When To Cover Tomato Plants

Knowing when to cover tomato plants is the single most important skill for protecting your precious harvest from an early frost. That sudden, unexpected chill in late spring or early fall can wipe out months of growth in a single night. This guide will give you the clear, actionable information you need to make the right call every time.

We’ll cover how to read weather forecasts, choose the best materials, and the proper techniques to tuck your plants in safely. With a little preparation, you can extend your growing season by weeks and enjoy ripe tomatoes long after your neighbors have given up.

This is the core question. The timing isn’t about a specific date on the calendar, but about understanding temperature thresholds and weather patterns. Tomato plants are tropical natives, and they simply cannot handle freezing temperatures.

The Critical Temperature Threshold

You need to cover your tomato plants when the forecast predicts a low temperature at or below 40°F (4°C). This is your trigger for action. Many gardeners mistakenly wait for a freeze warning, but that’s often too late.

Here’s why 40°F is the magic number:

Chilling Injury: Temperatures between 32°F and 50°F can cause chilling injury. This damages cell membranes, leading to stunted growth, leaf discoloration (a purplish or bronze tint), and reduced fruit set.
Frost Formation: Frost can form even when the air temperature reported by the weather service is slightly above 32°F. On clear, calm nights, cold air sinks, and the temperature at ground level can be 5 or more degrees colder than the reported air temperature.
Flower Protection: Pollen becomes inviable in cool temperatures, so covering on chilly nights protects new flowers and ensures they can still produce fruit.

How to Read the Forecast Correctly

Don’t just glance at the predicted low. You need to dig a bit deeper into the details.

Check the “Real Feel” or “Overnight Low”: Some weather apps give a more localized overnight low that accounts for radiational cooling.
Look for Clear Skies and Calm Winds: These are the perfect conditions for frost formation. A cloudy or windy night often provides some protection.
Note the Dew Point: If the dew point is close to the predicted low, humidity is high and frost is less likely. A large gap between dew point and air temperature on a clear night signals high frost risk.
Use a Local Thermometer: Place a thermometer in your garden at plant level. This gives you the only reading that truly matters.

Spring vs. Fall Frost Protection

Your strategy has a slightly different focus depending on the season.

In spring, you are protecting young, tender transplants. Your goal is to get them established. Covering is often a nightly ritual until nighttime temperatures consistently stay above 50°F.

In fall, you are protecting mature plants loaded with green fruit, trying to coax them into ripening. You might cover plants every cold night for weeks to extend the harvest. The effort is well worth it for those last few batches of ripe tomatoes.

Choosing the Right Covering Materials

Not all covers are created equal. The material you choose directly impacts the level of protection you provide.

Best Materials for Frost Protection

Floating Row Cover (Frost Blanket): This is the gold standard. Made of lightweight spun-bonded polyester, it lets in light and water while trapping heat. It can provide 2-8°F of protection depending on weight. Secure it well at the edges.
Bed Sheets or Blankets: A good second choice. Use old cotton sheets or lightweight blankets. Avoid plastic tarps or vinyl that touch the foliage, as they transfer cold directly.
Cardboard Boxes: Excellent for individual young plants. Simply place a box over the plant and weight it down with a rock.
Specialized Cloths: Products like Nylofume or polyethylene plastic films can be used but require a support structure to keep them from touching leaves.

Materials to Avoid

Plastic Tarps (Direct Contact): Plastic that touches leaves will make frost damage worse. If you must use plastic, create a tent structure with stakes so it doesn’t contact the plant.
Heavy Comforters: They can be too heavy and break delicate branches, especially on younger plants.
Newspaper: While better than nothing, it provides minimal insulation and is difficult to secure in wind.

Step-by-Step Guide to Covering Your Plants

Doing it right makes all the difference. Follow these steps to ensure maximum protection.

Preparation Before Dusk

Water the Soil: Moist soil holds heat better than dry soil. Give the ground a good watering in the late afternoon if it’s dry.
Gather Supplies: Have your covers, stakes, and weights (stones, bricks, soil staples) ready before it gets dark.
Prune if Necessary: For fall plants, you can gently tie up long, sprawling branches to make covering easier. Avoid major pruning now.

The Covering Process

Time it Right: Cover your plants in the late afternoon or early evening, just before the sun sets and the day’s heat begins to escape. Don’t wait until it’s already cold.
Drape and Secure: Drape your chosen material loosely over the plant. It’s crucial that the cover does not rest tightly on the leaves, as this transfers cold. Use stakes or cages to create a tent if needed.
Seal the Edges: Weigh down all edges of the cover thoroughly with soil, stones, or boards. This traps warm air rising from the ground and prevents cold wind from getting underneath. This step is often overlooked but is vital.
Cover to the Ground: Ensure the cover extends all the way to the soil on all sides, creating a sealed microclimate.

Morning Uncovering Protocol

Timing is Key: Remove the covers in the morning once the air temperature has risen above freezing and the frost has melted. Usually, this is by 9 or 10 AM.
Remove Completely: Take the covers off fully to allow the sun to warm the soil and the plant to receive full light and air circulation. Leaving covers on all day can cause overheating and promote disease.
Dry Your Covers: Shake off any moisture and allow your blankets or row covers to dry completely before storing or reusing them. This prevents mold and mildew.

Advanced Techniques for Severe Cold

For temperatures predicted to dip well below freezing, or for a hard freeze, you may need to combine methods.

The Layered Approach: Use a floating row cover directly over the plant, then drape a blanket or heavier cloth over that. The air gap between layers provides extra insulation.
Add a Heat Source: For potted tomatoes, move them into a garage or shed. For in-ground plants, you can place a jug of warm water (like a milk jug) under the cover with the plant. It will radiate heat slowly through the night.
Create a Mini Greenhouse: For a few special plants, surround them with a cylinder of wire fencing, fill it with dry leaves or straw for insulation, and then cap the top with a blanket on cold nights.

What to Do If You Forgot to Cover

We’ve all been there. If you wake up to a surprise frost, act quickly but gently.

Do NOT Touch Frozen Plants: Wait until the sun naturally thaws them. Handling frozen leaves and stems causes immediate cell rupture.
Water with Cold Water: Surprisingly, spraying the plants with cold water before the sun hits them can slow the thawing process and reduce damage. This is a trick orchardists use.
Assess Damage Later: After the plant has fully thawed, assess the damage. Leaves may turn black and wilted. Don’t prune immediately. Wait a few days to see if new growth emerges from the stems.

Long-Term Strategies for Frost-Prone Gardens

Planning ahead can minimize your nightly covering chores.

Choose Cold-Tolerant Varieties: Look for tomato varieties described as “early season” or “cold-tolerant.” They often have shorter days to maturity.
Use Walls and Fences: Plant tomatoes on the south side of a wall or fence. These structures absorb heat during the day and release it at night, creating a warmer microclimate.
Employ Season Extenders: Consider using a cold frame, hoop house, or Wall-O-Water from the start. These provide protection without daily covering and uncovering.
Smart Planting Dates: Don’t rush spring planting. Wait until soil is warm and nighttime averages are reliably above 50°F, even if the calendar says it’s time.

FAQ: Protecting Tomatoes from Frost

At what temperature should I cover my tomato plants?

Cover them when the forecast low is 40°F (4°C) or below. This buffer protects against chilling injury and unexpected ground-level frost.

Can I use plastic bags to cover tomatoes?

It’s not recommended unless you support the bag so no plastic touches the leaves. Direct contact will worsen frost damage. Fabric is a much safer choice.

How cold is too cold for tomato plants?

A light frost (32°F) will damage foliage and fruit. A hard freeze (28°F or below) will likely kill the entire plant, even with covering. Prolonged temps below 50°F stunt growth.

Should I water tomatoes before a frost?

Yes, watering the soil (not the foliage) in the afternoon before a frosty night helps. Moist soil retains and releases heat more effectively than dry soil, offering a few degrees of protection.

Can tomato plants recover from frost damage?

They can if the damage is not too severe. If just the leaves are damaged but the main stems are still green and firm, the plant may produce new growth. Give it some time before you decide to pull it out.

What’s the difference between a frost cloth and a sheet?

A frost cloth (floating row cover) is specifically designed for this job; it’s lightweight, breathable, and lets in light. A sheet provides insulation but blocks light and can get heavy if wet. Both work, but frost cloth is often more effective and easier to manage.

Final Tips for Success

Protecting your tomatoes from early frost is a blend of vigilance and simple action. Keep a close eye on the evening forecast from late summer onward. Have your covering materials organized and ready to go in a handy spot. Remember, it’s always better to cover unnecessarily one night than to regret not covering the next morning.

With these practices, you’ll gain confidence in knowing exactly when to cover tomato plants. Your reward will be vibrant, healthy plants that produce delicious fruit long into the autumn season, giving you the longest and most bountiful harvest possible from your garden.