Heat Stress vs Drought Stress in the Vegetable Garden - Ep. 308

[00:00:00] Alright my gardening friends, I want you to picture this with me. It is 2 o'clock on a July afternoon, you walk out to check the garden and your squash leaves are drooping so low they're nearly touching the mulch. Or your tomato plants look like they have just given up on the whole season. If your first thought every single time this happens is, I need to water it, like right now, hold on a second before you grab that hose.

[00:00:25] Because that dramatic afternoon collapse that you're looking at likely has absolutely nothing to do with how much water is in your soil. Welcome back to Just Grow Something, I'm Karin Velez, market farmer, horticulturist, and homesteader here in West Central Missouri. And every single summer, I get some version of the same message from listeners and from followers on social.

[00:00:48] My plants are wilting every afternoon, I'm watering every day, and they still look terrible by dinnertime, what's wrong with them, am I killing them? And most of the time the answer is, no, thankfully. Most of the time, what you're watching is a plant doing exactly what it's built to do under heat stress. And it has almost nothing to do with drought. But sometimes, it is a water problem.

[00:01:15] And telling the two apart is the difference between a garden that bounces back every afternoon, and one that slowly declines over a season because you either overwatered it into root rot, or underwatered it into permanent damage. So today we're covering three things. What's actually happening inside a plant's cells when the temperature spikes. The specific ways to tell heat stress wilting apart from drought stress wilting.

[00:01:44] And what heat stress does beyond the leaves. Because your tomatoes and your peppers and your squash can look perfectly fine and still be failing to set fruit. And that is a heat problem too. Let's dig in. Alright, before we get into it, quick shout out to everyone who's been emailing or DMing me photos of their gardens this week. Asking, is this normal? Basically, I love that you're sending pictures instead of just panicking.

[00:02:14] Keep doing that. Half the time I can tell you in one sentence whether you need to worry. And the other half, well, that's what today's episode is for. Also, shout out to those of you who have signed up for the Fall Garden Challenge. I hope that you are getting the gist of what should happen and when for planning and executing the Fall Garden. If you have not signed up, it is five days of emails and it is free.

[00:02:38] You can go to justgrowsomethingpodcast.com slash fall to sign up and you will start getting those emails right away. So let's start with the plumbing of our plants, so to speak. Because wilting only makes sense once you understand what a plant is doing with water all day long.

[00:03:04] Every leaf on your tomato plant, for example, has tiny pores called stomata. And through those pores, water constantly evaporates out into the air. This is called transpiration. It's how the plant pulls water and nutrients up from the roots in the first place. And it's how the plant cools itself, the exact same way sweat cools us. On a mild day, the amount of water leaving through those pores roughly matches what the roots can pull in from the soil.

[00:03:34] And the plant stays firm and upright. Heat changes that math. As air temperature climbs, the rate of transpiration climbs with it, sometimes dramatically. The roots are still delivering water at roughly the same pace, but the leaves are losing it much faster. For a few hours in the heat of the afternoon, demand simply outpaces supply. And the water pressure inside the plant's cells, what's called turgor pressure, drops.

[00:04:03] Turgor pressure is what keeps cell walls firm and a stem standing upright. When that turgor pressure drops, the plant droops. And that is wilting. So plants aren't just passively losing this battle either. They have a defense mechanism, and it's the stomata themselves. So under serious heat and water stress, there are guard cells around each one of those stomatal pores.

[00:04:32] And they actually close that stomata down, cutting off the water loss. You'll also see plants respond to heat by rolling or cupping their leaves. Tomatoes and corn are especially both known for this because it shrinks the leaf surface area exposed to the sun and to the wind. So if you combine that with closing the stomata, then this is the plant minimizing water loss on purpose.

[00:05:00] It's a very clever survival strategy. But there's a trade-off. Those same stomata are also how the plant takes in carbon dioxide for photosynthesis. So when they close to conserve water, photosynthesis drops too, sometimes substantially. So there was a recent study on tomatoes, and it was published in the journal Discover Plants. And it measured this directly.

[00:05:26] So under heat stress conditions, the tomato plants that they studied showed roughly an 11% drop in carbon dioxide assimilation, a 17% drop in the transpiration rate, and a 20% drop in the relative water content in the leaves. All of this alongside a 25% increase in electrolyte leakage.

[00:05:54] And this is a marker of stress damage at the cellular level, basically. So a heat-stressed plant is quite literally running a reduced version of itself and its operations until conditions can improve. But not every plant handles this the same way. How badly a given plant wilts also comes down to its root system.

[00:06:18] The severity of heat stress depends on duration, intensity, the time of year, the condition of the plant, and its immediate environment. So shallow-rooted crops, so lettuce, spinach, most of your leafy greens, only have access to moisture in the top few inches of the soil because that's where their root system is. And that top layer heats up and dries out the fastest, especially without mulch, all right?

[00:06:46] The deeper-rooted crops, like tomatoes and winter squashes, can tap into the cooler, moister soil that is several inches or more down. So that buys them a buffer, even on a really brutal afternoon. That's part of the reason why your squash and your cucumbers are usually the first thing that you see collapse in the heat. They have enormous leaves, and those leaves are losing water really quickly.

[00:07:12] And then you pair that with a root system that, especially early in the season, really hasn't fully established any kind of deep reach yet. So afternoon wilting during a hot stretch is, a lot of the time, just the plant protecting itself while it waits out the heat. That is a completely different situation from a plant that is wilting because the soil underneath it is genuinely dry.

[00:07:38] And figuring out which one you're looking at is exactly what we need to do. Okay, so wilting can mean two very different things. And watering is the right answer for one of them, and it's actively harmful for the other, okay? So let's make this simple. Do a test, right? I'm always talking about soil tests.

[00:08:08] This is a different type of a soil test. We're testing the soil, not the leaves. But we're testing the soil for moisture, okay? This is the fastest, most reliable test that you have, and it costs you nothing. Just stick your finger into the soil near the base of the plant, okay? Try to get it past your first knuckle, ideally all the way to your second knuckle, okay? If it's cool and it's damp down there, your plant has water available to it, and the wilting that you're seeing above ground is just a heat response, not a drought response.

[00:08:38] If the soil is dry more than an inch or two down, that is your sign that the plant genuinely needs some water. Your second test is just the clock, right? Timing is a really reliable indicator of this. Heat-stressed plants typically droop through the hottest part of the afternoon, but then they perk back up on their own once the temperatures start to drop in the evening because they're drawing on whatever moisture is available in the soil

[00:09:08] to restore that turgor pressure we talked about overnight. It's absorbing water in the evening. It's going to restore that firmness to the stems and the leaves so long as there is adequate soil moisture to draw from. So go back out there after dinner or go out first thing in the morning before the sun is back up. If your plant has recovered and it looks normal, then what you saw that afternoon was heat stress doing its job and you didn't need to give it any additional water.

[00:09:35] But if it's drooping already at 8 o'clock in the morning before the day has even had a chance to heat back up, that is a much stronger signal that you're probably dealing with an actual soil moisture deficit and it is time to water. Now if you want sort of an early warning system built right into your garden, pay attention to those squash and cucumber plants, okay? They have those really large, thin leaves and they lose water fast.

[00:10:03] And their root systems, especially on the younger plants, just aren't as deep or extensive as something like an established tomato plant. So that combination means that they're going to tend to show heat wilt earlier and much more dramatically than almost anything else that you're growing. So when you see your squash flagging really hard in the early afternoon, while everything else nearby, it still seems to look okay, that's often just squash being squash in the heat. It's not really a signal that your whole garden is in trouble.

[00:10:33] So why does this distinction between heat stress versus drought stress matter beyond just being technically correct? Because the consequences of guessing wrong runs in both directions. If you assume that every afternoon droop means it's a drought and you respond by watering daily, regardless of what's happening in the soil, you risk waterlogging that root zone.

[00:11:01] Okay, vegetable roots growing in poorly drained, saturated soil can very quickly become deprived of oxygen, which itself leads to wilting and yellowing and browning, which looks an awful lot like the very drought stress that you were trying to prevent. So you end up in this really weird loop. The overwatering causes symptoms that look like underwatering, which convinces you to water more, which makes the root problem just worse, right?

[00:11:29] Consistently soggy soil around the root zone also sets the stage for those root rot pathogens. And that is a completely separate and much harder problem to fix than just a few hours of afternoon droop. On the other side, if you assume that every bit of wilting is just heat and it's fine and you skip watering during a genuinely dry stretch, especially if you don't have a good layer of mulch on your soil,

[00:11:59] you can let a real moisture deficit run on for days without even noticing. And that's when you start seeing actual damage. So leaf scorch, blossom drop, fruit drop, stunted growth, okay? All things that don't reverse itself once that heat breaks. So the soil test and that evening check-in solves both problems at once. They take about 10 seconds and they tell you which situation you're actually in

[00:12:26] instead of leaving you to just guess based on how dramatic the plant looks at two o'clock in the afternoon. Now, one quick caveat here though, if you're growing in pots or grow bags or raised metal planters like the ones that I get from Planter Box Direct, you have to remember container soil heats up and dries out much faster than ground soil. And this is because there's less total soil volume. There is no thermal or moisture buffer there,

[00:12:56] or at least not as large of one as it would be if you're gardening in the ground. Plus those container walls themselves can absorb heat as well. So you really do want to check containers, especially the smaller containers, daily during a heat wave, sometimes twice a day for the really small pots because the finger test still applies. It just needs to happen more often.

[00:13:27] So, so far we've talked about leaves and stems, but heat stress doesn't stop there. And this can catch a lot of gardeners off guard because your plant can look completely fine and recover every evening exactly like it's supposed to, but still be failing to produce fruit. That is usually a pollination problem. And it comes down to what heat does to flowers and pollen specifically.

[00:13:55] So, for example, tomato flowers need to be pollinated within about 50 hours, 50 hours of opening or they'll abort. And that pollination depends on pollen that is viable and able to release properly from the anther. Heat interferes with that process.

[00:14:16] So when daytime temperatures exceed 85 degrees Fahrenheit or the nighttime temperatures exceed 70 degrees Fahrenheit, which is exactly what we are experiencing here right now, pollination suffers because the pollen becomes sticky or tacky. And it's not viable at that point. Daytime highs in the 90s Fahrenheit and above, and you pair that with those nights that aren't dropping to be cool enough,

[00:14:45] are likely just going to cause blossom drop or even fruit drop across an entire region during a heat wave. So here in West Central Missouri, we get stretches almost every summer where our overnight lows just aren't cooperating, like right now when we're finally getting the first tomatoes from the gardens. But it's not a fertility issue and it's not a watering issue. It is straightforwardly a temperature problem.

[00:15:12] And there isn't a spray or an amendment that is going to fix it, okay? What does help is selecting varieties that are known to hold up better under that heat and the humidity and maybe providing afternoon shade in the hottest part of the season so that the plant isn't fighting the peak sun on top of fighting the peak heat. Peppers also follow a similar pattern to like what tomatoes do.

[00:15:41] So New Mexico State University Extension is one of the leading research authorities on chili pepper production, as one might expect coming out of New Mexico, right? And they publish a guide specifically on environmental stress disorders in peppers. And that guide lists high temperature stress right alongside insufficient water, strong wind,

[00:16:06] high humidity, and nutrient imbalance as the primary triggers for flower drop in peppers. Any of those factors can interfere with pollination. And when the plant senses that pollination isn't going to succeed, it drops the flower rather than spending resources on something that's not going to become a fruit. So we do the same thing for peppers as we do for tomatoes. Keep the soil moisture consistent through a heat spell

[00:16:33] and don't panic if flowering slows or pauses entirely during the worst of a heat wave. It's the plant managing its own resources, and it typically resumes once the conditions sort of start to moderate a little bit. Now, squashes and cucumbers and all of their relatives play by a different set of rules, I guess, because they have separate male and female flowers on the same plant.

[00:17:02] So that means that the pollination has to happen between two flowers rather than with like self-fertile flowers like a tomato, right? That makes them dependent on pollinator activity. And yes, you guessed it, heat affects that too. High temperatures tend to reduce bee and other pollinator activity during the hottest part of the day. So that's the first problem. But if we go back to heat stress in general,

[00:17:27] then it shouldn't surprise you to learn that pollination problems from heat stress are more pronounced in urban gardens because you've got roads and buildings and other hard surfaces that absorb that solar energy during the day, and they radiate it back out as heat in the overnight. So that's going to keep the nighttime temperatures elevated in ways that mimic exactly the conditions that hurt tomato and pepper fruit set too.

[00:17:54] So if your garden sits near pavement or a south-facing wall or something, that heat island effect is very real, and it's worth factoring in when you're troubleshooting why you don't have fruit setting yet. Now, I don't want this to make you feel like your whole season is at risk every time we get a hot week because that's not the reality here. Most plants can make up for a temporary setback in fruit production with a fresh flush of flowering

[00:18:24] once those temperatures get back into a more comfortable range. So this is a pause. It's not a permanent loss. And it's exactly the kind of stress response that we've been talking about all episode. The plant protecting its resources during a rough stretch so it can come back strong once the conditions improve. Okay? And then one more thing that's kind of worth noting is that heat stress can also contribute to sun scald.

[00:18:53] So you've got exposed fruit, and it starts to develop a pale blistered patch from direct sun exposure. This happens a lot on tomatoes that have been really heavily pruned or peppers that aren't planted closely enough to their neighbors so they're not getting good leaf coverage. Heat can also lead to blossom and rot, which we know is a calcium-related disorder. And it shows up as those leathery brown spots on the bottom or on the sides of tomatoes and peppers and other fruits,

[00:19:22] especially when heat stress is paired with uneven watering. Now, I'm going to give Blossom and Rot its own full episode next week because it's always such a big topic in the summer and also because it deserves a deeper dive than what I can give it here. But I wanted you to hear how it directly connects to everything that we've covered today. So let's talk about what to actually do with all of this

[00:19:52] because the goal here isn't just to diagnose the problem and go, ah, okay, you know. It's to give your plants some support during a heat stretch without swinging into the kind of overcorrection that just creates new problems. So the first thing is to think about your watering, okay? Less frequent, more thorough waterings still apply during a heat wave. If your soil check tells you the ground really is dry,

[00:20:18] the fix isn't a daily light sprinkling, okay? It's a deep, thorough soaking that gets moisture well down into the root zone and it should still be done less frequently. Shallow, frequent watering trains those roots to stay near the surface, which is exactly the wrong direction during a heat wave because that surface soil is the first layer to heat up and dry out.

[00:20:44] Giving it a deep soak encourages those roots to go downward and it's going to chase that moisture into the cooler, more stable soil, which is precisely the resilience that you want to build in before like the next hot stretch hits, right? Because we know we're not facing just one of these. Timing matters here too. You want to water in the early morning whenever you can.

[00:21:08] So watering before the heat of the day sets in gives a well-watered soil just the chance to actually absorb that water and stay cooler throughout the afternoon. Soil that has already been baked dry by the sun and then gets watered tends to resist the absorption and the water just runs off of it instead. Morning watering also means that any water that does land on the foliage has time to dry,

[00:21:37] which matters for disease prevention, especially in a garden that is already stressed out, okay? Now, you know already that I am not shy about how much I love a good mulch layer, okay? And heat stress season is exactly where mulch earns its keep. A layer of organic mulch, straw, shredded leaves, wood chips, whatever it is, shades the soil surface directly, which slows the evaporation

[00:22:04] and it keeps that root zone temperature much more stable through the hottest part of the day. This is one of the reasons why black plastic mulch can be a bad idea in a lot of areas. Rather than cooling the soil, it absorbs that heat and it can actually raise the soil temperature, which is working against you at exactly the moment that you need the opposite. So if you use black mulch or landscape fabric to prevent the weeds in your garden,

[00:22:32] then try to protect your plants and the soil by putting an organic mulch on top of it to help block the absorption of that heat. Another thing that is a really useful tool during a sustained heat stretch is shade cloth, okay? So we're looking for about 30 to 50% shade rating for most vegetable crops, just enough to take the edge off that really intense sun and heat

[00:22:59] without blocking so much light that the photosynthesis also suffers along with it. If you don't want to spring for like actual agricultural shade cloth, then even a thin, light-colored bed sheet will actually do the trick. There are a couple of things that are easy to get wrong with shade cloth, though. The first is that shade cloth should never touch the plants directly, okay? Because the heat absorbed by the cloth itself can actually transfer straight through to any leaf or fruit that's pressed against it.

[00:23:28] And then cloth moving in the wind can damage the tender growth that it's resting on. So you want to rig it on a simple frame or over hoops. Even if you have some tall chairs or step stools with just the cloth clamped on, you can often hold it high enough for at least a small garden. The second thing is to remember that this is a temporary tool for the duration of just the heat spell, okay? This is not something that we want to leave up all season.

[00:23:55] You want to pull it back once the temperatures ease so that your plants can get back to their full light exposure for normal healthy growth and for fruit ripening. And then if heat stress is a recurring theme in your particular garden, and for a lot of us it increasingly is, then variety selection is actually worth factoring into next year's garden plan and your seed order.

[00:24:20] Varieties bred with better heat and humidity tolerance can noticeably reduce the flower and the fruit drop that we talked about. And if you're gardening somewhere with genuinely brutal sustained summer heat, you might try some alternative crops through the summer. So tropical squash varieties like calabaza and the seminal pumpkins, or swapping like a standard slicing tomato for cherry tomatoes that are a little bit more heat forgiving,

[00:24:48] or tomatillos, or the little current type tomatoes that are really heat tolerant, just during the worst stretch of the summer. You don't have to overhaul like your whole garden plan, but just knowing which of your crops are the heat sensitive ones, and which ones have more heat tolerant alternatives, gives you some options before like getting into the really bad summer just forces your hand at it.

[00:25:13] And I want to end with something that I think is genuinely hard for most gardeners, myself included sometimes, and that's doing less, right? Once you've confirmed that the soil has adequate moisture, and your mulch is in place, and you've got the shade cloth up if you need it, the most useful thing you can do during a heat wave is just to leave the plant alone,

[00:25:41] and let it manage itself through the worst of the afternoon, okay? Don't add extra water on top of the moist soil, you know, just in case, because the plant looks sad. Don't strip healthy leaves off of a tomato plant to reduce its workload. I've seen this advice, and it's the worst. Those leaves are shading the fruit underneath from sunscald, and it's still doing photosynthesis, even if it's doing it at a reduced rate.

[00:26:09] So don't strip off healthy foliage, okay? And don't reach for fertilizer to try to push new growth when you're in a heat spell. A stressed plant isn't positioned to use it well, and pushing tender new growth right as a heat wave peaks just gives the plant more thirsty tissue to support, and that's at the worst possible time, okay? We want to support the roots, manage the sun as best we can, and give the plant room to do what it already knows how to do.

[00:26:38] So let's do a quick recap. So your signs of heat stress wilt. The soil is moist an inch or two down when you check it with your finger. The wilting shows up in the hottest part of the afternoon. The plant looks normal again by late evening or the next morning. Your squash and your cucumbers are drooping earlier and harder than everything else nearby. And your leaves may be rolled or cupped or angled to reduce the sun exposure.

[00:27:08] Signs of it being drought stress wilt is the soil is dry more than an inch or two down. The plant is still wilted first thing in the morning before the day heats up. Wilting has been getting worse over several days rather than resolving each evening. And you have leaf edges that are browning, scorched, or crisp rather than just drooping. So if it's heat stress, do nothing to the soil.

[00:27:34] Consider shade cloth at about a 30 to 50 percent if that heat stretch is prolonged, and then check back at the soil level before watering again. If it's drought stress, water deeply and slowly until the moisture reaches well down into the root zone, ideally early in the morning, and add or refresh your mulch to help that moisture last. If flowers are dropping on tomatoes, peppers, or squash during a hot stretch,

[00:28:03] don't try to intervene with sprays or extra fertilizer. Keep the soil moisture consistent and expect a fresh flush of blooms once those temperature ease. If you're not sure, check again tomorrow morning. One data point rarely tells you as much as seeing a pattern over a day or two. And then finally, if you've watered and it doesn't seem to be heat or drought,

[00:28:29] but your plants are still wilting consistently, then it may be a disease that you're dealing with. So bacterial wilt in cucumbers, for example, which is spread by cucumber beetles, is a perfect example of this. But understand that if a plant is at the wilting stage because of some bacterial or viral disease, then nothing you would have done at the point you saw the symptoms likely would have saved that plant.

[00:28:55] So work through the identification steps the same way every time to figure out what you're dealing with. Don't ever take action in a panic over your plant. All right, just remember, your plants wilting on a brutal July afternoon are, more often than not, doing exactly what they're built to do. Closing down, conserving water, and waiting out the heat before bouncing back overnight. I mean, same plants, right?

[00:29:24] The soil test and the evening check-in are going to tell you which situation you're actually in. If it's heat stress or if it's true drought in about 10 seconds, okay? And that 10 seconds can save you from overwatering into root rot or underwatering into real damage. And when your tomatoes, your peppers, or your squash stop setting fruit during a heat wave, even though the plant itself looks fine, that is not a fertility problem or a watering mistake, okay?

[00:29:51] That is just pollen and pollination getting knocked out by temperatures that are outside their very narrow working range. And it resolves on its own once the weather breaks. I will be honest, every July I have at least one bed where I catch myself just staring at really droopy squash leaves. And I have to talk myself through my own advice before I grab the hose unnecessarily. It happens to all of us, okay? So just check the soil first.

[00:30:21] In the show notes, I'm linking every extension resource and research citation from today. If you're not already following the show in your podcast player or subscribed in YouTube, you might want to do that right now so that you don't miss any episodes. Next week, we're going to dig all the way into blossom end rot, which what it actually is, why heat and uneven watering make it so much worse, and what genuinely helps versus what's just garden folklore.

[00:30:50] You will not want to miss it. So set those notifications. Until next time, my gardening friends, keep on cultivating that dream garden, and we'll talk again soon.