As a gardener, few things make you rush outside faster than a sudden frost warning. You know you need to act, but the big question is always: what temperature should i cover my plants for frost? The simple answer is to get ready when temperatures are forecast to dip below 36°F (2°C), but the real strategy is a bit more nuanced. Protecting your delicate blooms and tender vegetables requires understanding frost types, plant hardiness, and the right materials to use. Let’s break it all down so you can sleep soundly on those chilly nights.
What Temperature Should I Cover My Plants For Frost
This is the core of frost protection. You should generally cover your plants when the forecast calls for a temperature at or below 36°F (2°C). However, this isn’t a single, magic number for every garden. A calm, clear night might see frost form at 37°F, while a breezy, cloudy one might prevent it until 32°F. The safest bet is to take action when there’s any frost advisory or when temps are predicted to hit the mid-30s.
It’s crucial to understand the two main types of frost. A “light freeze” occurs between 29°F to 32°F. This can damage tender plants but often spares hardy perennials. A “hard freeze” is 28°F and colder, which is widely destructive to most non-dormant plant tissue. Your covering efforts are primarily aimed at surviving light freezes.
Why Plants Need Protection From Cold
Frost damage happens when ice crystals form inside plant cells. As water in the cells freezes, it expands. This expansion ruptures the cell walls, leading to the blackened, mushy leaves you see after a frost. Covering plants works by trapping the heat that radiates from the soil, creating a warmer microclimate around the plant. It’s like giving them a cozy blanket that holds in the earth’s stored warmth.
Know Your Plant’s Hardiness
Not all plants need the same level of protection. Categorizing your plants helps you prioritize.
- Tender Plants: These are killed or severely damaged by light frost. Examples: basil, impatiens, peppers, tomatoes, most summer annuals. Cover them at or below 36°F.
- Half-Hardy Plants: They can tolerate a brief, light frost. Examples: lettuce, broccoli, pansies, snapdragons. Cover them if a hard freeze (below 28°F) is expected.
- Hardy Plants: These can handle freezing temps and often don’t need covering. Examples: kale, spinach, cabbage, most perennials and trees native to your zone.
The Best Materials for Covering Plants
What you use is as important as when you cover. The goal is to insulate while allowing moisture and some air exchange. Avoid using plastic sheeting directly on plants, as it can transfer cold and damage leaves it touches.
- Frost Cloth or Row Cover (Best Choice): This lightweight, woven fabric is designed for this job. It lets in light and water while providing 2-8°F of protection. You can leave it on for days.
- Bedsheets or Blankets: A good second option. Use medium-weight cotton. They provide decent insulation but must be removed during the day.
- Burlap: Excellent for wrapping shrubs or creating windbreaks. It’s breathable and provides good protection.
- Cardboard Boxes: Perfect for covering individual small plants or vegetable clusters overnight.
- Newspaper: Can be used as a last-minute layer under another cover for extra warmth, or to make little tents.
Step-by-Step Guide to Covering Your Plants
Doing it right makes all the difference. Follow these steps for the best protection.
1. Prepare Before Nightfall
Don’t wait until its dark and cold. Cover your plants in the late afternoon while daytime heat is still present in the garden. This traps that valuable warmth. Ensure your plants are well-watered earlier in the day; moist soil holds heat better than dry soil.
2. Secure the Cover Properly
Drape your chosen material loosely over the plants. The cover should extend all the way to the ground on all sides. Use rocks, bricks, or soil staples to anchor the edges tightly. This prevents cold wind from getting underneath and stealing the trapped heat. Make sure the cover isn’t taught against the plant’s foliage.
3. Remove the Covers Timely
Once the morning sun has warmed the air above freezing, remove the covers. Usually, this is by mid-morning. If you’re using frost cloth, it can often stay on if cloudy weather persists. Leaving heavy blankets or sheets on all day can cause plants to overheat or encourage disease due to lack of air flow.
Advanced Frost Protection Techniques
For extra-tender plants or severe cold snaps, combine covering with these methods.
Adding Heat Under the Cover
For a few extra degrees, you can add a heat source. Outdoor-rated Christmas lights (incandescent, not LED) strung under the cover provide gentle warmth. For a single plant, a jug of warm water placed under the cover can also help. Never use anything that could be a fire hazard near fabric.
Mulching for Root Protection
A thick layer of mulch (straw, leaves, wood chips) around the base of plants helps insulate the roots. This is especially critical for perennials and newly planted shrubs. The roots are often less hardy than the top growth, and mulching protects them from freeze-thaw cycles.
What NOT to Do When Protecting Plants
Common mistakes can cause more harm than good. Avoid these pitfalls.
- Don’t use only plastic sheeting: Plastic traps moisture and can “cook” plants in the sun if left on, or freeze leaves it touches.
- Don’t cover too early in the fall: Allowing plants to experience mild, non-damaging chill can help them harden off naturally.
- Don’t forget to uncover: Trapping heat on a sunny day is just as stressful for a plant as the cold was.
- Don’t prune frost-damaged growth immediately: Wait until spring to see where new growth emerges; the damaged material actually provides some insulation.
Reading Weather Forecasts Like a Pro
Gardeners need to look beyond just the air temperature. Pay attention to these key terms.
- Dew Point: Frost is more likely when the dew point is low. A large gap between air temp and dew point means drier air and potentially more radiational cooling.
- Wind Chill: While wind chill doesn’t affect plants the same as humans, a strong wind strips away the warm air layer around plants, making them more vulnerable.
- Sky Cover: Clear, calm nights are the biggest frost risk. Clouds act like a blanket, preventing heat from escaping into the atmosphere.
- Low Spot Awareness: Cold air is dense and settles in low areas. Plants at the bottom of a slope are in a “frost pocket” and will need protection first.
Recovering From Frost Damage
If you were too late or the cold was too severe, don’t despair. Many plants can recover.
- Resist the Prune: Do not cut off damaged parts immediately. They protect the lower stems from further cold.
- Water Gently: Once the soil thaws, water the plant if it’s dry. This helps reduce stress.
- Wait for New Growth: Be patient. Wait until consistent warm weather returns and you see where new buds form. Then, prune back the dead material to just above the new growth.
Long-Term Strategies for a Frost-Prone Garden
Planning can reduce your need to rush around with bedsheets.
Smart Plant Placement
Plant tender species on south-facing slopes, near walls (which radiate heat), or on raised beds. Avoid planting in known frost pockets. Using structures like fences or hedges as windbreaks can also modify your garden’s microclimate.
Choosing the Right Plants
The best defense is to grow plants rated for your USDA Hardiness Zone or one zone colder. Native plants are typically well-adapted to local temperature swings. Focus on hardy varieties for the bulk of your garden and save the tender exotics for containers that can be moved.
Investing in Season Extension
Consider permanent or semi-permanent structures. Cold frames, hoop houses, or even a simple wall of water (a water-filled teepee) can protect plants for weeks longer than uncovered ground. These are great for getting a jump on spring veggies or extending the fall harvest.
Frequently Asked Questions (FAQ)
At what temperature should I cover my flowers?
Follow the same 36°F rule for tender annual flowers like petunias, geraniums, and begonias. Hardy spring bulbs and perennials usually don’t need covering unless an extreme hard freeze is expected after they’ve broken dormancy.
Should I water plants before a frost?
Yes, watering the soil (not the foliage) during the day before a frost can help. Moist soil absorbs more solar heat and releases it slowly overnight, which can raise the temperature around your plants by a critical degree or two.
Can I use trash bags to cover plants?
It’s not recommended. Plastic bags don’t breath and can cause condensation that freezes on leaves. If you must use plastic, ensure it does not touch the plant by supporting it with stakes, and remove it first thing in the morning.
What temp is a killing frost?
A “killing frost” or hard freeze is generally considered to be 28°F (-2°C) or lower for an extended period. This temperature will kill the top growth of most annuals and tender perennials, ending the growing season for them.
How can I protect potted plants from frost?
Potted plants are more vulnerable because their roots are exposed. Move them to a garage, shed, or against a sheltered house wall. Grouping pots together and wrapping the sides with burlap or blankets can also help. Remember, the goal is to keep the root ball from freezing solid.
Is there a difference between frost and freeze?
Yes. Frost refers to the ice crystals that form on surfaces. A freeze refers to the air temperature dropping below 32°F. You can have a “freeze” without visible frost if humidity is very low, and you can see frost on the ground even if the official air temperature is slightly above 32°F due to microclimates.
Protecting your garden from frost is a key skill for any gardener wanting to extend the season and safeguard their hard work. By knowing what temperature to cover your plants for frost, using the right materials, and acting in time, you can save your delicate blooms and enjoy a healthier, more resilient garden. Paying close attention to the weather and understanding your own garden’s unique microclimates will make you even more effective. With this knowledge, you can face those cold forecasts with confidence, knowing your plants are safe and sound.