Tomatoes love summertime, and it’s not just because of the long daylight hours. They crave warmth too. Tomato plants thrive in temperatures ranging from 65-80°F (18.5-26.5°C).
As fall gives way to cooler temperatures, tomatoes slow their growth and development.
Once a tomato or part of a tomato freezes, it’s game over. Treat the frozen fruit like you would treat tomatoes that you just put in your freezer. If you discover any part of the tomato thawed out, you will most likely need to throw out the whole fruit.
Just because frosty conditions arrive does not necessarily mean your tomatoes will freeze. Your tomato plant’s ability to ride out the frost depends on several factors.
In this article, we will address the following:
- What Frost Is
- Variables That Affect Frost Injury in Tomatoes
- Protecting Tomatoes From Frost
- Identifying Tomato Frost Damage
Let’s first identify what frost is.
What is Frost?
The Oxford Dictionary defines frost as:
“a weather condition in which the temperature drops below 0° Celsius (= freezing point) so that a thin white layer of ice forms on the ground and other surfaces, especially at night.” (source)
This is all true, but there’s more nuance to it.
The National Weather Service lists a couple of other conditions that need to align with cool temperatures in order for frost to form. These include clear skies and no more than a gentle wind. Local topography also plays a role since cold air collects in low places like valleys. (source)
A freeze, on the other hand, involves freezing temperatures, but the conditions do not necessarily result in frost.
The dew point needs to be below freezing for frost to occur. Air warmed by the sun during the day can typically hold more moisture than cold air. As the temperature falls during the night, the air can’t hold as much water.
The point at which the air lets go of water vapor is called the dew point, and water must change its form. It either turns into a liquid (condensation) or a solid (frost).
Tomatoes are affected by freezing and frost is a sign of freezing temperatures. The extent to which freezing temperatures affect your tomatoes depends on a number of variables.
Can Your Tomatoes Survive a Frost Event?
Just because frost develops outside doesn’t necessarily mean the tomatoes will freeze. The tomato’s ability to survive frost depends on the following factors:
Some tomatoes withstand cold better than others. Growing a cold-hardy variety could give you the edge in the fight against frost.
Oregon Spring, for example, beats out other varieties when it comes to growing in the cool spring temperatures, though you still need to take precautions on frosty nights.
The Siberian variety is a great slicing tomato from Russia that manages to fruit set even in temperatures around 40°F/4°C.
A strong plant is less likely to freeze than an unhealthy, weak one. Though, new growth is more susceptible to cold injury.
Wet soil stays warmer than dry soil, and heavier soil retains water better than sandy soils. No matter what type of soil you have, give it a good soak before the freezing temperatures hit.
The presence of mulch and/or plant cover
Mulch and plant covers help tomato plants retain heat generated by the ground to prevent damage during a frost. Mulch is the most helpful in preventing late spring frosts as plants are closer to the ground.
How long the freezing temperatures last
The longer the freezing temperatures last, the more likely a tomato plant will freeze. Even with lower temperatures of 41°F/5°C for longer than a week or so, tomatoes can experience invisible damage that makes storage difficult. (source)
If you wake up to crystallized grass, you can surmise that the air temperatures dipped below freezing during the night. However, sometimes the air only reaches freezing at ground level, sparing the tomatoes that are more than a couple feet off the ground. (source)
Frost is usually accompanied by no more than light winds, but moderate to severe winds exacerbate freezing conditions whether or not frost is present.
If your frost or freeze is brief and not extreme, and your plants are operating under optimal conditions, it’s likely your tomatoes can stay on the vine a little longer.
How Do You Protect Tomatoes From Frost?
There are several strategies to deal with frosty, freezing conditions in the forecast:
For light, brief frost periods, mulch could provide enough protection. This strategy helps retain heat in the soil, roots, and base of the stem.
Wet soil retains heat better than dry soil. Therefore, water the plants well before the cold weather hits.
Use sheets, blankets, or plastic, and prop the covers up with stakes for support. The idea behind this is to retain the heat generated by the soil. If the weather conditions are too cold, covers will not be enough. Remove covers in the day so plants can absorb sun rays.
Add a light bulb.
If you’re concerned that the sheets alone will not keep the tomato plants above freezing, consider adding at least one light bulb under the covers as an additional heat source. (source)
It’s possible the weather will be too harsh and you can harvest the full-sized, light green tomatoes and let them finish ripening indoors over the course of several weeks. They won’t be quite as delicious as vine-ripened tomatoes, but they will be at least as tasty as tomatoes purchased from the store.
Missouri Botanical Garden recommends storing extra green tomatoes in a cool location with each tomato individually wrapped in paper and placed inside a larger paper bag. Then take out a few green tomatoes at a time to ripen in a warm room as you’re ready to eat them. Temperature is the key to ripening. (source)
Harvest the green tomatoes.
If the weather forecast looks too threatening and the tomatoes are not developed enough to ripen indoors, New Mexico State University suggests eating them as fried green tomatoes or canning them to make a green tomato relish. (source)
With a cold-hardy plant, extending the season a bit could be worth the effort. Other varieties that depend more heavily on warm weather to develop might not benefit much from the extra time you can buy them.
What Does Tomato Frost Damage Look Like?
Frost in both spring and fall can damage tomato plants.
Frost damage on stems and leaves appears as dark areas that later wither. If you’re unsure, damage becomes more noticeable the day after frost. (source)
Frost damage on tomatoes themselves results in lost vibrancy, browning, and shriveling.
Chill injury can also occur with temperatures less than 55°F/13°C, resulting in contorted fruit. Temperatures less than 50°F/10°C cause poor fruit set. (source)
Of course, if you have a variety like Siberian which can fruit set in 40°F/4°C weather, your story might be different. However, no matter how hardy your tomato variety is, it still can’t recover from being frozen.
Ultimately, frost is a sign of freezing temperatures which can damage tomato plants. However, underperformance is common with even cool conditions. If you live in a colder climate with a short growing season, consider varieties that mature quickly and are cold hardy.
If you’re hit with a surprise spring frost, do your best to keep your new plants warm.
Be aware of the first and last frost dates in your location. If you’re not sure when they’ll take place, the Almanac will inform you on the dates to expect them. Just type in your zip code.
May your growing seasons be lengthy and warm!