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266-How Heat Affects Tomato Plants and How to Protect Them, with Craig LeHoullier

| Care, Podcast

Tomatoes enjoy warm weather, but they don’t like it too hot. Extended periods of heat can stress out plants and lead to a number of problems. Joining me on the podcast this week to discuss how to overcome the tomato-growing challenges that arise in high heat is tomato expert Craig LeHoullier. 

Craig is the author of two books on gardening, “Epic Tomatoes” and “Growing Vegetables in Straw Bales.” He’s also the tomato adviser to the Seed Savers Exchange and a co-leader of the Dwarf Tomato Breeding Project. He’s been gardening since 1981 and he has a seed collection that includes more than 1,000 heirloom tomato varieties. Craig is also the co-instructor of our joegardener Online Gardening Academy™ course, Growing Epic Tomatoes.

 

Joe Lamp'l and Craig LeHoullier

Craig and I preparing to record for our Growing Epic Tomatoes course offered through the Online Gardening Academy. (photo: Carl Pennington)

 

Craig lives in Hendersonville, North Carolina, which is about four hours farther inland than his last home, in Raleigh, North Carolina. Having experienced summers in Raleigh for 28 years, the heat Hendersonville is experiencing right now — the town has already hit 90° several times this year — is nothing to him. But locals in Hendersonville are “just freaking out,” Craig says.

“But to those who have lived here for a long time, this is kind of unprecedented,” he says. “And I’m getting a lot of questions from people all over the country: ‘We’re roasting. My tomatoes are baking. What do I do?”

We’re receiving many questions from our Growing Epic Tomatoes enrollees, and I can’t remember a time when this topic has been top of mind for so many.

“When you think about the central nature of tomatoes, in terms of people’s gardens, if people start losing their confidence in growing tomatoes, if the weather causes an inability to succeed, that’s going to depress an awful lot of gardeners,” Craig says. “So I think the good news is we have some tactics and some plans.”

Before proceeding any further with our discussion of growing tomatoes in extreme heat, I want to take a moment to remind you that I have a new book coming out in September, and it’s available for pre-order now. The title is “The Vegetable Gardening Book: Your complete guide to growing an edible organic garden from seed to harvest,” and I’m very excited for you to read it. It’s chock full of insider tips and new-to-you information that will help you step up your gardening game and tackle challenges. 

And on tap for 2023 is my new Online Gardening Academy™ premium course, Organic Vegetable Gardening. Sign up for the waitlist here.

How Heat and Humidity Are Detrimental To Tomato Flowers

“This is the time of the year where you really need to know your plants and be in your garden because tomatoes — being a tropical crop — can handle heat,” Craig says. “The question is, are we doing the things that can help them handle heat the best? So they’re going to have increased watering needs because of that. They’re going to have increased needs for food. And the big bugaboo: the fact that tomato pollen doesn’t do really well when it gets to 90, 95 or above for extended periods of time, and especially when it’s really humid.”

In high humidity, tomato pollen can get sticky and just never releases. The dreaded blossom drop is a concern at this point. Plants that were beautiful and full of flowers can, a couple of weeks later, lose their flowers and not produce any fruit.

Even when humidity is low, if the temperature is high enough, the heat will render tomato pollen sterile. Fortunately, this only becomes a problem during extreme heat waves. On just a regular day when the high temperature reached 95° there will still be hours in the morning when the temperature is only 75°. It’s when a high temperature is sustained all day that the flowers never have a window to pollinate successfully during their life cycle.

If you don’t see as many flowers or as many tomatoes setting on your plants compared to your experience last year, blossom drop may be the culprit. Think about what temperatures you’re dealing with right now, and there’s a good chance that the reason that is occurring is that the temperatures this year are so much hotter and that has a direct impact on fruit set.

When someone tells Craig that their plants have no fruit, he asks them if there were flowers. If they can’t recall, that means they really weren’t paying enough attention to their garden and making those all-important frequent observations. 

“If you were to look at your plants every day, you’d undoubtedly find that there were flowers there, they opened, but then they shriveled and they dropped off without forming a tomato,” Craig says. “So rare — extremely rare — is the plant that doesn’t bloom; there’s a problem with the seed that produced that variety, you’ve got a mule of some sort, it’s a genetic issue. But more often than not, the flowers are there and it’s too hot for them, for the pollen to activate and fall from the flower and touch the pistil and have that pollination.”

 

Joe inspecting his tomato plants

Tomatoes may be a warm weather crop, but during extended periods of high heat they will be subject to stress that creates a host of problems. (photo: Tobi McDaniel)

 

Feed Tomato Plants Properly for More Flowers and Fruit Set

Getting the proper nutrient balance and feeding the plants correctly are vital steps to encourage more blooms. Ongoing fertilization is also important. Tomatoes are heavy feeders and certainly want to start off with the right soil with some good organic nutrients in it. If you are growing tomatoes in a container filled with bags of potting mix or container mix, that mix will usually have slow-release synthetic fertilizer in it that is designed to last through the growing season. But keep in mind, in extreme heat, you are watering more often, and that frequent watering will cause some of those nutrients to leach out. So you may still need to add fertilizer. 

The three macronutrients in fertilizer are nitrogen, phosphorus and potassium, known by their symbols on the periodic table as NPK. For flower production, phosphorus is the most important nutrient, so look for a fertilizer with a higher middle number on the bag, such as a 5-10-5, during the flower and fruiting phase.

Nitrogen encourages vegetative growth — the green and lushness. Too much nitrogen will mean lots of green growth but very few flowers and little fruiting. Potassium contributes to the overall health and vigor of plants. 

Choose Small Tomatoes For Better Self-Pollination

Tomatoes have what are called “perfect flowers.” Each blossom has both male and female parts, which allows them to pollinate themselves. A visit from a pollinating insect is not required.

But when high heat sets in, the pollen in the blossoms clumps up, and the flowers can’t self-pollinate. 

Craig notes that tomato plants with smaller fruit are less affected by blossom drop. So cherry tomatoes up to golf-ball size tomatoes are the safest bet for growers in really hot climates.

“For whatever reason, the pollen doesn’t seem to be quite as affected by the high heat and humidity,” he says.

Sadly, it’s the biggest tomatoes — those that we imagine when we think of summer garden tomatoes that can cover a whole sandwich with one slice — that are most affected by blossom drop. 

“When it gets to 90 or above for extended periods, the pollination doesn’t happen as efficiently, and you get the highest level of blossom drop on those tomatoes,” Craig says. 

Still, there are interventions gardeners can make to help those tomato plants succeed even in high heat.

 

Tomato blossom

Tomatoes have “perfect flowers” that have both male and female parts. They don’t require the assistance of pollinating insects. (photo: Tobi McDaniel)

 

How to Improve Tomato Pollination

VegiBee is like an electronic toothbrush without the bristles, designed specifically for pollination. The VegiBee vibrates at its tip, and when it touches a blossom, it causes the pollen to release.

It’s surprising how much pollen is contained within the flower that you can’t see because it’s under the petals, but wow, it pours out. 

The blunt side of an electronic toothbrush can also be used for this purpose, and you can even just flick the flowers with your finger. Just don’t flick the flowers so hard that they fall off the plant. 

On humid days, especially after it rains, buzzing and flicking flowers will have poor results. 

As mentioned above, tomato flowers don’t need pollinating insects to set fruit. However, tomato plants can sometimes use our help to increase production.

“Everything the tomato needs to form fruit is right on that single flower, so it’s just about getting the mechanism to work under challenging conditions,” Craig says.

The window of opportunity to help a tomato flower self-pollinate is short. Tomato flowers will start out pale yellow, then all of a sudden they open and appear butter yellow. Once they open, there is a day or two of opportunity for pollination. If pollination is successful, the yellow flowers begin to shrivel up and the little tomato ovary is formed inside. 

Craig gets impatient and tugs at the shriveled petals, pulling them off to reveal whether or not there is an ovary there. (Sometimes he pulls off the whole flower and stem, ruining it, he admits.) When pollination doesn’t work, the area where the flower attaches to the stem, a little knuckle, turns yellow and eventually falls off.

“It’s not anything to do with disease. It’s not anything to do with the health of your plant. It’s just all about the conditions not being right for that particular flower to pollinate that day,” Craig says. “And because tomatoes, of course, produce lots of flowers, you can’t let a few dropped blossoms depress you.”

Though bees and other pollinating insects are unnecessary for tomato pollination, they are still helpful. Craig says if you find a tomato flower that looks chewed on, that means a bee has visited it and likely pollinated it. This could be an issue for a seed saver who wants to avoid unexpected hybrids, but for the average gardener, it’s no issue at all.  

“Don’t be alarmed if even in perfect conditions, some blossoms drop,” Craig advises. “Because if a plant is vigorously blossoming and producing a lot of tomatoes, it’s almost like the tomato plant says, ‘I can only do so many of these,’ and it will abort some flowers.”

Watering Tomatoes in High Heat Conditions

Internet sources often say the way to get the best-flavored tomatoes is to withhold water from your tomato plants, but Craig says this is an urban legend. Withholding water does not concentrate the flavor.

If you are growing in a traditional in-ground garden that holds moisture well, you can get away with not watering as often. But if you are growing in a container, raised bed, grow bag or straw bale, you need to water more often during periods of high heat and drought.

If your tomato plants looks visibly wilted in the middle of a hot day, it’s stressed and won’t be happy. 

“It will be a guaranteed invitation to blossom end rot,” he says. “You need to provide water that the plant needs when it needs it and forget about all of these other theories about ‘I’m gonna dilute my flavors.’ What you’re trying to do is keep your plant alive and healthy and producing, and that is the number one priority when it comes to watering.”

In fact, Craig says he’s had the best-flavored tomatoes of his life after growing them in containers or straw bales and watering them frequently. 

“If the plant is happy, it will produce the fruit that it is supposed to produce according to the genetics of that particular variety,” he says. “… The plant will take what it needs. It will just ignore what it doesn’t need, and the tomatoes will come out just fine.”

Using the “finger test” you can determine if your soil needs more water. If you stick your finger an inch or two into the soil and it comes out clean, the soil is too dry. Your finger should come out dirty as the moisture helps soil stick to your finger.

Craig waters his containers once a day with a garden house, counting to 30 seconds at each container before moving on to the next. 

Drip irrigation is an option that gives your garden little sips of water all day rather than a bug gulp daily. 

In-ground gardens can hold a lot more water than you’d expect, especially a garden with heavy clay soil. Yes, the weather may be hot, but the soil could still be waterlogged if you are watering too often. If the roots sit in water that is very slow to drain, your plants may wilt from drowning rather than dehydration. 

Whatever you do, avoid overhead watering and avoid splashing soil onto your plants. Water gently at the base of plants, taking care to ensure that the foliage doesn’t get wet. Wet foliage invites disease.

 

efficient water use

I love using emitter tubing, soaker hoses and drip irrigation in my garden and landscape. Plus, I use inexpensive timers to monitor when the water is on and off. It’s the ultimate in efficient irrigation. I can’t recommend it enough.

 

Preventing Blossom End Rot

A growing brown spot on the bottom of a growing tomato is not a disease, but blossom end rot. It’s a physiological problem caused by stress on the plant. It’s usually the earliest fruit on the plant that experience blossom end rot. 

If you’ve grown tomatoes before, chances are you have seen blossom end rot. There are a lot of solutions offered to prevent blossom end rot that aren’t based in reality. You’ll often hear that your plant needs more calcium, but the fact is it is rare for soil to be deficient in calcium. What really causes blossom end rot is inconsistent watering.

When growing tomatoes in containers or straw bales, watering regularly and frequently to prevent the growing media from drying out completely is essential. Raised bed gardens and in-ground gardens hold more moisture between waterings, so blossom end rot is seen less often but still happens. 

You can add mulch to a container to retain moisture between waterings, and Craig notes that some straw bale gardeners add an impermeable wrap around their bales to reduce water loss. 

When the growing medium dries out, the plants don’t have water available to help them take up calcium. When they can’t transfer calcium, blossom end rot occurs. You can add all sorts of calcium to the soil and apply blossom end rot eliminator sprays and see no results.

“If you’re not using regular watering, and if you’re allowing your plant to become stressed, the plant literally will not be able to access all of the calcium you’ve put there,” Craig says.

If there is a 100-degree forecast, check your plants at noon, he recommends.  If they are wilted, that’s a sign of stress. Give them a drink, even if that means watering twice a day.

Plants naturally fold their foliage to reduce their surface area in high heat, so a little bit of wilt is of course going to happen. But extending wilting of whole plants — you don’t want to see that in your garden, Craig says.

If you just pay attention to plants, it’s amazing what signs you can pick up on. 

 

Cherokee Purple Blossom end rot

Blossom end rot on a Cherokee Green due to stress from lack of needed water. (photo: Craig LeHoullier)

 

Added Disease Pressure in Extended Heat

During prolonged periods of hot weather, plants are more vulnerable to diseases — particularly in humid areas. 

You may have had great, healthy-looking tomatoes that suddenly experienced a bloom of Septoria leaf spot. Leaves become covered with brown speckles, which is alarming for tomato growers.

Septoria, early blight (it looks like concentric rings with a yellow aura around them, Craig says) and other disease agents thrive in humid weather. They can easily latch onto damp foliage and then proliferate. 

I can testify — living in the hot humid south of Atlanta, Georgia  — that after you get a few days of high humidity or after a rain shower, diseases are just popping up everywhere in the garden. Pathogens splash up from unmulched soil and ride in on the wind.

The lowest foliage, the inner foliage and the rear of the plant — where the sun doesn’t reach — are where fungal diseases tend to show first. Those areas stay wet the longest and don’t experience the disinfecting power of sunlight. 

Remove diseased foliage as you see it to prolong the life of your plants. If you don’t remove it, the disease can spread like wildfire — so don’t leave it till the weekend. Take care of it as soon as you notice it.

“It’s just one of the things that you have to endure to have as good a garden as possible, is to take on some of these tasks when it’s not terribly pleasant to do them,” Craig says.

 

Septoria Leaf Spot

Tomato plants are susceptible to foliar diseases in heat and humidity.

 

How to Lower the Temperature in the Garden

Shade cloth can reduce the temperature underneath it.

“What you’re trying to do is block the rays of the sun directly on the plant to create a micro-climate,” Craig says.

However, if you install shade cloth too tightly on plants with no openings for heat to readily escape, you can actually make the microclimate hotter rather than cooler. There should be openings at the ends for airflow, and there should be some space between the shade cloths and the plants.

Craig also advises determining the arc of the sun over your garden to help you recognize when the hottest part of the day will be so you can erect a shade cloth at the appropriate time.

Students in our joegardener Online Gardening Academy Growing Epic Tomatoes course have found a 5°F temperature reduction on average when using a 70% shade cloth 5 feet above the plants.

Another option is to build a hoop house to reduce the temperature and also reduce the rainfall onto plants. Reducing how much water comes into contact with the foliage will reduce occurrences of fungal diseases.

 

Shade cloth in a raised bed garden

Shade cloth reduces the temperature underneath, which can be welcome relief for tomato plants in excessively hot weather.

 

How to Overcome Fruit Cracking & Quick Ripening

High heat can accelerate ripening, and necessary frequent watering can lead to fruit cracking. 

To harvest tomatoes before they become overripe and/or crack, watch for the breaker stage. This is when the tomatoes are blushing and about half ripe. Once they reach this point, they will be able to ripen the rest of the way off the plant, indoors. Once the fruit are picked, you won’t have to worry about them cracking from taking up more water than they can handle. And you can also enjoy them before animals in your garden do. 

Tomatoes picked at the breaker stage will be just as flavorful as vine-ripened tomatoes.

 

Lillian's Yellow tomato

Lillian’s Yellow is prone to cracking, so it’s best to harvest them during the breaker stage. (photo: Craig LeHoullier)

 

How Gardeners Can Protect Themselves in High Heat

Craig says he can get so caught up in gardening that he can spend hours outdoors in the searing sun.

It’s important to wear sun protection, take breaks in the shade or indoors, and stay hydrated. Yes, gardening is fun and it is hard to pull ourselves away, but we have to think about our health and safety.

Arrhythmia, heat exhaustion, heat stroke and sunburn are a few of the risks of working outdoors for extended periods. 

Craig dresses lightly, wears a hat, uses sunscreen and brings bottles of ice water into the garden with him. He has a shady corner where he stops to cool off every half hour or 45 minutes. 

He says to be aware of what your body is telling you and be aware of your heart rate and whether you’re feeling okay.

“Just be safe out there,” he says. “We all want to have great gardens, but our great garden will be nothing without us to tend it. So you have to take care of yourself.”

I really enjoy tending my garden early in the morning with a coffee, when it’s cooler, the birds are singing and it’s otherwise quiet and peaceful. The end of the day is another option — perhaps with a glass of wine in hand. 

Wearing moisture-wicking synthetic fabrics rather than moisture-absorbing cotton shirts will also keep you cool.  

 

Joe Lamp'l watering plants

A baseball cap and a moisture-wicking shirt will help to keep you cool while working in the garden.

 

How to Take Care of the Garden While on Vacation

In addition to taking care of our physical health, we have to think about our mental health too, and that means taking a vacation every once in a while. As much as we hate to be away from our gardens, there will be times when we take a break and need to make other arrangements to keep it watered.

If you have a drip irrigation system, you’ll be in good shape. If you don’t, take a look at the weather forecast while you’ll be away. If it’s going to be cool and rainy, there may not be much for you to worry about. If it’s going to be hot and dry, there are watering arrangements to be made.

Even if your watering situation is taken care of by drip irrigation or Mother Nature, you will still need someone to pick the fruit that you can’t. This will mean teaching your garden’s temporary caretaker about the breaker stage. If your neighbor picks your tomatoes at the breaker stage and leaves them on your counter, they will be ripe and ready for you to enjoy when you walk back through the door. 

“I’ve had some of my vacations be filled with angst because I know that the garden is suffering while I’m away, and I know that the person who’s watching my garden does not have really good gardening savvy and confidence,” Craig says. “So you’re not really sure what you’re going to find when you get home.”

Craig has even left vacation and drove home mid-week, to harvest tomatoes, check on the garden, and bring tomatoes back to his vacation spot to eat.

“When people are really avid gardeners, they pour their hearts into this, and you don’t want to have one week when everything goes south ruin all of your gardening plans,” Craig says.

 

Basket full of tomatoes

It can be hard going on vacation knowing that you will be missing out on the bounty for your garden. Enlisting an educated friend or neighbor to take care of your garden while you are away will bring some peace of mind.

 

I hope you enjoyed my conversation with Craig LeHoullier. If you haven’t listened yet, you can do so now by clicking the Play button on the green bar near the top of this post.

How do you arrange for watering and harvesting when you are away from your garden? Let us know in the comments below.

Links & Resources

Some product links in this guide are affiliate links. See full disclosure below.

Episode 003: Growing Epic Tomatoes with Craig LeHoullier

Episode 004: Heirloom Tomatoes: Past, Present and Future with Craig LeHoullier

Episode 047: Tomato Seedling Mistakes with Craig LeHoullier

Episode 056: Tomato Care Checklist with Craig LeHoullier

Episode 064: Tomato Growing Season Lookback: Lessons Learned With Craig LeHoullier

Episode 066: Tomatoland: The Dirty Truth of the Tasteless Tomato, with Barry Estabrook

Episode 094: How to Start and Care for Seedlings Indoors: My Steps for Success

Episode 095: Tomato Seed Starting Update: Innovations and Inspiration, with Craig LeHoullier

Episode 099: Understanding Crop Rotation: The Basics and Beyond, with Jack Algiere

Episode 115: Understanding Tomato Diseases and How to Deal With Them

Episode 146: Catching Up With Epic Tomatoes Author Craig LeHoullier: Big Changes and New Opportunities

Episode 148: Gardening in Straw Bales: An Easy & Inexpensive Solution to Make Growing Food More Accessible for All

Episode 150: The Story of Seed Savers Exchange, with Co-founder Diane Ott-Whealy

Episode 173: Starting a New Tomato Garden: Lessons Learned, with Craig LeHoullier

Episode 195: Identifying and Controlling Garden Pests Organically

Episode 204: Hardening Off and Setting Plants Up for Success in Spring

Episode 208: Growing Epic Tomatoes: Our Just-Released Online Course Preview, With Craig LeHoullier

Episode 216: Tomato Disease Prevention & Control: Tried and True and What’s New

Episode 236: Gardening in Grow Bags: Answers to All Your Questions

Episode 238: Peat Moss: Examining the Challenges of Its Ongoing Use in the Face of Climate Change

joegardener Tomato Care Checklist free resource

joegardener blog: Busted – Top Five Tomato Growing Myths

joegardener blog: How Do I Grow Tomatoes

joegardenerTV YouTube: How to Top Tomatoes – What to Do When Tomato Plants Get Too Tall

joegardenerTV YouTube: Sunscald – What Happens When Tomatoes Are Overexposed

joegardenerTV YouTube: How to Save Tomato Seeds

joegardenerTV YouTube: The Ultimate Tomato Cage in 5 Simple Steps

joegardener Online Gardening Academy™: Popular courses on gardening fundamentals; managing pests, diseases & weeds; seed starting and more.

joegardener Online Gardening Academy Organic Vegetable Gardening: My new premium online course membership opens in 2023. Sign up for the wait list here.

joegardener Online Gardening Academy Beginning Gardener Fundamentals: Essential principles to know to create a thriving garden.

joegardener Online Gardening Academy Master Seed Starting: Everything you need to know to start your own plants from seed — indoors and out. 

joegardener Online Gardening Academy Growing Epic Tomatoes: Learn how to grow epic tomatoes with Joe Lamp’l and Craig LeHoullier. 

joegardener Online Gardening Academy Master Pests, Diseases & Weeds: Learn the proactive steps to take to manage pests, diseases and weeds for a more successful garden with a lot less frustration. Just $47 for lifetime access!

joegardener Online Gardening Academy Perfect Soil Recipe Master Class: Learn how to create the perfect soil environment for thriving plants.

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joegardenerTV YouTube

Growing a Greener World®     

GGW Episode 803: Epic Tomatoes with Craig LeHoullier

Craig LeHoullier: Heirloom Gardening for All

Craig LeHoullier Instagram

Dwarf Tomato Breeding Project

“Epic Tomatoes; How to Select and Grow the Best Varieties of All Time” by Craig LeHoullier

“Growing Vegetables in Straw Bales: Easy Planting, Less Weeding, Early Harvests” by Craig LeHoullier

The New York Times: “Are Your Tomatoes ‘Epic’? If Not, Here’s What You Should Be Doing”

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PittMoss – Our podcast episode sponsor and Brand Partner of joegardener.com Use promo code JOEGARDENER at checkout to receive 20% off your order!

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Disclosure: Some product links in this guide are affiliate links, which means we get a commission if you purchase. However, none of the prices of these resources have been increased to compensate us, and compensation is not an influencing factor on their inclusion here. The selection of all items featured in this post and podcast were based solely on merit and in no way influenced by any affiliate or financial incentive, or contractual relationship. At the time of this writing, Joe Lamp’l has professional relationships with the following companies who may have products included in this post and podcast: Rain Bird, Corona Tools, Milorganite, Soil3, Greenhouse Megastore, PittMoss, Territorial Seed Company, and TerraThrive. These companies are either Brand Partners of joegardener.com and/or advertise on our website. However, we receive no additional compensation from the sales or promotion of their product through this guide. The inclusion of any products mentioned within this post is entirely independent and exclusive of any relationship.

About Joe Lamp'l

Joe Lamp’l is the creator and “joe” behind joe gardener®. His lifetime passion and devotion to all things horticulture has led him to a long-time career as one of the country’s most recognized and trusted personalities in organic gardening and sustainability. That is most evident in his role as host and creator of Emmy Award-winning Growing a Greener World®, a national green-living lifestyle series on PBS currently broadcasting in its tenth season. When he’s not working in his large, raised bed vegetable garden, he’s likely planting or digging something up, or spending time with his family on their organic farm just north of Atlanta, GA.

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