Knowing what temperature should i cover my plants for frost is a common question for gardeners. The answer depends less on a single number and more on the type of plants you have and how long the cold lasts.
A light frost might only nip tender annuals, while a hard freeze can damage even some hardy perennials. Protecting your plants is about understanding these differences.
This guide will give you the specific temperature thresholds, plant-specific advice, and the best covering techniques to use.
What Temperature Should I Cover My Plants For Frost
The general rule is to cover plants when temperatures are forecast to drop below 36°F (2°C). However, this is just a starting point. You must consider the type of frost and the hardiness of your plants.
A light frost, or “killing frost,” typically happens between 29°F to 32°F (-1.7°C to 0°C). A hard freeze is more severe, with temperatures falling below 28°F (-2°C) for several hours.
Here is a simple temperature guide to follow:
- 36°F (2°C) and falling: Start monitoring forecasts closely. This is when tender plants begin to be at risk, especially in low-lying areas of your garden where cold air settles.
- 32°F (0°C): Cover all tender plants. This includes vegetables like tomatoes, peppers, beans, and cucumbers, as well as tropicals and most annual flowers.
- 28°F (-2°C) and below: Cover all but the most cold-hardy plants. Even some cool-season crops can be damaged by a prolonged hard freeze. Consider providing extra insulation with layers of cover.
Understanding Plant Hardiness Is Key
Not all plants react to cold the same way. Categorizing your plants is the first step to effective protection.
Tender Plants
These are the most vulnerable and should be covered first. They are often annuals or tropicals.
- Examples: Impatiens, begonias, tomatoes, peppers, eggplants, basil, cucumbers, squash, and citrus trees.
- Protection Threshold: Cover when temps are forecast at or below 36°F (2°C).
Half-Hardy Plants
These can tolerate a brief, light frost but will sufer from a hard freeze.
- Examples: Lettuce, broccoli, cauliflower, carrots, peas, and some perennial herbs like rosemary (in colder zones).
- Protection Threshold: Cover when a hard freeze below 28°F (-2°C) is expected.
Hardy Plants
These plants are adapted to cold and generally do not need covering. They often require a period of cold to thrive.
- Examples: Kale, spinach, brussels sprouts, cabbage, pansies, and many native perennials and trees.
- Protection Threshold: Usually none needed unless temperatures are extremely and unusually low for your region.
How To Properly Cover Your Plants
Using the right materials and techniques is as important as timing. The goal is to trap heat radiating from the soil.
Best Materials For Frost Protection
Choose materials that provide insulation without damaging the plant.
- Frost Cloth or Row Covers: These are the best option. They are lightweight, allow light and water through, and can provide 2°F to 8°F of protection.
- Bedsheets or Blankets: A good second choice. Use old cotton sheets or lightweight blankets. Avoid plastic directly on plants, as it can transfer cold and damage foliage.
- Cardboard Boxes or Plant Pots: Excellent for covering individual small plants or shrubs. They provide good air insulation.
- Mulch: A thick layer of straw, leaves, or wood chips can protect plant roots and crowns. This is great for perennials and root vegetables.
Step By Step Covering Instructions
- Water the Soil: Damp soil holds heat better than dry soil. Water your plants lightly during the afternoon before an expected frost.
- Cover Before Dusk: Apply your covers in the late afternoon while daytime heat is still trapped in the garden. This seals the warmth in.
- Drape to the Ground: Drape your fabric or material so it reaches the soil on all sides. Use stakes, rocks, or bricks to anchor it, creating a tent of insulating air.
- Remove in the Morning: Once the sun is up and air temperatures are above freezing, remove the covers. This prevents overheating and allows plants to recieve sunlight.
Factors Beyond The Thermometer
Several other conditions influence frost formation and damage. Pay attention to these as well.
Garden Microclimates
Your yard has pockets of warmer and cooler air. Frost often forms first in low spots where cold air drains and settles. Plants on a south-facing slope or near a wall may be protected longer.
Wind and Cloud Cover
A calm, clear night is ideal for frost formation. Wind mixes the air and can prevent frost. Cloud cover acts like a blanket, trapping heat. A forecast for clear, still skies increases frost risk.
Duration of Cold
A brief dip to 31°F may cause less damage than four consecutive hours at 32°F. Check the forecast for how long the freezing temperatures are expected to last.
Special Considerations For Different Plant Types
Protecting Vegetable Gardens
Your edible garden needs careful attention. Use row covers supported by hoops for entire beds. For individual plants like tomatoes, a tomato cage wrapped in burlap or frost cloth works well. Root crops like carrots can often stay in the ground if mulched heavily with straw.
Protecting Potted Plants and Containers
Pots are especially vulnerable because the roots are exposed on all sides. For a light frost, group pots together against a sheltered wall and cover them. For a hard freeze, its best to move them into a garage, shed, or porch.
Protecting Trees and Shrubs
Young trees and tender shrubs (like hydrangeas or fig trees) may need protection. Wrap the trunk with burlap or use a frost blanket to cover the entire canopy. Avoid letting coverings touch the foliage directly.
What To Do If You Forgot To Cover Plants
If you discover frost damage in the morning, do not panic. Do not immediately prune the damaged parts. Wait until the sun has warmed the plant and the frost has thawed naturally. Then, assess the damage. Brown, wilted leaves may indicate damage, but the stems and roots might still be alive. Give the plant time to recover before cutting anything back.
Long Term Frost Preparedness
Planning your garden with frost in mind can save you effort. Plant tender crops after your area’s average last frost date. Choose plant varieties rated for your USDA Hardiness Zone. Incorporate cold frames or a greenhouse for extending the season. Keep frost protection materials handy as the season changes.
Frequently Asked Questions
At What Temperature Should I Cover My Tomato Plants?
Tomato plants are very tender. You should cover them when temperatures are expected to fall below 40°F (4°C). They can suffer chilling injury even above freezing. For a frost below 32°F (0°C), ensure they are well-covered.
Can I Use Plastic To Cover Plants From Frost?
It is not recommended to use plastic sheeting directly on plants. Plastic can transfer cold to leaves and cause damage if it touches them. If you must use plastic, drape it over a frame so it does not contact the foliage, and remove it first thing in the morning.
How Cold Is Too Cold For Covered Plants?
Even covered plants have limits. A quality frost cloth may only provide 4-8 degrees of protection. If temperatures drop into the low 20s Fahrenheit (-6°C) or below for an extended time, covered tender plants may still be damaged. In such severe freezes, moving potted plants indoors is the safest option.
Should I Water Plants Before a Frost?
Yes, watering the soil around your plants a day before a frost can help. Moist soil retains and releases heat more effectively than dry soil, creating a slightly warmer microclimate around the plant. Avoid wetting the foliage itself.
What Is The Difference Between a Frost Advisory and a Freeze Warning?
A Frost Advisory means conditions are favorable for frost formation (temps 33°F-36°F). A Freeze Warning means temperatures are expected to drop below 32°F. A Freeze Warning requires more urgent action to protect sensitive vegetation.