Not a metaphor.
The purpose of this post.
This post is a brief summary of “blossom drop”, an important way in which hot summer temperatures can interrupt the flow of ripe tomatoes from the garden.
Briefly, high temperatures can prevent pollination — the topic of this post — and, completely separately, delay ripening.
That’s often hard to perceive, because it takes around two months for the typical tomato to go from blossom to ripe fruit. Today’s dearth of ripe tomatoes could be due to a spell of hot weather 60 days ago.
For delayed ripening — where you can see plenty of green tomatoes on the vine — there’s not much to do but wait. If it’s hot enough, tomatoes will not initiate ripening. They get stuck at the “mature green” phase. (But once ripening starts, a tomato will continue to ripen regardless of subsequent high temperatures. I think.) Standard advice for protecting plants from the heat — provide mid-day partial shade and plenty of water — appears to increase yields modestly. Presumably, that’s because the tomato plants spend less time stressed by the highest temperatures.
For failed pollination due to heat — where baby tomatoes are failing to form due to blossom drop or poor fruit set — I read the literature as suggesting that shaking your tomato blossoms, briefly, every couple of days, may improve fruit set and yield. The reason being that tomato pollen gets sticky in hot, humid weather, and so takes more effort to dislodge so that it can go on to fulfill its fertilization duties.
Or you can just let your tomatoes grow, and take what you get. It’s all good.
The consensus seems to be that delayed ripening is an annoyance, but does not reduce overall yield. By contrast, failed pollination appears to generate an outright loss of fruit for the growing season. It’s a case of tomatoes delayed versus tomatoes denied.
So, how hot is hot, exactly?
Simple enough question, right? Read on.
First, I know the answer to the question — or, at least, my answer to the question above, for failure of green tomatoes to ripen (Post G22-043). As I understand it, tomatoes need nighttime temperatures of 70F or lower in order to initiate ripening. A few years back, a string of nights in excess of 70F led to a gap in ripe tomato production, as a bunch of tomatoes got stuck at the “mature green” phase. And that temperature (70F, maybe 72F) seems to be the most commonly-cited one for “failure to ripen”.
Source: See (Post G22-043).
Second, for “blossom drop” or reduced fruit set, or poor pollination, my reading of the table below is that, for the typical tomato, pollination success tails off if you have successive days where daytime highs exceed 85F and nighttime lows exceed 70F. (Each flower has a roughly two-day window to be fertilized, so presumably, it takes a couple of days in a row to prevent pollination entirely.)
Based on what I read, you get your best tomato fruit set when temperatures stay warm, but daytime highs are below 80F. By the time you get to 93F daytime highs (95F in other sources, not shown), it’s hot enough to sterilize the pollen that the tomato produces.
All of the above could be modified by planting heat tolerant tomatoes. That said, one seemingly credible reference says that planting heat-tolerant tomatoes will only get you a few degrees of temperature leeway, compared to more standard tomato varieties. Heat-tolerant, not heat-proof.
At least two other weather-related factors matter, for blossom drop / poor fruit set. The first is humidity. Tomato pollination drops in high temperatures because the pollen becomes sticky, and won’t release from the flower’s anther. High relative humidity just makes it all that much stickier. And, at least as I read it, still air/lack of breezes. The idea being that a breezy day provides more force for releasing pollen.
The upshot is that successive hot, dry, breezy summer days should result in less tomato blossom drop than the same number of hot, sultry, still days.
For summer in Virginia, that’s more of a theoretical oddity than a practical consideration.
(There are further and more esoteric heat-related issues. As noted above, a high enough temperature (typically cited as ~95F) is said literally to sterilize tomato pollen, so presumably no amount of shaking will fix the problem once it gets that hot. Separately, photosynthesis for most food plants stops somewhere in the mid-90s F. So overall plant productivity falls off on very hot days, not just for tomatoes, but for more-or-less all food crops. On top of that, high nighttime temperatures raise the plant’s respiration rate — and so, increase the fraction of photosynthetic activity that merely goes toward keeping the plant alive, as opposed to producing fruit. And above 95F or so, tomatoes can no longer produce lycopene – the red in red tomatoes — and so red tomatoes are reported to ripen to an orange color if they ripen during a prolonged stretch of very hot days).
Background on tomato blossoms
Tomatoes don’t need insects for pollination. Around 96% (U MD) to 99% (Journal of the American Horticultural Society) of field-grown (outdoor, non-hothouse) tomatoes are self-pollinated.
That’s by design. The tomato’s design, I mean. Each tomato blossom contains both male and female parts, and under most conditions, the action of wind shaking the flowers dislodges enough pollen within the blossom to result in pollination (fertilization), and the resulting fruit set.
If grown in a windless environment (e.g., a greenhouse), tomatoes need something to shake their pollen loose. Reportedly, commercial growers frequently use bumble bees for that. (Those are social (not solitary) bees, so you can maintain nests of bumble bees as you would honeybees.) Those insects “buzz pollinate” tomatoes by biting the flower and shaking the pollen out of it, and apparently, bumble bees are much better at pollinating tomatoes than any other insect, or any mechanical means (reference), such as using a mechanical device to buzz the flowers by hand (e.g., an electric toothbrush or equivalent), or a wind machine of some sort. Or just plain grabbing the plant and shaking it.
Interestingly, there are hints in the on-line discussion and in the scholarly literature that staking your tomatoes reduces wind-driven pollination by reducing the movement of the flowers. At least one scholarly source pointed to what we now know as the Florida weave (a.k.a., stake-and-twine trellising) as particularly likely to reduce successful tomato seed pollination. Less formally, I’ve seen the same thing said about growing them next to a solid, wind-stopping object such as a house or wall. No idea if any of that is true, or if true, to what extent it reduces yield.
Here’s an interesting thing. For field-grown (outdoor) tomatoes, research has long shown that an extra bit of pollen-shaking (beyond that from the wind) modestly improves both fruit size and overall fruit yield. Buzz-pollinating bees like bumble bees are best, but mechanical vibrators and wind machines are known to work. If I had to pick a number out of thin air, for the effect of using a vibrator or blower to increase field-grown tomato yield, I’d go with 20% to 30% as a typical reported increase in yield, with considerable variation around that figure. The mechanism seems to be that, on average, the more seeds successfully fertilized, the bigger the resulting tomato. (Although I note that does not explain why the overall yield would go up, instead of just giving you fewer, bigger tomatoes with the same solar energy input. But that’s what the numbers say — both bigger tomatoes and higher total yield). Below is a table showing impact on the weight of individual tomatoes, from field-grown (wind-pollinated) tomatoes subject to some form of supplemental pollination treatment. (Note that this is the weight of individual fruits, not overall yield.)
Source: Hazel Cooley, Mario Vallejo-Marín, Buzz-Pollinated Crops: A Global Review and Meta-analysis of the Effects of Supplemental Bee Pollination in Tomato, Journal of Economic Entomology, Volume 114, Issue 2, April 2021, Pages 505–519, https://doi.org/10.1093/jee/toab009 You might also be tempted to ask, how in the hell did they determine which flowers the bees had buzzed? Did they produce and then analyze the world’s most boring time-lapse video? The short answer is that when a bumble bee buzz-pollinates a tomato flower, it leaves a visible mark (scar) where it grabbed the lady-parts of the flower in its mouth. Experts can then determine which flowers were bee-bit. Or so I am led to believe, though I have not observed this first-hand.
In a nutshell, as a way to pollinate tomato flowers, buzz-pollinating bumble bees are best, but mechanical vibrators (e.g., electric toothbrush) or blowers also increase yields, relative to wind pollination alone.
The final fun fact is that tomato flowers only live a couple of days after they open. Within that time span, they either get pollinated, or die trying. (I was young once. I can relate.) If still not pollinated within that time span, they wither. Hence the term “blossom drop”.
Conclusion: A treatment for heat-related blossom drop?
As I read the literature, during hot spells, you can try giving your tomatoes some obvious types of heat-related aid, such as partial shade and lots of water. In addition, if you live in an area with habitually hot and humid summers, or otherwise expect a lot of excess summer heat, you can try growing more heat-tolerant varieties. These typically are given as cherry tomatoes (generically), as well as varieties of full-sized tomatoes specifically bred to be heat tolerant.
But you can also try something non-obvious, to reduce blossom drop or lack of fruit set during hot spells: Shake your tomatoes.
Shelby et al. (1978) reported that yield of tomatoes grown under unfavorable conditions of high temperature was markedly improved by supplemental hand pollination. He concluded that insufficient pollination probably was the major cause of yield reduction.
Source: Cited in https://journals.ashs.org/hortsci/view/journals/hortsci/34/5/article-p846.xml
Aside from the time involved, the only real downside here is that if you don’t routinely shake your tomatoes anyway (and some people do), you have to anticipate (or at least notice) your periods of heat-related blossom drop. If you only notice something is amiss once your ripe tomato production tails off, well, you’re about two months too late to do anything about it.
In any event, this is all apropos of our latest hot spell here in Northern Virginia. I’ve now gone a couple of days without picking ripe tomatoes, and I’m pretty sure the heat is to blame. My current lack of eating tomatoes is due, I think, merely to some delayed ripening. We recently went about a week with no nights below 70F. But I think I’ll be giving my plants a daily shake for a while, just for good luck pollinating in this continued hot weather.
Source: Weather underground.
