven’t made it to the “breakers” stage get stuck at “mature green”.
At least, that’s my best guess for what I’ve been observing in my garden this year.
(Completely separately, tomatoes won’t set fruit under excessive daytime heat. See Post G24-021, on tomato blossom drop and heat. Both of these effects — blossom drop and non-ripening — depend somewhat on the variety of tomato being grown, with some tomatoes (e.g., Floradade) having been bred to perform somewhat better in high heat.)
The issue
If it gets too hot, tomatoes won’t ripen. That’s a well-established fact. But that seems to depend critically on the minimum 24 hour temperature, that is, on nighttime lows. By report and by my analysis, lack of tomato ripening is associated with nighttime temperatures consistently in excess of 70F. Here’s the contrast of July 2020 (no ripening) and July 2021 (ripening), in my area:
Source: Calculated from historical weather data from NOAA, for Dulles International Airport.
We’re now entering another late July with nighttime lows above 70F.
I took that opportunity to document what’s going on with my early-season tomatoes. I took a snapshot of one of my tomato beds day-before-yesterday, looking to track prominent individual tomatoes over time. A few had some tinge of red on them, but most appeared green.
Today I took a second snapshot. Before you see it, choose the most likely narrative:
- The spiteful leprechaun in my garden switched my unripe tomatoes for ripe ones.
- I finally turned off the no-red filter on my camera.
- A whole bunch of tomatoes that (probably) had already started to ripen continued to ripen.
- This whole “tomatoes won’t ripen” thing is just untrue folklore.
I believe the answer is c. So far. With a bit of explanation below.
Here are the two photos, one taken 7/19/2022, one take 7/21/2022:
If you look at tomatoes 2, 5, 7, and 8 (numbered in bottom photo), you can see that the ones that were already visibly ripening continued to ripen. (For the record, see below, I’ll note that there’s some color on the cluster of tomatoes labeled 6 above.)
Vine-ripened tomatoes?
That continued ripening, of tomatoes that had already started to ripen, despite the heat, was to be expected, I think. That’s why I noted some color on my tomatoes when I took the original photograph.
Tomatoes are climacteric, that is, once they begin ripening, they will continue to ripen, even after you pick them. That certainly happens in my kitchen, which is warmer than 72F right now. It’s therefore no surprise that tomatoes that have already started to ripen will continue to ripen, even if temperatures remain above 72F.
This harks back to an early post on “vine-ripened” tomatoes (Post G21-038). Plainly put, “vine-ripened tomatoes”, aren’t. Tomatoes legally sold in the grocery stores as “vine-ripened” were probably picked just past the “mature green” phase, labeled “breakers” below. Probably, they were only mostly green, I guess you’d have to say.
Source: Ripening Tomatoes, Marita Cantwell, Dept. Plant Sciences, UC Davis,
Fruit Ripening and Retail Handling Workshop, Postharvest Technology Center, UC Davis, March 18-19, 2013
Without getting into the issue of whether or not tomatoes ripened off the vine can taste as good as those ripened on the vine, the point is, once they hit “breakers”, they will eventually ripen to full red. That will occur whether they are on or off the plant.
The reason that some think that you get the same flavor either way is that the tomato plant mostly seals off the link (the abscission zone) between plant and tomato at that point. The debate over flavor centers around that weasel-word “mostly”. But, for sure, you can get an acceptable-tasting tomato if you pick it at the breakers stage and let it ripen.
The rock-hard pale-pink not-entirely-unlike-tomato things that grocery stores sold, in my youth? Those were picked at mature green, and ripened artificially with ethylene gas. That gas may or may not have been used to speed ripening of “vine ripened” tomatoes.
The hypothesis restated, or tomato lags.
The upshot is that the statement linking high temperatures and tomato ripening needs clarification.
In you look into this issue, you will see it stated that consistently warm temperatures will prevent tomatoes from ripening.
But what I think this actually means is that consistently warm temperatures will prevent tomatoes from entering the “breakers” stage. Anything with a tinge of color on it — at the “breakers” stage above — should continue to ripen. Just as it would if you pulled it off the vine and took it inside.
Restated:
Consistently warm temperatures will prevent mature green tomatoes from ripening.
The upshot is that you never see this as a clear-cut issue. You, the home gardener, will see the impact of warm nights on ripening only with some time lag. And then without any sharp cutoff in tomato production.
Once nighttime temperatures are consistently above 70F (maybe 72F), you will continue to pull ripe tomatoes out of your garden. But what you don’t realize is that these are all tomatoes that were already at the “breakers” stage or later. Those will eventually taper off. But they won’t be replaced, because anything that was still at “mature green” or younger will remain stuck there.
And then, on the other side of that warm spell, your tomato production will pick up again, with a lag. Once nights cool off enough, tomatoes will again begin to transition from mature green to “breakers”. And then will ripen from there.
So it’s really no wonder that this link between high temperatures and non-ripening tomatoes is not universally noticed. First, we don’t get a spell of hot nights every year. But mostly, the gap in the tomato harvest doesn’t coincide with the string of hot nights. It lags it by whatever time it takes for tomatoes to go from “breakers” (first hint of color) to the point at which you normally harvest them.
Harvesting and looking forward
After taking today’s picture, I pulled all the ones that I thought would ripen up well enough, off the vine. (There’s still one cluster with just a hint of color.) Most of what you now seen hanging should be at the “green mature” phase or earlier.
Here they are today, pre-picking and post-picking. One cluster (6) has some color on it, I’m pretty sure all the rest of what remains is fully green. We’ll see how long those remain in that state, as temperatures should remain above 70F in this area through more-or-less the end of July.
Stay tuned for more thrilling updates.
