Narrow strips along paved surfaces need tough, low-growing plants that tolerate reflected heat and occasional foot traffic. But when the forecast calls for freezing temperatures, even the toughest plants need protection. If you are wondering what temperature should i cover my plants for frost, the short answer is 32°F (0°C), but the real work begins before that.
Frost can form even when the air temperature reads 36°F or 37°F, especially on clear, calm nights. You need to watch the forecast closely and act early. Covering your plants at the right time makes the difference between a healthy spring garden and a sad pile of wilted leaves.
What Temperature Should I Cover My Plants For Frost
Most tender plants suffer damage when temperatures drop below 32°F. However, frost damage can occur at 36°F if conditions are right. The ground radiates heat upward on clear nights, and that heat escapes into the atmosphere. When the air near the ground cools to 32°F, ice crystals form on plant surfaces.
You should cover your plants when the temperature is forecast to hit 36°F or lower. This gives you a buffer. By the time the thermometer hits 32°F, the damage may already be done. Covering early is always safer than covering late.
Understanding Frost Types
Not all frost is the same. You need to know what kind of frost is coming to protect your plants properly.
- Light frost: Occurs when temperatures drop to 29°F to 32°F. Tender plants may die, but hardier plants survive.
- Hard frost: Occurs when temperatures drop below 28°F. Most annuals and tropical plants will not survive without protection.
- Freeze: Occurs when temperatures stay at or below 32°F for several hours. This is the most dangerous for your garden.
You need to check your local forecast for the “low” temperature, but also pay attention to the dew point and wind speed. A windy night can actually reduce frost risk because the air mixes and prevents cold air from settling. A calm, clear night is the most dangerous.
Which Plants Need Covering
Not every plant in your garden needs the same level of protection. Some plants are naturally frost-hardy and will shrug off a cold night. Others are extremely sensitive and need immediate action.
Tender plants that absolutely need covering include:
- Tomatoes and peppers
- Basil and other tender herbs
- Cucumbers and squash
- Impatiens and begonias
- Citrus trees (especially young ones)
- Succulents and tropical houseplants
Hardy plants that generally do not need covering include:
- Kale, spinach, and lettuce
- Broccoli and cabbage
- Peas and carrots
- Pansies and snapdragons
- Most native perennials
If you are unsure about a specific plant, look up its hardiness rating. The USDA hardiness zones give you a good starting point. But remember, a plant rated for zone 7 might still need protection in zone 6 during an early or late frost.
Best Materials For Covering Plants
You have several options for covering your plants. Some materials work better than others, and some can actually cause more harm than good.
Good covering materials include:
- Frost cloth or row cover: This is the best option. It is lightweight, breathable, and lets light through. You can leave it on for several days.
- Old bedsheets or blankets: These work well for one or two nights. Make sure they are dry before you put them on the plants.
- Burlap: This is breathable and provides good insulation. It is especially good for shrubs and small trees.
- Plastic sheeting: This is tricky. Plastic does not breathe, so it can trap moisture and cause condensation. If you use plastic, make sure it does not touch the leaves directly. Use stakes or hoops to keep it off the foliage.
Materials to avoid:
- Metal or foil (conducts cold)
- Wet newspaper (falls apart and offers little insulation)
- Black plastic (absorbs heat during the day but can trap cold at night)
Whatever material you choose, make sure it extends to the ground on all sides. You want to trap the heat that radiates from the soil. If there is a gap at the bottom, cold air will get in and defeat the purpose.
How To Cover Plants Properly
Covering plants is not just about throwing a sheet over them. There is a right way and a wrong way. Follow these steps to give your plants the best chance of survival.
- Water the soil first. Moist soil holds more heat than dry soil. Water your plants thoroughly in the afternoon before the frost is expected.
- Wait until late afternoon or early evening. You want to trap the heat that has accumulated during the day. If you cover too early, you trap cold air.
- Drape the cover loosely. Do not wrap the plant tightly. You want an air pocket between the cover and the leaves. This air pocket provides insulation.
- Secure the edges. Use rocks, bricks, or soil to hold the cover down. This keeps the warm air inside and prevents the cover from blowing away.
- Use hoops for tall plants. If you are covering tomatoes or peppers, use stakes or wire hoops to keep the cover from touching the foliage. Contact with the cover can transfer cold to the leaves.
- Remove the cover in the morning. Once the sun is up and the temperature rises above freezing, take the cover off. This prevents overheating and allows pollinators to reach your plants.
One common mistake is leaving the cover on for too long. If the temperature warms up during the day, the cover can trap heat and cook your plants. Remove it as soon as the frost threat has passed.
Additional Protection Methods
Covering is not the only way to protect your plants from frost. You can combine multiple methods for extra protection, especially if the forecast is for a hard freeze.
Mulching: A thick layer of mulch around the base of your plants helps insulate the roots. Use straw, leaves, or wood chips. Apply the mulch after the soil has warmed up in the spring, or before the first frost in the fall.
Cloches: These are individual covers for single plants. You can buy them or make them from plastic bottles, milk jugs, or glass jars. Cut the bottom off a plastic jug and place it over the plant. Remove the cap during the day to let heat escape.
Water walls: These are rings of plastic tubes that you fill with water. The water absorbs heat during the day and releases it at night. They are excellent for tomatoes and peppers.
String lights: Old-fashioned incandescent Christmas lights can provide a small amount of heat. Wrap them around the plant before covering. Do not use LED lights because they do not produce enough heat.
Heat sources: For very sensitive plants, you can use a small space heater or a heat lamp in a greenhouse or cold frame. Never use these outdoors without proper safety precautions.
What To Do After A Frost
Even with the best protection, some plants may suffer damage. Do not panic. Wait until the morning after the frost to assess the damage. The plants may look wilted and droopy, but that does not mean they are dead.
Follow these steps after a frost:
- Remove the covers in the morning as soon as the temperature rises.
- Do not prune damaged foliage immediately. The damaged leaves may actually protect the healthy parts of the plant from further damage.
- Wait 2-3 days before pruning. This gives the plant time to recover and shows you which parts are truly dead.
- Water the plants lightly. This helps them recover from the stress.
- Apply a light fertilizer to help the plant regrow.
If the plant is severely damaged, you may need to replace it. But many plants can recover from a light frost if you give them time.
Regional Considerations
The temperature at which you should cover your plants depends on your region and your local climate. A gardener in Florida will need to protect plants at different temperatures than a gardener in Minnesota.
In warm climates, tropical plants are common. These plants have no tolerance for cold. You should cover them whenever the temperature drops below 40°F. In colder climates, you are likely growing hardy plants that can withstand lower temperatures.
Pay attention to your local frost dates. The first frost in the fall and the last frost in the spring are the most dangerous times. You can find your local frost dates from your county extension office or online gardening resources.
Microclimates also matter. Areas near buildings, walls, or large bodies of water tend to be warmer. Low-lying areas are colder because cold air sinks. If your garden is in a low spot, you may need to cover your plants at higher temperatures than your neighbor who gardens on a hill.
Signs Of Frost Damage
It helps to know what frost damage looks like so you can act quickly. The most common signs include:
- Wilting or drooping leaves
- Dark, water-soaked spots on leaves
- Browning or blackening of leaf edges
- Translucent or glassy appearance on leaves
- Soft, mushy stems
Some plants show damage immediately, while others take a day or two to show symptoms. If you see these signs, do not give up. The roots may still be alive, and the plant can regrow from the base.
Frequently Asked Questions
Is 40 Degrees Too Cold For Plants?
For most tender plants, 40°F is the threshold where damage can begin. Tropical plants and warm-season vegetables should be covered or brought indoors when the temperature drops to 40°F. Hardy plants can usually tolerate 40°F without any problem.
Should I Cover Plants At 39 Degrees?
Yes, if you have tender plants, you should cover them at 39°F. This is especially important if the forecast calls for clear skies and calm winds. These conditions can cause frost to form even when the air temperature is above freezing.
Can I Use A Tarp To Cover Plants From Frost?
You can use a tarp, but it is not the best choice. Tarps are heavy and can damage plants. They also do not breathe, which can trap moisture and cause rot. If you must use a tarp, make sure it does not touch the leaves and remove it as soon as the temperature rises.
How Long Should Plants Stay Covered?
Plants should stay covered only as long as the frost threat exists. Remove the covers in the morning once the temperature rises above freezing. Leaving covers on for too long can cause overheating and other problems.
Does Spraying Plants With Water Prevent Frost Damage?
Spraying plants with water before a frost can actually help in some cases. The water releases heat as it freezes, which can protect the plant tissue. However, this method is risky and only works if you keep spraying continuously. It is not recommended for home gardeners.
Final Tips For Frost Protection
You now know what temperature should i cover my plants for frost, but here are a few more tips to keep your garden safe.
Always check the forecast multiple times a day. Weather can change quickly, and a forecast that says 35°F in the morning can drop to 28°F by nightfall. Sign up for weather alerts on your phone to stay informed.
Keep your covers clean and dry. Store them in a place where they are easy to grab when you need them. You do not want to be searching for a sheet in the dark at 10 PM.
Consider building a cold frame or a low tunnel for your most valuable plants. These structures provide permanent protection and make it easy to cover and uncover your plants as needed.
Finally, do not forget about potted plants. Potted plants are more vulnerable to frost because their roots are exposed to cold air on all sides. Move potted plants indoors or against the house for extra warmth. You can also wrap the pots in burlap or bubble wrap to insulate the roots.
Frost protection is not complicated, but it does require attention and timing. By knowing the right temperature and using the right materials, you can keep your plants healthy and productive through the cold nights. Check the forecast, prepare your covers, and act early. Your plants will thank you with strong growth and a bountiful harvest.
Remember that a little effort goes a long way. A few minutes of covering your plants can save you from losing your entire garden. So next time the forecast calls for cold, you will know exactly what to do. Cover at 36°F or below, use breathable materials, and remove the covers in the morning. That is the simple formula for frost protection success.
Your garden is an investment of time, money, and effort. Protect it wisely. The temperature threshold is clear, and the methods are simple. Now you can face the frost with confidence, knowing that your plants have the best possible chance of surviving the cold.