how to keep tomato plants warm

Every year, gardeners get excited to start their tomato plants, in the hopes of a great harvest. However, if the weather does not cooperate, a frost in late spring or early fall can damage your plants or fruit. So, what is the lowest temperature tomato plants can tolerate? Tomato plants can tolerate temperatures above freezing (over 32 degrees Fahrenheit) and survive. A temperature of 32 degrees Fahrenheit or below will result in a frost or freeze that will kill unprotected tomato plants. A temperature below 40 degrees Fahrenheit will cause stunted growth, wilted leaves, and pitting of fruit on tomato plants. Any temperature below 50 degrees Fahrenheit will lower pollen production. Any temperature below 55 degrees Fahrenheit will cause dropped flowers and decreased fruit quality. Of course, there are some ways to protect your tomato plants from the cold, both at the start and the end of the growing season. In this article, we’ll talk about how much cold tomato plants can tolerate, and how they are affected at different temperatures. We’ll also look at ways to protect young tomato plants from spring frost and mature tomato plants from fall frost. (By the way, you can check out all kinds of cold protection resources on this page.) Let’s get going. What Is The Lowest Temperature Tomato Plants Can Tolerate? Tomato plants can tolerate temperatures above freezing (over 32 degrees Fahrenheit, or 0 degrees Celsius). However, temperatures of 32 degrees Fahrenheit or below can result in a frost or freeze, which can kill unprotected tomato plants. A tomato plant can survive temperatures above freezing, but temperatures below 55 Fahrenheit can cause damage. Although tomato plants can survive temperatures above freezing, they will still suffer negative effects from exposure to temperatures below 55 degrees Fahrenheit (13 degrees Celsius). When it comes to cold, the temperature and duration of the cold will determine how tomatoes are affected. The longer a tomato plant is exposed to cold temperatures, the worse the effects will be. The table below summarizes the temperatures that will negatively impact tomato plants and the effects you might see: TemperatureRange(degreesFahrenheit)EffectsBelow 32(frost orfreeze)Brief exposureleads to deathof plant and fruit damage.33 to 40Long exposurecauses chillinginjury: stuntedgrowth, wiltingleaves, and pitson fruit.41 to 50Long exposurecauses chillinginjury. Alsolowers pollenproduction inflowers.51 to 55Long exposurecauses stuntedgrowth, loweryields, flowerdrop, and poorfruit flavorand texture.Effects of various temperatures on tomato plants. You can also see my illustration of ideal temperature ranges for tomato plants here. At 32 degrees Fahrenheit or lower, a frost or freeze occurs. According to the Colorado State University, frost will kill most warm-weather plants, including tomatoes. Frost is not good for tomato plants - they will only survive with cold protection, such as row covers or cloches. After freezing damage, the leaves and stems of your tomato plant will get darker. Later, the damaged parts will wilt and turn brown. You may be able to provide cold protection to help tomato plants survive a frost or freeze on a cold night (more on this later). At 40 degrees Fahrenheit or lower, tomato plants will sustain chilling injury after prolonged exposure. The effects include stunted growth of plants, wilting of leaves, and pitting of fruit. The plant will also be less hardy and more susceptible to disease. At 50 degrees Fahrenheit or lower, flowering tomato plants will produce less pollen, decreasing fruit set and yield. Also, you may see catfacing (a scarring deformity) of fruit that forms later in the season. You can learn more about catfacing and other common tomato deformities, along with what causes them, in my article here. At 55 degrees Fahrenheit or lower, tomato plants will begin to experience stunted growth and lower yield of fruit later in the season. Vivipary (sprouting of seeds inside tomato fruit) may also occur. According to Penn State University, flowers may also drop off of the tomato plant. According to the University of Illinois Extension, the texture and flavor of the fruit will decline at these temperatures. Cold soil may also prevent tomato plants from absorbing the necessary nutrients. For example, cold soil may cause a phosphorus deficiency in tomato plants, which causes their leaves to turn purple. Is 45 Degrees Too Cold For Tomatoes? A temperature of 45 degrees Fahrenheit (7.2 degrees Celsius) may not cause severe, immediate damage to your tomato plants, especially if you protect them. However, it can cause them to produce less pollen during the flowering stage. Cold temperatures can cause tomato flowers to produce less pollen, which will reduce fruit yield later in the season. This will reduce pollination and fruit set, decreasing your tomato yield later in the season. When tomatoes do appear, you may see catfacing (a scarring injury of the fruit, thought to be caused by cold exposure). You can prevent poor pollination by keeping your tomato plants just a bit warmer. You can do this with row covers (more on this later). Is 40 Degrees Too Cold For Tomatoes? A temperature of 40 degrees Fahrenheit (4.4 degrees Celsius) will cause damage to your tomato plants. However, it will not kill them directly. Instead, prolonged exposure to this temperature will cause chilling injury, reducing the vigor of the plant. This will decrease the plant’s hardiness, making it more susceptible to disease. Chilling damage will also cause stunted growth, wilted leaves, and pitted fruit. If the weather calls for an extended period of temperatures at 40 degrees Fahrenheit, it might be a good idea to harvest your fruit. You can protect tomato plants from some of the cold damage that occurs at temperatures of 40 degrees Fahrenheit or below. This will require the use of protective measures, such as row covers (more on this later). At What Temperature Should I Cover My Tomato Plants? Row covers offer 3 to 5 degrees Fahrenheit of cold protection. According to the University of Idaho Extension, they are good for protecting against short burst of frost, but not a prolonged freeze. If temperatures threaten to drop down to 27 to 30 degrees Fahrenheit for one night, a row cover just might save your plants. Adding cloches to cover young plants can provide even more cold protection (more on this later). If temperatures threaten to get much colder than 27 degrees Fahrenheit, it might be time to harvest your tomatoes and let them ripen indoors. Since prolonged temperatures below 55 degrees Fahrenheit can cause damage, you can consider covering tomato plants when nighttime temperatures consistently dip to 50 degrees Fahrenheit or below. How To Protect Tomato Plants from Cold & Frost Now that we know when to protect tomato plants against cold and frost, it’s time to talk about how you can do it. It starts at the beginning of the season, with choosing the right tomato varieties. Choose The Right Tomato Varieties First things first – that means choosing the best tomato varieties for your particular climate. Depending on where you live, the USDA plant hardiness zone map will tell you what zone you are in. A lower zone number means that it will be more difficult to grow tomatoes that need a long season. If you live in a colder region with a short growing season, consider cold tolerant and fast-maturing tomato plants to help you stay ahead of Mother Nature. Choose the right tomato variety and your plants will resist cold and produce fruit before the growing season ends. The only drawback is that cold tolerant and fast-maturing tomato varieties tend to have smaller fruit. So, if you want larger tomatoes, you might need to build a greenhouse! Cold Tolerant Tomato Plants A little research will reveal some cold-tolerant tomato varieties that can thrive in your region – even when cold threatens. Here are a few cold-tolerant tomato varieties: Cold Set – this cold-tolerant tomato is determinate and ripens in 65 days, producing red tomatoes that are 4 inches wide. You can learn more about Cold-Set tomatoes on the Reimer Seeds website.Glacier – this cold-tolerant tomato is determinate and ripens in only 55 days, producing red tomatoes that weigh 2 to 3 ounces. You can learn more about Glacier tomatoes on the Reimer Seeds website.Northern Delight - this cold-tolerant tomato is determinate and ripens in 65 days, producing red tomatoes that weigh 1 to 2 ounces. You can learn more about Northern Delight tomatoes on the Reimer Seeds website.Polar Baby - this cold-tolerant tomato is determinate and ripens in 60 days, producing deep red tomatoes. You can learn more about Polar Baby tomatoes on the Reimer Seeds website.Polar Star - this cold-tolerant tomato is determinate and ripens in 65 days, producing red tomatoes that weigh 3 to 4 ounces. You can learn more about Polar Star tomatoes on the Reimer Seeds website. Fast Maturing Tomato Plants If you would rather stay ahead of the cold by growing your tomatoes quickly, then you can pursue that option. Here are a few fast-maturing tomato varieties: Fourth of July - this fast-maturing indeterminate tomato produces mature fruit in only 49 days, producing lots of red tomatoes that weigh 4 ounces. You can learn more about Fourth of July tomatoes on the Burpee website.Siberian – this fast-maturing heirloom tomato from Siberia, Russia is determinate. It can set fruit at temperatures of 48 degrees Fahrenheit! The fruit ripens in only 48 days, producing lots of bright red tomatoes that weigh 2 to 5 ounces. You can learn more about Siberian tomatoes on the Reimer Seeds website.Sub-Arctic Plenty – this fast-maturing tomato is determinate and ripens in just 42 days, producing clusters of red tomatoes that weigh 2 to 4 ounces. You can learn more about Sub-Arctic Plenty tomatoes on the Reimer Seeds website.Tiny Tim – this fast-maturing tomato plant only grows to about 1 foot high. It yields mature fruit in 45 days, producing small globe-shaped red tomatoes that are about one-half inch across. You can learn more about Tiny Tim tomatoes on the Victory Seeds website. Planting Schedule For Tomato Plants After you decide on the tomato varieties you want to grow, it will be time to consider your planting schedule. First, look at the time to maturity for your plants (you can find it on the seed packet or on the website where you bought the seeds from). You will also need to find the frost dates in your area. The Farmer’s Almanac has a tool that you can use to find the dates for your region. For example, let’s say I want to grow Fourth of July tomatoes, which mature in 49 days. The first chance of frost in my area (just south of Boston, MA) is October 4th. So, I will want to harvest all of my tomatoes by October 3rd. Let’s assume four weeks’ worth of harvest for my tomatoes (28 days). So, I will need 77 days (49 days for the plants to mature, and 28 days of harvest). Then, I work 77 days backwards from there: 3 days in October, 30 days in September, 31 days in August, and 13 days in July. That means I would want to plant my tomato seeds by July 18 at the absolute latest. Of course, my tomato plants would not do very well if the nighttime temperatures get into the 40s towards the end of the season. Planting earlier than this would be much better for my harvest. In fact, you can transplant tomato plants outdoors in May where I live. For more information, check out my article on when it is too late to plant tomatoes. Start Tomato Seeds Indoors For tomato seeds, the soil temperature needs to be 50 to 95 degrees Fahrenheit to achieve germination (65 to 85 degrees Fahrenheit is ideal). However, the soil in your garden may not get that warm until later in the season. You have two choices to bypass this limitation: Buy established tomato plants from a garden centerStart your own tomato seeds indoors For the second option, the question is: when should you start the seeds? It will depend in part on the variety of tomato plant you choose. However, a good guideline is to start the seeds indoors 6 weeks before the last chance of frost, and transplant them 2 to 3 weeks after the last chance of frost. This means that the plants will be indoors for 8 to 10 weeks total. Start tomato seeds indoors 6 weeks before the last frost date, and transplant them outdoors a few weeks after the last frost date. You can find the last frost dates using this map from the Farmer’s Almanac. For instance, say the last frost date in my areas is May 9, and I would like to start my tomato seeds indoors 6 weeks (6x7 = 42 days) before then. Working backwards 42 days, we find: 9 days in May, 30 days in April, and 3 at the end of March, for 42 days total. That means I would want to start my tomato seeds indoors on March 28 (31 days in March – 3 days at the end of March = March 28). Remember: the soil temperature should be kept at 65 to 85 degrees Fahrenheit for ideal tomato seed germination. Now, let’s assume I want to move the seedlings outside 3 weeks (3*7 = 21 days) after the last frost date. Adding 21 days to the last frost date (May 9), we get a date of May 30 (9 + 21 = 30). One last thing to remember about seedling care: you can either use artificial lights or sunlight from a window to grow your plants. If you put tomato seedlings near a window, make sure they don’t get a cold draft from a leaky window. Otherwise, tomato growing season might end before it really begins! Watch Weather Forecasts Before Transplanting Tomatoes You might be doing a great job when it comes to choosing tomato varieties and your planting schedule. However, nature may have something different planned. The frost dates are usually fairly reliable. However, they are only an average. Every so often, a year with strange weather patterns will bring spring frosts much later than average. For that reason, you should plant to track short-range and long-range weather forecasts for your area. If a period of unseasonable cold is approaching, keep the tomato seedlings indoors for a little longer before transplanting outside. Apply Cold Treatment To Tomato Plants (Harden Off) You can also use cold treatment to toughen up your tomato plants before transplant. The goal here is to briefly expose them to cooler temperatures than they would like (not freezing or damaging temperatures!). Ideally, you should place the tomato plants in a spot with lots of sun and a temperature in the 50’s Fahrenheit) for a few weeks. The cold temperature encourages tomato plants to put more energy into developing roots, stems, and branches to support later growth. In the short term, cold temperatures will slow the growth of a tomato plant. Over the long term, cold treatment yields you more flowers and tomatoes. It also makes the plant stronger and more resistant to cold at the other end of the season, when fall frost threatens. If you live in an area with a short growing season, there might not be time for cold treatment. However, if you enjoy a long growing season and want to increase your tomato harvest, cold treatment is an experiment you can try. Cold treatment is one part of a broader plant strengthening regimen called hardening off. The goal is to make plants cold hardy and to help them adapt to outdoor conditions after being started indoors. You can learn more about hardening off seedlings in my article here. Use A Cold Frame For Tomato Seedlings In some cases, you would like to move your seedlings outdoors a little sooner. In other cases, your spouse yells at you for keeping too many plants indoors. In either situation, a cold frame will help to keep tomato plants warm. A cold frame is a short structure, often made of wood. It has a glass top, which means that it acts like a mini greenhouse. Some cold frames open automatically when the temperature gets high enough, and then close again when it gets low enough. When your seedlings are mature, you can put them directly in the cold frame. Before you do that, install a thermometer in the cold frame and monitor it every day to make sure it stays warm enough for the seedlings. You can also put transplants in a cold frame to toughen them up with cold treatment and hardening off. Always remember to watch for frost in the weather forecast! Use A Greenhouse For Taller Tomato Plants When it comes to protecting your tomato plants from cold during the growing season, a greenhouse is one of the best options out there. You can your own greenhouse or hire help to do it for you. Another option is to buy a pre-fabricated greenhouse. A greenhouse is a good way to keep both young and mature tomato plants warm during cold weather. Before putting transplants in the greenhouse, monitor the temperature with a thermometer. If nighttime temperatures inside the greenhouse are consistently above 55 degrees Fahrenheit, then you can think about making the move from indoors to the greenhouse. It it is any colder, you might want to wait (or use an extra cold protection measure, such as cloches). Once you move your tomato plants to the greenhouse, keep the door and any vents closed at night to trap heat inside. Otherwise, you might lose your tomato plants on a single cold night! Remember that high temperatures can also adversely affect tomato plants. A greenhouse is more than capable of generating stifling hot temperatures on sunny days, so keep an eye out. Watch the weather forecast often. If it calls for a hot, sunny day, then leave the greenhouse door open before you leave the house. Use Cloches For Younger Tomato Plants A cloche is a cheaper, easier alternative to a greenhouse or cold frame. A cloche is a cover used to protect plants from cold, pests, and wind. Historically, a cloche was made of glass, shaped into a bell-like dome to cover plants. It was used to keep plants warm and extend the growing season for cold-tolerant plants (like tomatoes) to get a better harvest. You can use a plastic bottle (of any size) with the top removed as a cloche to keep tomato plants warm. However you don’t need to get as fancy as that. You can use a homemade plastic cloche to protect your plants from cold. Another option is a wire cloche, often used to protect plants from garden creatures (such as rabbits). However, a wire cloche will not keep plants warm, unless you wrap a blanket or row cover over it. The top part of a cloche should have a ventilation hole. This hole will let your plants breathe on a hot day by allowing extra heat to escape. If you would like to make a cloche, start by collecting empty plastic gallon containers of milk or water (one per plant). Then, cut out the bottom of each container, and put one container over each plant. The cap on the top part of the gallon container will act as the vent. Take the cap off if the day will be extra hot. The only drawback of using a cloche in the garden is that your tomato plant will soon outgrow it. However, a cloche is a great way to keep plants warm if they are transplanted early in the season, or if unexpected cold comes to your area. You can learn more about cloches and how to use them in my article here. Use Row Covers For Taller Tomato Plants Sometimes, your tomato plants are too tall for cloches, but they will still need cold protection. This is likely towards the end of the season, as fall frosts approach. In this case, row covers will help to keep your plants warm. The most basic way to use a row cover is to simply drape them over your plants. Another way is to bend a flexible plastic rod into a half-circle. Then, stick each end into the ground on opposite sides of the row of plants. A row cover hanging over plastic hoops will help to protect an entire row of tomato plants from cold (and insects!) Once you have enough of these half-circles along the row of plants, hang a row cover over the hoops. This is sometimes called a low-tunnel or high-tunnel (depending on the height), and it is a good alternative to cloches for younger plants. Row covers are great because they can protect your plants from pests at the same time that they protect from cold. Just be sure to take them off if it gets too warm – this will help to keep your tomato plants from overheating. If you decide to use a plastic tarp instead of a cloth row cover, be careful about putting it directly over plants. Otherwise, the leaves or stems will freeze wherever they come into contact with the plastic (due to freezing condensation). To prevent this problem, drive stakes into the ground around your tomato plants, and then hang the plastic tarp over the stakes. The stakes you use to hold up a tarp or row cover will need to be at least as tall as the tomato plants. Whether you use tarps or stakes, you will need to weigh down the edges so that the wind doesn’t blow them out of place. Otherwise, you won’t be able to trap heat underneath the covers, and there won’t be much benefit to your plants. Conclusion Now you know how cold tomato plants can tolerate, and how various cold temperatures affect tomato plants. You also know when to protect your tomato plants from cold and how to do it. If you are preparing for the upcoming gardening season, check out my article on what to do before planting tomatoes. You can check out all kinds of cold protection resources on this page. I hope you found this article helpful. If so, please share it with someone who can use the information. If you want to read some of my most popular posts, check out the “Best of GreenUpSide” page here. Enjoy! ~Jonathon