If you grow tomatoes, you know they love warmth. But what is too cold for tomato plants? Understanding this is key to protecting your investment from frosty temperatures that threaten survival.
Tomatoes are tropical natives. They simply cannot handle cold weather. Even a slight chill can stunt their growth and ruin your harvest.
This guide will give you the exact temperatures to watch for. You’ll learn how to protect your plants and recover them if they get too cold.
What Is Too Cold For Tomato Plants
This is the core question. The answer depends on the plant’s stage of growth and the duration of the cold.
For all stages, the danger zone begins at 50°F (10°C). Below this, problems start.
The Critical Temperature Thresholds
Here’s a simple breakdown of temperatures and their effects:
- Below 50°F (10°C): Growth slows dramatically. Pollen fails to develop properly, leading to blossom drop and no fruit. Plants become stressed and vulnerable to disease.
- 40°F to 50°F (4°C to 10°C): Chilling injury occurs. Leaves may turn purple or develop a bronze sheen. Recovery is slow, and the plant will be set back for weeks.
- 33°F to 40°F (0.5°C to 4°C): Light frost damage. Leaves and tender stem tips turn black and wilt. Fruit can be damaged, leading to rot.
- 32°F (0°C) and Below: A killing freeze. Ice forms inside plant tissues, causing cells to rupture. This damage is usually fatal, especially for young plants. Mature plants may lose all foliage.
Remember, soil temperature matters just as much as air temperature. Cold soil, below 60°F, prevents roots from taking up nutrients like phosphorus. This is why leaves sometimes turn purple in cool springs.
Why Cold is So Damaging
Tomatoes aren’t built for cold. Their cell sap is mostly water. When it freezes, ice crystals form and literally shred the cell walls from the inside out.
Cold also shuts down their metabolic processes. They can’t photosynthesize or move water effectively. This stress invites fungal and bacterial diseases to take hold.
Protecting Tomatoes from Cold & Frost
You have several effective tools to shield your plants. The best method depends on how cold it will get and for how long.
Before You Plant: Smart Timing
Prevention is the best strategy. Don’t rush your tomatoes into the ground.
- Wait until nighttime temperatures are consistently above 50°F.
- Check your local average last frost date, but use it as a guide, not a guarantee. Weather can be unpredictable.
- Warm the soil for 1-2 weeks before planting by covering it with black plastic. This gives seedlings a much better start.
Covering Your Plants (The Most Common Method)
When a cold night is forecast, cover your plants! This traps heat radiating from the soil.
What to Use:
- Frost blankets or row covers (best option)
- Old bedsheets or lightweight blankets
- Burlap or cardboard boxes (for small plants)
- Specialized plastic cones (sometimes called “hot caps”)
What NOT to Use: Plastic sheeting alone. If it touches the foliage, it transfers cold directly and can cause worse damage. If you must use plastic, drape it over a frame so it doesn’t touch the plants.
How to Cover Correctly:
- Cover plants in the late afternoon before the sun goes down and heat escapes.
- Drape the material loosely over the plant, allowing for air space.
- Secure the edges to the ground with rocks, soil, or pins to trap warm air.
- Remove the covers in the morning once temperatures rise above freezing to prevent overheating.
Other Effective Protection Techniques
For extra cold nights, combine covering with these methods:
- Watering: Wet soil holds heat better than dry soil. Water the base of your plants deeply in the afternoon before a frost. (Do not wet the foliage).
- Mulching: A thick layer of straw or leaves around the base insulates the roots and helps keep soil warmth in. Apply it after the soil has warmed in the season.
- Heat Sources: For potted plants, move them into a garage or porch. In the garden, you can place jugs of warm water under the covers or use outdoor-rated Christmas lights (incandescent, not LED) for a tiny heat boost.
What to Do If Your Tomatoes Get Cold Damaged
Don’t panic if you see wilted or blackened leaves after a cold snap. Act quickly to assess and help.
Assess the Damage
Wait until the day warms up. Gently touch the stems. If they are still firm and flexible, there is hope. If the main stem is mushy and collapsed, the plant is likely dead.
Look for green growth at the base or along lower stems. This is a sign the plant is trying to recover.
Immediate Recovery Steps
- Do Not Prune Immediately: Damaged leaves actually protect the inner tissue from further stress. Wait until new growth starts.
- Water Gently: Give the plant a moderate drink of lukewarm water if the soil is dry. Cold-damaged roots are fragile.
- Provide Afternoon Shade: For a day or two, shield the plant from harsh afternoon sun while it’s in shock. A piece of shade cloth or even an umbrella helps.
- Hold Off on Fertilizer: Do not fertilize a stressed plant. Its roots cannot absorb it, and the salts can cause more harm.
Long-Term Care After a Chill
In about a week, you can prune out the clearly dead, blackened foliage. Make clean cuts back to healthy, green tissue.
Monitor for disease. Stressed plants are targets. Apply a preventative organic fungicide like a copper spray if conditions are damp.
Be patient. Recovery can take 2-3 weeks. New growth may emerge from leaf nodes lower on the stem. If the main growing tip is dead, the plant will often send out new leaders.
Special Considerations for Different Stages
Seedlings and Transplants
These are the most vulnerable. “Hardening off” is non-negotiable. This process acclimates indoor-grown seedlings to outdoor conditions over 7-10 days.
- Start by placing them in a shaded, sheltered spot for just an hour or two.
- Gradually increase their time outside and expose them to morning sun and a light breeze.
- Bring them in anytime temperatures dip near 50°F during this process.
Flowering and Fruiting Plants
Cold is a major problem during bloom. Pollen is infertile below 55°F, and blossoms will drop without setting fruit.
If cold hits when fruit is on the vine, you might save it. Mature green tomatoes can be harvested before a hard freeze and ripened indoors. They will not develop flavor further, but they will turn red.
Never refrigerate tomatoes! Temperatures below 55°F cause chilling injury, making them mealy and flavorless. Store them at room temperature out of direct sunlight.
FAQ: Your Tomato Cold Questions Answered
Can tomato plants survive 40 degree weather?
They can survive, but they will not thrive. At 40°F, they experience chilling injury. Growth stops, leaves may discolor, and the plant becomes very stressed. Protection is highly recommended at these temperatures.
At what temperature should I cover my tomato plants?
You should cover them anytime temperatures are forecast to dip below 40°F. If your plants are young or not fully hardened, consider covering them below 45°F to be safe. It’s better to be cautious then to lose them.
Will a frost kill tomato plants?
A light frost (temps just below freezing for a short time) will kill the leaves and tender growth. A hard freeze (below 32°F for several hours) will usually kill the entire plant, especially if it’s not established. Mature plants with woody stems might regrow from the base if the roots survive.
How do I know if my tomato plant is dead after frost?
Wait a day. Scratch the bark on the main stem with your fingernail. If you see green underneath, the plant is still alive. If it’s brown and dry all the way through, and the stem is completely limp, the plant has died. Don’t pull it up to quickly; sometimes suprises happen.
What is the lowest temperature for tomato plants in Celsius?
The lowest safe temperature is about 10°C. Danger starts at 4°C (chilling injury). A light frost at 0°C causes damage, and temperatures of -1°C or lower typically cause fatal freeze damage. These thresholds are why gardeners in cooler climates must be so vigilant.
Planning for a Successful Season
The best defense is choosing the right varieties and planting times for your climate.
- Early Varieties: Look for cultivars labeled “early season” or “cold-tolerant.” Examples include ‘Early Girl’, ‘Stupice’, ‘Glacier’, and ‘Siberian’ types.
- Season Extension: Use walls of water, cold frames, or a simple hoop house to create a warmer microclimate. These can let you plant 2-3 weeks earlier in spring.
- Container Growing: Growing in pots allows you to move plants to a sheltered location when cold threatens. This is a huge advantage.
Paying attention to the weather forecast in spring and fall is part of the gardeners job. A little preparation makes all the difference between a saved crop and a total loss. By knowing exactly what is too cold for tomato plants, you can take confident action to ensure they survive and produce a bountiful harvest for you all season long.