If you live in a place with clear seasons, checking the weather forecast every day is a gardening habit that you should do every other year.
Changes in temperature that happen quickly in late spring or early fall can stunt growth, make flowers or fruit look funny, or, worst of all, kill your frost-sensitive plants before they should.
To make tomatoes as healthy as possible, give them lots of light and warmth. This is also true for other warm-season vegetables like peppers and eggplant.
Tomato plants are very sensitive to frost; even cool temperatures that don’t get below freezing can hurt tomato plants, young and old, in the long term.
Tomato plants are warm weather lovers. When nighttime temperatures dip below 50°F, it can spell trouble for your tomato crop. Cold stresses the plants, making them more susceptible to disease and reducing fruit set and yields. Knowing when to cover your tomatoes can help protect your plants and extend your harvesting season. In this complete guide, we’ll look at optimal temperatures for tomatoes, signs of cold damage, and best practices for covering your plants at night.
Optimal Temperatures for Tomato Plants
Tomatoes thrive when daytime temperatures are between 70-85°F and nighttime lows stay above 55°F. Once temperatures exceed 90°F during the day or drop below 50°F at night, tomato growth and production suffers.
Temperatures below 50°F prevent pollen from germinating and fertilizing the flowers. Without pollination and fertilization, the flowers wither and drop off without setting any fruit. Photosynthesis also decreases dramatically when temperatures drop into the 40s. This leads to reduced growth and smaller, lower quality fruit.
Cold nighttime temperatures early in the season can set back plant growth. Young plants are more susceptible to chill injury than established plants Mature plants can usually tolerate a light frost down to 32°F, but any temperatures in the 20s will likely kill the entire plant
Signs of Cold Damage in Tomato Plants
It’s easy to spot cold damage in tomato plants. Here are some telltale signs to look out for:
- Wilted, curled leaves that fail to recover during the day
- Bronzing and purple discoloration on leaves
- Flower drop without fruit set
- Smaller, misshapen fruit
- Cracked or catfaced fruit
- Split stems
- Increased susceptibility to disease and pests
Once you see these signs of cold stress, it’s time to take action to protect your plants at night.
When to Cover Tomato Plants
There’s some debate among gardeners about the ideal temperature to start covering tomato plants. Some say to cover any night below 50°F while others wait until closer to 40°F. The general guidance is
- Cover plants when lows are predicted to drop between 40-45°F
- Definitely cover plants when lows are forecast below 40°F
For best results, cover plants before sundown to trap residual heat from the soil and prevent shock to the plants Remove covers in the morning once temperatures climb above 40°F.
Row covers, hot caps, and cloches are the most common methods used to cover tomato plants. Let’s look at how each works:
Row Covers
Row covers are lightweight fabric sheets that drape directly over plants to create a protective barrier. The material allows air, light, and water to pass through while trapping heat. Row covers can boost temperatures by 2-4°F. Use hoops or tunnels to keep the fabric from touching the plants. Look for row covers rated to at least 25-30°F for cold protection. Remove covers during the day to prevent overheating.
Hot Caps
Hot caps are miniature greenhouses that fit over individual plants. These walled plastic domes easily increase temperatures by 5-10°F compared to uncovered plants. Ventilate the caps on sunny days to prevent scorching. Remove the caps completely once nighttime temperatures are reliably above 50°F.
Cloches
Cloches are bell-shaped glass or plastic covers that fit over one or more plants. They can increase temperatures by as much as 20°F! Make sure to provide ventilation to prevent extreme heat buildup during the daytime. Remove cloches when frost danger has passed.
When to Stop Covering Tomato Plants
It’s safe to stop covering tomato plants once nightly lows remain consistently above 50°F. This usually occurs 4-6 weeks after your last spring frost date. But keep an eye on long range weather forecasts. Unusual cold snaps can happen well into late spring. Be prepared to cover plants any night temperatures threaten to drop into the 40s.
The goal is to minimize nights below 50°F. But don’t stress if your plants experience a few chilly nights early in the season. Mature tomato plants can handle a little cold shock, as long as it’s not an extended cold spell or extreme lows.
Just remember, covers aren’t needed once nightly temperatures normalize above 55°F. At that point, your tomato plants will thrive in the warmer weather.
Tips for Covering Tomato Plants
Follow these tips to successfully cover and protect tomato plants from cold damage:
- Wait to transplant seedlings until nightly lows are at least 50°F.
- Harden off plants for 7-10 days before transplanting.
- Cover plants before sundown to retain residual heat.
- Use hoops or supports to prevent fabric from touching plants.
- Monitor weather forecasts and cover any nights below 45°F.
- Ventilate or remove covers during the day to avoid overheating.
- Stop covering once lows consistently stay above 50°F.
- Mulch around transplants to insulate roots and retain heat.
- Water plants well before covering to prevent dessication.
- Consider using heat retention technologies like Wall O’ Waters for extra protection.
Grow Frost Tolerant Varieties
Beyond covers, another strategy is to select inherently cold tolerant tomato varieties:
- Oregon Spring: reliable setter down to 45°F
- Glacier: sets fruit at 38°F
- Sub Arctic Plenty: one of the earliest to fruit
- Cold Set: starts flowering at 38°F
- Mountain Magic: thrives in cooler climates
- Legend: great flavor in cold conditions
These varieties can better cope with chilly spring nights, reducing the need for extensive covering. But all tomato plants still need protection when temperatures threaten to drop below 40°F.
Enjoy an Extended Tomato Season
Paying close attention to temperatures and properly covering your tomato plants enables an earlier start and longer harvest season. While tomatoes relish heat, they can still thrive when properly protected from cold snaps. Just be ready to cover plants anytime cold looms in the forecast. Your reward will be bountiful tomato harvests lasting from early summer through late fall.
How to Protect Mature Tomato Plants from Early Fall Frost
Just like young tomato plants need light and warmth to grow leaves, flowers, and fruit, older tomato plants need certain temperatures for fruit to ripen.
Between 68°F and 77°F (20°C and 25°C) is the best range of temperatures for tomato fruits to turn from green to red.
Temperatures below 50°F (10°C) can destroy some of the ripening enzymes within the fruit. When green tomatoes are out in these cold weather more often, they are less likely to ever ripen.
Tomatoes that are turning red on the vine won’t be hurt by the cold as much as green ones, but the quality of the fruit will suffer because it will lose flavor, become less firm, and last less time.
Add a Plastic Tarp
It will keep you a little warmer on colder nights if you put a plastic sheet on top of the fabric clothes.
When adding a plastic layer, make sure it is not touching any of the plants’ foliage. In freezing temperatures, any plastic that comes into direct contact with plant tissues will freeze.
What is the lowest temperature tomato plants can tolerate?
FAQ
How cold is too cold for tomato plants at night?
Do I need to cover tomato plants at 40 degrees?
What temperature do tomatoes need to be covered?
What is the lowest temperature tomato plants can tolerate?
Should you cover tomatoes at night?
It is important to keep in mind that when you are covering your tomatoes at night, it’s important to remove the covers during the day when temperatures are warmer. This will allow the plants to get some much-needed sunlight. Using fabric or pest-proof mesh cloth which will allow more air circulation.
Are tomatoes still good to eat after a frost?
Yes. However, if the tomatoes have frozen even a little, it’s advisable not to eat them raw since the texture of the fresh tomato will have changed significantly when thawed. You can either cook them immediately after picking or place them in the freezer, whole, diced, or mashed, to use for cooking at a later period.
When should you cover tomato plants?
You want to cover the plants when temperatures are expected to be between 40 and 50 degrees F during the day and 20 to 30 degrees F at night. If you expect low temperatures much lower than this, it’s best to harvest the fruit. Tomatoes don’t like cold weather, although they can survive during short cold snaps.
How do you keep tomato plants warm at night?
Using fabric or pest-proof mesh cloth which will allow more air circulation. Alternatively, you may like to grow your tomato plants in a greenhouse. This will keep them warm at night. You can also put a layer of mulch around the bottom of your tomato plants to help keep the roots warmer during the night.