Managing Heat Stress in Garden Plants - Ep. 207

[00:00:00] It's the middle of July and is the time of year when heat stress in plants can be a significant challenge for gardeners. If you're gardening in the peak of the summer or just in a region with consistently high temperatures,

[00:00:13] heat stress can lead to wilting, reduced yields, poor fruit quality, and even cause our plants to just up and die. But we can do something about it. Lots of things actually. Today on Just Grow Something, we're talking effective management of heat stress in our plants, including preventative measures

[00:00:31] and actions to take after our plants are possibly affected. We'll talk all the strategies that we can take to help maintain a healthy garden despite the heat. Let's dig in. Hey, I'm Karin. I started gardening in a small corner of my suburban backyard and now 18 years later

[00:00:49] I've got a degree in horticulture and operate a 40-acre market farm. I believe there is power in food and that everyone should know how to grow at least a little bit of their own.

[00:00:58] On this podcast, I share evidence-based techniques to help you plant, grow, harvest, and store all your family's favorites. Consider me your friend in the garden. So grab your garden journal and a cup of coffee and get ready to just grow something.

[00:01:21] So it is the middle of July and honestly, it hasn't been nearly as bad here as it usually is. By now, Mother Nature has generally turned the faucet off like weeks ago and the temperatures would have normally soared into the upper 90s or low 100s Fahrenheit.

[00:01:36] But we've had much more rain this year and the temperatures have only eaked into the low 90s a couple of times so far. But I know it's coming so it's time to prepare the garden to be resilient. Heat stress in our plants

[00:01:51] happens when the temperatures basically exceed what the plant's optimal range is. And so this causes a bunch of different physiological disruptions. It means the plant can't function the way it normally would.

[00:02:05] For most of our summer vegetables, the optimal growth occurs when they- the temperatures are between 70 and 85 Fahrenheit or 21 to 29 Celsius. Anytime the temperature starts to go above that range, we're going to start to see some signs of heat stress in our plants.

[00:02:24] The first and most obvious one is wilting and this is generally due to excessive water loss. Now, this is not the same as the plants sort of wilting over in the hottest part of the day

[00:02:37] to reduce their leaf surface exposure because they're doing that at that point to just prevent further transpiration from happening. You don't want to have the water evaporating from the leaf surface so they will wilt over. But generally speaking,

[00:02:52] once the temperatures start to cool off in the evening and into the overnight, they will bounce right back up again. Wilting due to excessive water loss is not going to recover very quickly. So you're not going to see those plants back out. They're

[00:03:06] going to stay wilted at that point. Another symptom is leaf scorch. This is the browning or burning of the leaf edges. You might see them getting even crispy. Blossom drop. So this is when

[00:03:19] the flowers fall off of the plant without setting any fruit. They just fall off for seemingly no reason. This is due to the heat and then fruit disorders like blossom and rot in our tomatoes

[00:03:31] and our eggplant and even our peppers and our zucchinis and stuff. This is not generally going to be due to a lack of calcium in the soil. We talk about blossom and rot being a lack of calcium,

[00:03:46] but it's technically a lack of calcium in the plant. In the extreme heat, this is often as a result of heat stress or inconsistent water just causing a lack of calcium movement. So it can't get

[00:04:02] into the plant or it can't move within the plant into the fruit where it is needed. So you may not necessarily have a lack of calcium. It very well may be due to the heat or a lack of water.

[00:04:13] And then you also may see some very obvious stunted growth in your plants. They either will slow to a crawl or completely stop development altogether and just sort of seem like they're

[00:04:26] in this sort of, I don't know, stasis. So what do we do to prevent heat stress from getting to our plants at all? Really what we want to do is sort of create an environment in the garden

[00:04:39] that minimizes extreme temperature swings and that sort of supports the plants being resilient on their own. And the first thing to do is to look for varieties that are bred for heat tolerance.

[00:04:55] That can significantly reduce the risk of heat stress. There are a lot of varieties out there that have been bred to be able to withstand a lot of high heat. So look for varieties that are labeled

[00:05:05] as heat tolerant or heat resistant. There are tomatoes and peppers and eggplant, even lettuces that have been bred to withstand higher temperatures, even though that's not something that we would typically grow here in the middle of the summer unless they are very, very tiny and they are

[00:05:22] underneath some shade. But in areas where the heat doesn't get quite as extreme, you actually can grow some of those varieties of lettuce throughout the summertime. You can also look for local seed companies that are offering bioregionally adapted seed and plant varieties.

[00:05:42] This means that those cultivars have been grown in your area for multiple seasons, standing up against the weather and the pests and the diseases that are unique to your region and they have thrived to be able to reproduce. You know, when we're saving seeds, we want to save

[00:06:00] the best of the best. And so when you do that, those plants are going to be better adapted for growing in your unique summer heat situation than ones that are maybe grown elsewhere. So if you're in the Kansas or Missouri region, I highly recommend the Buffalo Seed Company.

[00:06:16] I have had stellar results with their seeds and they are bioregionally adapted for this area. So you can find them at thebuffaloeseedcompany.com. The second thing that we can do to help prevent the heat stress from even setting in is to apply mulch. I constantly preach mulch. This

[00:06:35] is one of the reasons why applying mulch around your plants can help conserve the soil moisture, number one, but it also is going to help regulate that soil temperature so it's reducing the impact of these extreme heat events. I recommend organic mulches that are light in color.

[00:06:54] So straw or light colored wood chips are particularly effective. They're going to reflect the heat back off while keeping that soil cool. You can use compost. Compost tends to be a little

[00:07:04] bit darker so it may not be quite as effective, but it's still going to help more than not having anything at all. You want to make sure that the mulch is applied about two to four inches deep

[00:07:16] around the base of your plants. Just make sure that you're not putting them in direct contact with the stems because we don't want them to rot. Now is a good time to be double checking.

[00:07:25] Even if you did apply mulch early in the spring or the beginning of your gardening season, now is a good time to check and make sure that you still have as thick of a layer as

[00:07:34] you think you do because again these are organic mulches so they're going to start to break down over time and you may not have as much right now as you think. I don't recommend

[00:07:44] in areas with extreme heat like this that you use black plastic mulch or black landscape fabric. We have used it in the past and generally speaking the plants that do the best for us

[00:07:56] with those types of mulches are the ones that are grown in the very early spring so our lettuces, for example head lettuces, they've done very very well. But when we have used them for things like tomatoes, it tends to at least out in the field and the wide

[00:08:13] open. It tends to make those root zones very very hot and our plants are just as affected by the soil temperatures as they are the air temperature so we have actually gotten away

[00:08:24] from using too many of those types of mulches unless we are also putting some sort of organic mulch over the top. So we'll use it as a weed barrier but then we'll use an actual organic

[00:08:35] mulch to kind of reflect that heat off. The next thing to think about is watering practices. I say it you know all the time it supplies for all the plants in your garden. You want less frequent but more thorough waterings. This is even more important when we're talking

[00:08:51] about heat stress. You want to water deeply to encourage that deep root growth so you want to saturate the soil down to the four to six inch layer. This is going to help the plants

[00:09:02] access moisture from deeper soil layers during heat waves and you want it to be deep because the closer that water is to the surface the faster it's going to evaporate and then the plants aren't even

[00:09:17] actually going to get to take advantage of that. This is also one of the reasons why we want to water earlier in the day so if we can water early in the morning it's going to reduce that

[00:09:26] evaporation and it's going to help the plants get really hydrated before they have to face that heat of the day. And then if you can use either a drip irrigation system or soaker hoses or

[00:09:39] low to the ground types of sprinklers that will put moisture sort of right at the root zone and allow it to soak in better. This is also going to minimize that water loss. It's also going to

[00:09:53] reduce the risk of foliar diseases because you're not splashing the water up onto those leaves and if you live in a very humid climate you know those leaves are already getting that sort of

[00:10:05] moisture from the air and that is just a breeding ground for foliar diseases so we don't want to add anything on top of that. So irrigating or watering at the soil line is going to be the

[00:10:15] best idea. Now you can provide also some shade solutions to help protect your plants from excessive heat and like really intense sunlight. Shade cloth is a good option. You can have this erected earlier in the season to help to reduce the temperature. It's also going to

[00:10:35] help reduce the light intensity. These cloths come in a wide variety of shading levels so they'll either say it shades at a certain thing or it allows a certain level of light. So if it's you

[00:10:50] know it says 50% then obviously it's going to let in about 50% of the light. Some of the time that will say it's a 70% which means it's letting in 70% of the light. You know you can go as high as

[00:11:02] 90%. So generally speaking you know I mean depending on the time of the year but 70% of the light coming through is going to be good. 50% is good too. It's going to help to shade those and cool them

[00:11:17] quite a bit. You just don't want it to be so dark or so thick that it's not letting any of the light through and it's just sort of trying to grow in the shade at that point. You can use

[00:11:26] row covers for this also. The lightweight row covers that we use for frost cloth in the winter time can also provide shade. You just want to make sure that it's elevated up enough above

[00:11:39] the plants that you get the airflow because those things if you put them like down like a tunnel over top of those plants it's actually just going to trap the heat in and that's

[00:11:49] going to make it even worse. So they need to be open and they need to be up high enough to where you're going to get that airflow under there. And then you know think about interplanting. We

[00:11:58] talk about growing taller crops with lower growing ones and you can do this in a manner that it's strategic so that the taller ones will provide natural shade for the shorter maybe more heat sensitive plants. This does require some forethought. So these are things that you

[00:12:17] need to think about when you're in your initial planning stages of your garden in the winter or in the early spring keeping in mind what those summer conditions are going to be like. Now what happens? What should we do when we see our plants undergoing some heat stress?

[00:12:35] You know despite our best efforts it's very likely going to happen no matter what. So if your plants are wilting or they are showing signs of heat stress check the soil moisture first before you immediately decide to water them. Again we have to learn to determine

[00:12:50] the difference between when they are wilting over to reduce the surface area just to protect itself from you know further about water evaporation versus them wilting due to heat stress or not

[00:13:05] having enough water. So check the soil moisture first and if you do see that that soil is dry below the 3 inch mark then go ahead and water. Water them immediately make sure that the water

[00:13:18] reaches that root zone and not just the surface where it's going to evaporate you want it down at that 4 to 6 inch level so you may have to run that water for a while and in very severe cases

[00:13:28] you might actually have to water twice a day until those stress symptoms start to subside. It may not be an immediate uptake of water. It all depends on how stressed they have gotten.

[00:13:39] The second thing you can do is throw up some temporary shading so you know it doesn't have to be a fancy shade cloth. It could be old bed sheets or you know burlap or you know if you have shade

[00:13:50] cloth great, if you have row cover great just support whatever you're using as a shading material up with stakes or some other type of the structure because you don't want them to be

[00:14:00] in direct contact with the plants that can cause some physical damage we also don't want them that close to the plants because that's going to impede the airflow and maybe trap more of the heat.

[00:14:10] We want the air to flow but we want to block some of that sun. Something else that you can do is to spray them very lightly with water okay a light mist of water on the foliage can actually help

[00:14:25] cool plants through evaporation. You want to do this kind of earlier in the day. You don't want to do it like right at the hottest part of the day because you might actually damage

[00:14:34] the plants at this point but just think about how good it feels for you to stand out in the sun and maybe just have a misting of water on your face. Now translate that into what it's going to

[00:14:42] feel like on those leaf surfaces so early in the morning or late in the afternoon just to kind of give them a little bit of a break and that will actually help to cool those plants a little

[00:14:53] bit and alleviate some of that stress. You can also alleviate some of the stress by doing some pruning and some thinning now obviously we want to do this in the early

[00:15:02] morning when the plants aren't already you know warm from the heat of the day and they're not already stressed but if you can remove some of the foliage you're going to actually reduce the plants water

[00:15:14] needs at that point and it's also going to help improve the airflow around that plant which is going to help to cool the plant down. You can also take a look at the way that your

[00:15:24] plants are growing and if they seem like they're maybe a little overcrowded for the current conditions then thinning them out is also going to reduce that competition for water and nutrients

[00:15:38] and it's also going to provide a little bit more airflow so this is all going to help with you know any kind of recovery from heat stress that they're already facing. It also will help

[00:15:47] to prevent any further stress from occurring. You also need to try to see how well you can manage your soil so you know adding organic matter to the soil is going to improve its water

[00:16:01] holding capacity and of course it also adds nutrients at the same time but that water holding capacity is important during times when you know it's extreme heat events so if we can add organic

[00:16:13] matter at the beginning of the season or in the off season to help improve the soil that is going to help in the overall. If you're doing a raised planter or raised garden bed and it seems like

[00:16:26] you need to somehow improve that soil structure and you need to do it fairly quickly you can actually incorporate vermiculite or perlite into those beds. You can do this in the in the garden

[00:16:36] beds too it's not going to cause any harm. We've talked about what perlite and vermiculite are they are natural substances they actually help to retain moisture and they also help to improve

[00:16:48] the soil structure so if the compost isn't doing it or you don't have access to a ton of compost you can actually get large bags of vermiculite or perlite and you can work that into your soil

[00:16:58] and then also consider growing cover crops during the off season that's going to help improve the soil structure too and it's also going to help with the moisture retention. Now some of this

[00:17:07] takes you know some effort it takes some forethought and then some of this you know takes effort in the garden to manage this stuff and I know the last thing that we may want to be doing

[00:17:17] is spending energy in the hot afternoon so doing these things in the early morning or in the evening is your best bet so be prepared to get out there when the conditions are more forgiving know what

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[00:20:05] or a one-time purchase. Okay, well let's talk long-term strategies for managing heat stress. If you garden in an area that frequently has high temperatures or you know that your summertime gets excruciatingly hot before it starts to cool off then there are some long-term strategies

[00:20:25] that you can use to sort of mitigate those circumstances. You can actually create your own kind of microclimate in your garden. This is going to give you a localized environment that helps protect your plants from extreme heat. One of those things that you can do is

[00:20:45] actually plant hedges or install fences or trellises where plants can be grown that will shade your garden beds at an appropriate time of day during the summer. Now this does take some planning and

[00:21:00] it actually takes some observation on your part during this summer and looking at where the sun is hitting, taking notes about when the temperatures get to be the most extreme and all of these things

[00:21:17] in your garden journals that you can go back. This includes drawing these things out. If you can give yourself a little drawing of your garden and duplicate it so you have sort of three

[00:21:29] different versions of it and you can monitor where the sun is hitting and what's growing and how those plants are reacting to the sun at the beginning part of the season and then smack

[00:21:40] dab in the middle and then again at the end or if it's really just the summer that you're worried about, the beginning part of the summer and then the middle and then the end. That is going to

[00:21:49] give you an idea of where you could put these sort of wind breaks or I guess sun breaks for lack of a better term, these hedges or fences or trellises that you can use to grow things

[00:22:00] on that are going to help protect some of those other plants. The other thing to think about is living ground covers so growing perennial plants in the garden that act as a living mulch that you

[00:22:13] can plant into, that is going to protect that soil. It's going to protect the soil without you having to constantly replenish mulch. You just have to make sure it's something that's not going to compete with the other things that you actually are growing in those beds. Believe

[00:22:28] it or not, I am working on this with perennial mints in many of my raised planters and I know that like sends alarm bells off to most people because we're always told don't put mint anywhere

[00:22:40] that you don't want it to live forever. I'm using them in my raised planter box direct planters and because they're not going to escape that cultivation if they do they're just going down in the grass and I don't care if it's going in the grass that's fine but

[00:22:54] the plants that I typically put into some of those planters are taller growing plants that are not going to be competing with the mint. The mint will stay low growing and then I do much taller

[00:23:05] like zucchinis or I put tomatoes in there or peppers or whatever. I probably would not use mint in a bed that I was going to grow nothing but leafy greens or, you know, lettuces or, you know, low growing greens like arugula. That would definitely be competition so you

[00:23:23] have to think about how you're going to use those beds. Clover is good for this too. You can also plant annuals if you don't want to put a perennial in there, you can plant annuals that do the same

[00:23:32] thing they just don't stay in place permanently. Anything that you can do to help protect that soil, you know, if the mulch isn't really doing as good of a job as you might hope. And the other

[00:23:44] option is reflective mulches. We talked about the organic mulches being a light color and that helping to reflect some of that heat. But there are mulches that use reflective materials like white plastic mulch, so white instead of black. That can help keep the soil temperatures lower. There are

[00:24:03] also light colored paper mulches that can also be used. We have used those before in our onions. If the organic mulch that you've got access to just isn't very effective for you, you have

[00:24:15] other options. Another thing that we can look at in the long term is some sort of soil moisture monitoring. If you are finding that you are constantly battling keeping your garden watered and you really feel like you're having a hard time sort of tracking those moisture levels and,

[00:24:35] you know, going out and sticking your finger in the dirt just isn't working and you feel like you're always watering, there are soil moisture sensors that can help you track these

[00:24:43] things. Some of them are very, very simple and it's just sort of an analog reading and you go down and you check it. Some of them are like really fancy-smancy and like you can hook it into your

[00:24:55] irrigation and it will automatically turn things on and then there's everything in between. So different technologies, wide gambit of options for them but it can prevent you from under watering and having to go out and do those sort of emergency waterings if you're just struggling

[00:25:14] with your plants constantly, you know, suddenly wilting over because something happened and they didn't get enough water so you can consider using some sort of a soil moisture monitor. And then one thing that we may not think about when it comes to heat stress is making sure

[00:25:31] that we have an integrated pest management strategy, right? So our plants, if they are healthier, are going to be more resilient to heat stress. So if we don't have a bunch of insects that are

[00:25:46] predating on them, breaking down their defenses, then they can focus their energies on being resilient to that heat stress. So reducing the pest pressure which, you know, is going to eliminate some of that extra stress. So crop rotation, making sure that we're disrupting those

[00:26:03] pest life cycles, using biological controls like the beneficial insects that we can bring in, whether you're bringing them in or you're planting things to kind of attract them to the garden. And then of course inter-planting or companion planting, making sure that you are growing

[00:26:17] plants together that are mutually beneficial to each other in terms of pest control and growth. Those are all things that you can do to help manage the pest pressure, which will, you know, sort of leave the energy of your plant available to be able to combat the heat.

[00:26:34] And then I really do want to talk about that sort of genetic resiliency component again. If you can grow plants that are closer to being native in your area, you will have a better,

[00:26:49] you'll have better luck getting them to survive extreme heat. So this goes along with what we said about bioregionally adapted seeds and plants. If a plant is a cultivated variety of something

[00:27:00] that grew wild natively in your area, it is much, much more likely to do well in your area than something that originated in a completely different climate from yours. So consider that when you're

[00:27:15] planting your garden. I know we all have our favorites, right? If you are a tomato lover, you want to grow tomatoes. Well, it's very likely that there are tomatoes that have been adapted

[00:27:25] to grow much better in your area than some of the other varieties. So once again, if you can find a company that is local to you that is growing those seeds or if you can figure out

[00:27:40] what the sort of area of origin is for the plants that you want to grow and find some that were originating in climates that were very similar to yours, it's going to help you have those plants be more resilient and more resistant against that heat stress.

[00:28:01] So if your plants got heat stress and then you employed all the measures you possibly could to get them to bounce back, you're going to know if they have recovered because you're

[00:28:10] going to see a few signs. They're going to regain their firmness, they're no longer going to appear wilted at all times, they should have bounced back. You will start to see healthy new leaves and shoots appearing and you should see the resumption of normal flowering and fruiting patterns.

[00:28:28] Even if your plants don't go through actual heat stress and look like they're being damaged, they still may have some of these symptoms that cause them to stop growing and stop flowering

[00:28:42] and that sort of thing. As the weather starts to cool off a little bit, you should also see the resumption of a lot of these activities. It's not necessarily a death sentence to those

[00:28:52] plants if they just kind of sit in a holding pattern for a little bit trying to get themselves through the worst of the heat. So we just want to kind of help them in that situation. So

[00:29:06] constantly monitoring the weather forecast and checking our soil moisture levels, making sure that we are anticipating when that next round of heat is going to hit will give us the ability

[00:29:18] to be able to give our plants a little bit of attention and that way we can maintain a productive garden even through the hottest summer conditions. Until next time my gardening friends, keep on cultivating that dream garden and we'll talk again soon. Thanks for listening to another

[00:29:33] episode of Just Grow Something Podcast. For more information about today's topic and to find all the ways you can get in touch with me or support the show, go to justgrowsomethingpodcast.com. Until next time my gardening friends, keep learning and keep growing.