How to Protect Your Plants from Frost

Frost can be devastating to your plants. How can you minimize any damage from unexpected frost?

A late spring frost is every gardener’s nightmare. Frost can kill an entire garden of plants.

You know your zone and keep an eye on the weather. You know you shouldn’t start planting your garden too soon. But temptations abound with a pop-up garden center in every supermarket parking lot.

The winter has been long, and you’re eager to get started, but frost is a real possibility. You are well aware that your garden can be ruined before the season even begins.

Frost forming the small leaves of a plant

What is Frost?

Frost is dew that has frozen. It’s the ice that forms on roofs and lawns, car windshields and patio furniture. It can also be found on garden plants’ leaves, flowers, and fruits.

Some plants are unaffected by frost, while others are killed.

There are various levels of frost. When air temperatures are 42°F (5.5°C) or lower, a light frost can form. A hard frost occurs when temperatures drop below 28°F (-2°C) for at least four hours. At these temperatures, a killing frost can last for more than 4 hours.

A freeze occurs when temperatures reach 32°F (0°C) but there is insufficient moisture in the air to form frost.

How does frost form at 40°F when water freezes at 32°F? The location of the thermometer is critical. Because cold air sinks, the air temperature may be above freezing, but the air temperature at ground level, where your plants are, may be colder. A single degree can make a significant difference!

These are the conditions that are perfect for frost to develop: 

  • Temperatures below 42°F(5.5°C)
  • Calm wind and clear skies 
  • Foggy conditions.  If the temperature is cold enough, the moisture in the fog can turn icy.

When the air freezes, the water in plant cells can freeze, causing the cells to rupture. A plant will die as a result of this.

Gardeners, of course, are constantly monitoring the weather forecast. And while most people are aware of the first and last frost dates in their area, freezing temperatures can catch you off guard at times. 

Forecast dates are based on 30-year averages, so if your last frost date is April 30, there may be frost a week later!

Some plants dying due to frosting

Plant for Your Zone

Cold damage is most common in plants that are not cold hardy in your USDA hardiness zone. 

NOAA, the Farmer’s Almanac, or your county extension service can help you determine your zone and frost dates.

Plants that are hardy in your area, such as tulips and crocus, daylilies and coneflowers, as well as shrubs and shade trees, will tolerate a few unseasonably warm days. These plants will recover even if the foliage is slightly damaged.

Frost can damage the blossoms of flowering trees and shrubs such as magnolias. Fruit tree blossoms can also become blighted, destroying your crop. The trees will survive, but the flowers will not, resulting in fruit this year.

Of course, you want to grow peppers and petunias regardless of your zone. Knowing your frost dates allows you to plant when it is safe.

Green leaves covered in frost

Don’t Plant Too Soon

“Plant when all danger of frost has passed,” as it says right on the seed packet.

We’d all like to have juicy, ripe tomatoes by July 4th. However, planting too early, while the soil is still cold, will not result in a higher yield. Soils in the spring are frequently cold and wet. Seeds, as well as tender seedlings and transplants, can rot.

Wait until a frost is no longer expected and the soil has warmed, as difficult as it may be. A soil thermometer is an excellent tool for any home gardener.

A soil thermometer placed on a damp soil

Move Plants Inside

You couldn’t resist the garden center’s display and purchased hanging baskets, vegetable plants, and annuals. And now the forecast for tonight calls for freezing temperatures! You can protect the plants from the cold by moving them to a slightly sheltered area, such as a shed or garage.

Water in the Late Afternoon

Watering your plants when a freeze is predicted may seem counterintuitive, but it can protect them. A moist soil retains four times the heat of a dry soil.

Water in the afternoon, when the soil has had time to absorb solar rays. As the soil releases its warmth in the evening, it will help keep the plants warm.

Watering beautiful flowering plants

Provide Air Movement

When the air is still and calm, frost forms. Using a fan to create a little wind can help prevent frost from forming.

Make sure the fan is weatherproof and that any power cords you use are properly grounded and approved for outdoor use.

Warm the Air with a Light Bulb

An incandescent light bulb labeled for outdoor use can warm the air around your plant just enough to prevent frost formation.

LED lights do not generate heat and should not be used. Some gardeners do this with an old string of holiday lights.

However, do not start a fire in your firepit for this purpose. Cold air is drawn in by a fire. You may have seen photos and stories about fires started in orange groves to protect the crop, but the situation is very different in a home garden.

Cover Your Plants with Sheets and Blankets

If frost is only expected for a few nights, this is the simplest and quickest option. Row covers or horticultural fleece make it simple to cover a row of tender vegetable seedlings, a blossoming fruit tree, or a large container of tender annual flowers, and old sheets and blankets work just as well.

premature plants covered with sheets and blankets

Make sure the covering reaches the ground when you cover the plants. As the ground releases heat into the night, this traps the warmer air inside.

Sticks or stakes can be used to keep the fabric off the plants, and if it’s windy, bricks or rocks can be used to keep the covering in place.

To give hanging baskets more protection, place them on the ground before covering them.

A gardener placing a sheet or blanket to cover plants

Inverted clay flower pots (cover the drainage hole), cardboard boxes, or layers of newspapers can also be used as coverings.

Avoid covering your plants with plastic. Plastic does not allow moisture to escape, and trapped moisture can cause your seedlings to freeze.

After the temperatures have warmed up, uncover your plants in the morning.

Cluster Your Containers

When it’s cold outside, we all know that cuddling up helps us stay warm. Plants are no exception. If you can, group your containers close together so they can help each other stay warm. Add a covering to protect against the cold.

Large clay pots or jars used for planting

Use a Cloche Over Individual Plants

Cloches made of glass can be placed over individual seedlings to protect them from the cold. These coverings are very attractive, elevating your frost protection to a high level of style, but they can be costly. The same can be said for hot caps.

Gallon jugs or 1 or 2 liter bottles with the bottoms cut off work just as well if you only need to cover your plants for a night or two. Make sure the bottle is large enough not to touch the plant.

Keep your caps on. If frost is expected for more than one night, the caps can be removed during the day to keep things from heating up during the day. You don’t have to remove the protection entirely.

When using cloches, hot caps, or bottles and jugs, keep an eye on the daytime temperature. Temperatures can quickly rise under glass, and it is all too easy to “cook” your plants.

Assorted vegetables in the vegetable garden
young plants shoots covered in recycled plastic bottles

Build an Insulating Barrier

Chicken wire cages filled with leaves and wrapped in burlap or other fabric can protect delicate shrubs such as tropical hibiscus.

To keep the air around young transplants warm, place gallon jugs filled with hot water around them.

A plastic full of dried figs and leaves

Use Microclimates

A microclimate is a small area with a distinct growing climate from the rest of the region. This is the area of your yard that is frequently warmer (or colder) than the rest of the garden.

A microclimate is determined by many factors, including soil type and moisture, wind, and topography, and the subject can become quite complex.

But for now, let’s say you can spot a warm microclimate when the shrubs on the south side of the house leaf out before the shrubs on the north side. It’s where the tulips bloom first and the lilies bloom first.

Warmer backyard microclimates occur in front of south-facing walls and fences, or in a sunny southwest corner protected from the wind.

A cold frame is a tried-and-true microclimate. A cold frame is a mini greenhouse made of wood, brick, or stone with a removable glass cover that is a great place to shelter young plants until they are ready to move out into the garden. Cold frames are also ideal for hardening off seedlings.

several glass notbed set up in the backyard

Watch Out for Frost Pockets 

These colder microclimates are commonly referred to as frost pockets. Because cold air sinks and warm air rises, a low area of your yard may become frosty while higher areas do not.

Plant the vegetable garden away from potential frost pockets.

Accept Nature’s Rhythms 

As autumn approaches and the days grow shorter, the first frost date approaches, and you brace yourself for the unavoidable.

Covering tender plants can help you extend the growing season, but a hard freeze is inevitable.

Many vegetables, such as tomatoes and peppers, continue to ripen after they are harvested. Half hardy vegetables such as bok choy, cauliflower, and radishes can withstand cold temperatures.

Cold hardy vegetables such as kale and spinach, carrots, and broccoli can withstand temperatures as low as 20°F (-6.6°C)! Many of these vegetables taste better after they’ve been frozen.

Lift tender bulbs and tubers like dahlias and gladiolas after the top growth has been killed by the cold.

Winter mulches will protect tender perennials from frost heave. Anti-desiccants will protect broadleaf and needled evergreens from the drying effects of cold winter air.

Wrapping young trees that are susceptible to frost crack also protects them.

A garden with little to no plants covered in frost
Alaine Connolly
Alaine has been working way too hard in horticulture since 1992, beautifying golf courses, resorts, and hotels. She is a part time landscape designer who works full time caring for a 28,000 square foot public garden. At home, she maintains her own 400 square feet plot. Alaine lives in northern Illinois - zone 5b.
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