The Burning Question Baffling Chili Growers

This is the strangest phenomenon to have happened in gardening for years – super-hot chili peppers coming off what should be mild varieties, and even hot and mild chilies picked from the same plant. It’s been happening across the globe this summer, and it has gardeners baffled. So what’s going on here?

The obvious answer is it’s been a scorching summer, and heat makes chili peppers hotter, right? Except that’s not the full picture...

What is ‘Hot’ – and Why?

To understand what’s going on, we need to understand what makes a chili hot – or not. Heat isn’t stored evenly through the flesh. Instead, almost all of it is concentrated in the pale, spongy tissue the seeds are attached to, called the placenta.

This tissue produces compounds called capsaicinoids (primarily capsaicin and to a lesser extent dihydrocapsaicin) which are secreted in tiny cavities just under the surface. When you eat one, those molecules lock onto receptors on your mouth – the same sensors that fire when you touch something genuinely hot. It’s why chili ‘burns’; as far as your nervous system is concerned, it isn’t lying to you!

And here’s the first surprise: a chili pepper plant doesn’t just decide on a level of heat and stick to it. Its genetics set a ceiling – a maximum heat potential if you like. How close any individual fruit gets to that ceiling depends on what happens to it while it’s growing. Which is where the plot thickens!

You see, capsaicin evolved as a defense. Mammals have receptors that feel the burn of these fruits, so they tend to avoid eating them. Birds, on the other hand, barely register capsaicin at all, so they can eat the fruit, and disperse the seeds intact. Birds are clearly tougher than they look!

So capsaicin is a chemical defense system, and a plant under threat can turn it up. The question is: what counts as a ‘threat’ to a chili pepper?

Turning up the Heat

Let’s test the obvious hypothesis first: does heat alone turn up the dial?

Researchers have found that fruit grown at around 86°F (30°C) can contain noticeably more capsaicin than fruit grown at a cooler 75°F (24°C), and there’s a real biochemical reason behind this. The enzymes that build capsaicin, and the genes that switch them on, are themselves sensitive to temperature. One gene in particular, called Pun1, which sits right at start of the capsaicin-making process, acts like an ‘on switch’, becoming more active when heated.

So far, our simple assumption holds up: warmer equals hotter.

But then researchers pushed the temperature further, up to around 97°F (36°C), and something unexpected happened: capsaicin production fell. The heat was disrupting photosynthesis and scrambling the very genes that switch on at lower temperatures.

This is our first big myth-buster: we’re not dealing with a thermostat where more heat means…more heat. It’s more like a bell curve. There’s a sweet spot, somewhere in the high seventies to mid-eighties Fahrenheit (mid-twenties to low thirties Celsius) where the pathway is firing on all cylinders. Push past it, into genuine heat stress, and the whole system starts to shut down.

And then there’s drought…

The Real Culprit - Drought

A heatwave is never just high temperature. It’s also intense sun, plus rapidly evaporating soil moisture, which all add up to one thing plants really care about: water stress. And water stress turns out to matter enormously for capsaicin production.

Multiple studies restricting irrigation on chili peppers have found measurable – sometimes substantial – increases in capsaicinoid concentration along with real rises in the enzymes that build the compound. A thirsty plant, it seems, treats ‘I might not survive this drought’ as a cue to invest more heavily in chemical defense.

This is almost certainly the missing piece of the puzzle here. It’s that classic combination of warmth, light, and dry soil that pushes a plant into exactly the kind of stress that seems to turn capsaicin production up a gear.

But – and this is important – more stress doesn’t automatically mean hotter fruits. Researchers testing several chili varieties under four different watering schedules found that while some varieties got noticeably hotter under drought conditions, others simply produced fewer, smaller and sadder fruits, and with less capsaicin overall because, after all, a struggling plant has less energy to spend on the chemistry that produces it.

So the old gardening tip to ‘stop watering your chilies to make them hotter’ isn’t wrong, exactly, it’s just missing the bigger picture. We’re not turning a dial here. Restricting water is gambling with your plant’s stress tolerance, and every variety of chili plays its chips differently.

Hot and Mild Chilies on the Same Plant

Here’s where we can finally solve our opening mystery: the same plant producing a mild chili, and then a hot one. The temptation is to think of ‘the plant’ as having one mood at a time: hot day – hot plant – hot chilies.

But that’s not how it works. Capsaicin isn’t produced evenly across the plant’s whole lifecycle. It’s mainly formed during a specific developmental window, while a particular fruit is expanding, and that window can last for a few weeks.

Indeed, a plant fruiting steadily over two or three months isn’t running in one strict, open-and-closed fruiting window; it’s running successive fruiting windows simultaneously – one for each fruit – because each fruit is living through a different slice of the summer.

Picture it like this:

• Fruit A starts expanding during a mild, damp week, giving it a mild to moderate level of heat.
• Fruit B begins swelling right in the middle of a heatwave, with the soil drying fast. The stress turns it ferociously hot!
• Fruit C on the other hand expands two weeks later during a cool, cloudy spell. It winds up being a mild one.
• Then fruit D starts filling out during a severe drought, in extreme heat, giving us a rather small and sad-looking fruit.

In theory these fruits can all ripen and get picked in the same week due to the many variables at play, even on the same plant. Genetically they’re completely identical, but taste them and it’s as if they’ve come from different plants because, in a biochemical sense, they sort of have.

And that’s the answer to this whole mystery. How hot a chili ends up depends on the weather diary written into its capsaicin production.

So just to be clear, heatwaves are not turning mild pepper varieties into hot ones. Genetics still set that upper ceiling. A pepper that genuinely lacks the biochemical machinery for capsaicin, like most sweet peppers or mild varieties of chili peppers, isn’t going to become dangerously hot, no matter how brutal the summer is. Genetics sets the ceiling. Environment decides how close you get.

But a lot of ‘mild’ varieties aren’t incapable of heat – they simply don’t usually produce that much of it. Under the right stress, though, they can move a long way up their own personal scale, from ‘playfully tingly’ to ‘surprisingly fiery’, reaching much closer to the ceiling it’s always had.

Chilies Under Too Much Stress

Where I am, like so many places in the world, this past summer gave us a near-perfect natural experiment. Here we recorded our hottest summer on record, with repeated heatwaves and severe drought – just the sort of ‘moderate stress’ conditions to nudge chili plants into ramped up capsaicin production.

But it’s been a double-edged sword. Where temperatures pushed past that sweet spot into prolonged, severe heat, some plants would have shown the opposite effect: dropped flowers, poor fruit development, and less capsaicin, not more. Which means this summer likely produced both some of the hottest home-grown chillies gardeners have ever tasted, and some of the most disappointing…and sometimes from the very same plant!

So here’s a little experiment to try for next summer: each time a fruit starts to form, make a note about it and the weather conditions. Then when picking time comes, taste the fruits side by side. You could even record everything in our free-to-use Garden Journal, which automatically maintains a record of the weather on any given day, including that all-important temperature. Just tag your notes for each variety so you can keep track of how they perform. And honestly, I’d love to hear what you find in the comments.

If you want help planning what to grow next, peppers sweet and chili included, our Garden Planner can help map out your whole season for you. Try it out for free – no credit card details required!

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