For the second spring in a row, I’m stuck with tomato plants indoors in May because the overnight temperatures are still too low for planting out.
In case you’re in the same boat and wondering what’s the minimum overnight temperature for planting tomatoes? I’ve put together a quick guide to help you out.
Let’s take a look.
What’s The Minimum Overnight Temperature Tomato Plants Will Tolerate In Springtime?
When you plant tomatoes outdoors (or in an unheated greenhouse or polytunnel) you need to make sure the threat of cold weather has passed.
Tomatoes thrive in warm weather. They don’t like the cold at all.
If there’s one thing I’ve learned over the years, it’s that springtime weather can catch us out, so it’s better to wait than to rush ahead.
Often spring days are sunny and warm, and you think, great I can plant my tomatoes out at last! But if your nighttime temps are too low, you’ll damage your tomato plants.
- Your daytime temperature should be at least 65°F (18°C).
- Your overnight temperature should be at least 55°F (13°C).
- Your soil temperature should be at least 60°F (15.5°C).
This isn’t just for planting day, this is for the entire season! Check your weather forecast for the next couple of weeks to get a general idea of what your overnight lows are going to be.
Weather forecasts are unreliable the further out you go, but the nighttime lows should be more reliable than the forecast for rain or sunshine.
What Happens To Tomato Plants When The Overnight Temperature Is Too Low?
When young tomato plants are exposed to cool temperatures in the spring, they’ll suffer setbacks.
- If the temperature is far too low, the tomato plants will die.
- Lower than optimal temperatures can result in stunted growth, reduced flowering, and deformed fruit.
- Flowering tomato plants exposed to low temperatures will suffer poor pollen formation and flowers will drop off.
Even when the temperature isn’t low enough to kill your tomato plants, low temperatures can still cause damage that lasts all season.
Learn more: How to germinate tomato seeds faster.
What Temperature Kills Tomato Plants?
Tomato plants will survive temperatures that drop as low as 33°F (0.55°C).
Below 33°F, tomato plants will suffer from frost damage. Watch out for cold wind at slightly higher temperatures too.
On frost burned tomato plants, stems and foliage will be a darker color. The entire plant will wilt and the stem and leaves will turn brown. The damage might not be apparent right away, but if your plant has been damaged, you’ll definitely see browning by the following day.
Can Tomato Plants Come Back After Frost Damage?
If all the foliage is brown, it’s unlikely that the plant can recover.
If the growing tip wasn’t killed, your tomato plant could shake off the damage as long as it isn’t subjected to another chilly night.
Sometimes the lower leaves will escape frost damage because they’re closer to the warm soil. Then it’s possible the plant will continue to grow from a side shoot even if the growing tip died.
Let damaged leaves drop off naturally.
If the stem is damaged below the lowest set of leaves, the plant doesn’t really have a hope of recovery.
Growth Problems In Tomatoes Caused By Low Temperatures
Don’t make the mistake of thinking your tomatoes will be fine if your nights are slightly above 33°F (0.55°C).
Even when tomato plants don’t die after exposure to cold temperatures, they won’t thrive. Growth will be slower and fruit production will suffer.
When it’s too cold, your tomatoes will simply wait for the warmer weather to arrive before they grow. You don’t gain anything by planting out too soon. Instead, you’re risking plant damage and a smaller crop.
But when you plant tomatoes in the right conditions, your tomatoes will grow like gangbusters.
Most years I get volunteer tomatoes spring up from fruit that dropped and rotted the previous year. Sometimes I leave them to grow if they’re not in the way of other plants.
I usually notice them at the end of May or early in June. And in the warm weather they quickly catch up to the plants I’ve sown, potted up, transplanted and fussed over since March.
Holding your tomatoes back a few weeks until the weather is warm enough won’t hurt them. Just make sure you re-pot them into larger pots with fresh compost or give them a feed.
Will Low Overnight Temperatures Really Make That Much Difference?
I know how tempting it is to plant tomatoes outdoors once the days are warm and sunny. But, if your overnight temps are too low, you’ll wish you waited.
A few days ago I was out in the glorious afternoon sunshine weeding my onion beds and I noticed how warm the moist soil was. Immediately, my thoughts turned to the tomato plants patiently waiting indoors under grow lights.
Even though I know better, I was almost tempted to push my luck and plant them out.
Planting tomatoes out too early is an easy mistake to make. The damage from sub-optimal temperatures between 33°F to 41°F (0.5°C to 5°C) will result in stunted growth, wilted plants, pitted foliage and leaf loss. Tomato plants exposed to chilling damage are also more susceptible to disease.
Colder nighttime temperatures result in decreased photosynthesis during the day. Photosynthesis is the process plants use to turn sunlight into energy. With less energy available, plants can’t develop, strong root systems and overall plant growth will be slower.
Damage caused by nighttime temperatures below 50°F (10°C) in the 4 to 5 weeks before your tomato plants form flower clusters can result in catfaced tomatoes.
This is what a catfaced tomato looks like. Ugly as they are, you can still eat them.
When night time temps are below 60°F (15.5°C), tomato plants mature enough to flower will put out lots of flowers – lots of flowers that fail to set fruit.
Minimum Overnight Temperature For Planting Tomatoes Chart
| °F | °C | Effect On Tomato Plant |
|---|---|---|
| Below 33°F | Below 0.55°C | Frost damage – wilting, browning, leaf drop. Plants will usually die. Slight chance of recovery. |
| 33°F to 41°F | 0.55°C to 5°C | Chilling damage – stunted growth, wilting, leaf loss, increased disease susceptibility. |
| Below 50°F | Below 10°C | Blossom drop, misshapen fruit, catfacing. |
| Below 60°F | Below 15.5°C | Profuse flowering with failure to set fruit. |
How Can You Protect Tomato Plants From Cold Nights?
Sometimes, even when you’ve waited for the cold nights to pass before planting tomatoes outdoors, an unexpected late frost or cold snap can really scupper your tomato growing efforts.
So it’s best to be prepared.
Know Your Weather
Check your weather forecast every day for the overnight low. That way, you won’t get caught by surprise and you’ll have time to protect your plants in the evening before the temperature drops.
Protect
Horticultural fleece (floating row covers) will keep frost off your tomato plants. The lightweight insulator also traps some of the heat rising from your soil, which creates a slightly warmer microclimate for your plants.
Another easy way to protect smaller tomato plants from a late frost is by using a large bottle or milk jug as a cloche. Just cut the bottom off a milk jug or large plastic bottle and pop it over the tomato plant.
Warm
Placing bottles of hot water around your plants (under the fleece) will raise the temperature of the microclimate. The water bottle shouldn’t touch the plants, though.
Any kind of plastic bottle will work. It’s always handy to keep a stash of empty 2 liter water bottles around – they’re perfect for setting among your plants on a chilly night.
Rocks can also help to make a warmer microclimate at night around plants. Rocks aren’t a last-minute thing you can nip out and set up, though. You’ll need to get hold of rocks ahead of time and set them around your plants to absorb heat from the sun during the day and slowly release the warmth at night.
Plant An Early Tomato Variety Instead Of Planting Too Soon
Another option for dealing with the uncertain temperatures in the spring is to plant an early tomato variety as well as your main crop variety.
The main reason we’re so eager to get our tomatoes planted outdoors is so we can start harvesting delicious home-grown tomatoes as soon as possible.
But as you now know, risking cold nights will do more harm than good. A great way to get an earlier harvest, is by growing an early variety that gives you ripe tomatoes 50 to 60 days after transplanting.
- Siberia – 50 days
- Early Girl – 52 days
- Beaverlodge – 54 days
- Early Wonder – 54 days
- Red Robin – 54 days
- Stupice – 55 days
- Latah – 55 days
Tiny Tim – 50 days. This is a super small dwarf cherry tomato, you can easily grow in a pot. Just pick it up and bring it indoors if it’s going to be a cool night.
Minimum Overnight Temperature For Planting Tomatoes Recap
- Your daytime temperature should be at least 65°F (18°C).
- Your overnight temperature should be at least 55°F (13°C).
- Your soil temperature should be at least 60°F (15.5°C).
Below 33°F (0.55°C) tomato plants will suffer from frost damage.
Overnight temperatures between 33°F to 41°F (0.5°C to 5°C) can often result in stunted growth, wilted plants, pitted foliage, leaf loss, and increased disease susceptibility.
Damage caused by nighttime temperatures below 50°F (10°C) includes blossom drop, catfacing, and misshapen fruit.
A burst of cooler temps when your tomatoes are mature enough to flower can result in a heavy flowering, with those flowers failing to set fruit.
Before you plant your tomatoes outdoors, make sure the cool nights have passed and have an emergency protection strategy ready in case there’s a sudden drop in overnight temps after planting.
Thanks for reading: What’s The Minimum Overnight Temperature For Planting Tomatoes?
