How to Protect Plants from Frost

A sudden dip in temperature can spell disaster for tender plants, seedlings and flowers. Learn how to make a kit for your garden so you can quickly and easily protect plants from frost.

Living in Ontario, Canada, it's not unusual for us to get snow in April. What's difficult is that there might be a week or two of warm weather that tricks our garden into thinking it's safe to wake up, and then a cold snap comes around and kills everything.

While a sudden cold spell is annoying, it doesn't have to be detrimental to your plants. In fact, some edible plants (like carrots and kale) actually benefit from frost as it improves their flavour!

You'll want to cover your cold-sensitive plants though. You've put time and money into those plants, so don't let sub-zero temperatures ruin them!

Here are some easy ways to protect your plants from frost.

Prepare Ahead of Time: Create a Frost Kit

When I worked at the garden centre, we had a "frost kit" for code geen situations where the temperature was expected to drop to below 0℃/32˚F overnight. The kit had everything we needed to protect the plants from frost.

I hated code geenshifts, because it meant being outside in the cold for hours while tenting everything in the garden centre that couldn't be moved indoors!

Protecting your own garden won't be nearly as bad, I promise. If you keep these supplies somewhere that's easy to pull out at a moment's notice, it won't take you long to protect your plants.

You'll Need:

• a container for storage (make sure it has a lid if you're going to leave this outdoors)
• frost fabric/drop clothes/old bedsheets or blankets/old towels (plastic tarp is ok but not the best choice)
• scissors
• rocks/bricks/weights
• stakes
• plastic containers (old juice bottles, buckets, pots etc.) to be used as cloches (optional)

Keep you frost kit in a place where you can grab it at a moment's notice. I'll explain how to use it in a minute.

Since flash frost is usually an issue for overnight temperatures, you'll be following the next set of steps before the sun goes down, and then reversing them in the morning once the temperatures are above 0℃/32˚F.

If you're not sure if you're likely to have frost or not, you should check your first and last frost dates.

Water Your Plants Before The Temperature Goes Down

Did you know that moist soil actually retains more heat than dry soil? Give your plants a good drink before the frost hits to help them stay warmer.

Which Plants Should You Protect from Frost?

First, remember that plants that love sun hate frost! Vegetable plants like tomatoes, eggplants, peppers, and beans won't be able to tolerate frost. The same is true for tropical plants and many annuals (like begonias and impatiens).

Second, while a plant might be considered "cold weather tolerant" that doesn't mean that frost won't damage it. Radishes can generally survive a hard freeze, but only once they're matured. Tiny seedlings will still need to be protected.

Also flowers like pansies, tulips and daffodils that we associate with cold weather can benefit from being protected from frost, as the cold weather can damage buds and flowers.

Move Plants in Containers Indoors

Anything in a pot or container (that isn't too big or heavy) can be tucked into a garage or shed. The shelter from wind, rain and snow is enough when the temperature is just below 0℃/32˚F (frost).

If you're expecting a hard freeze (temperatures below -2℃/28˚F), move the containers into your home instead. However, a sudden extreme change in temperatures can also hurt your plants, so place them in a cool place (a basement works if you have one).

If the weather is above 0℃/32˚F during the day, the plants can come back out to get light and then go back into the garage overnight again as needed.

Drape Plants That Must Stay Outside

This is where your frost kit comes in!

Loosely drape the material over your plants, using the stakes to support the weight. Frost cloth is generally light enough that it won't hurt the plants by resting on them, but something heavier, like a towel, might injure the plants if placed directly on top of them.

Plastic tarp, while usually lightweight, should always be lifted above the plants not placed directly on them. If possible, use a layer of cloth under the plastic to protect your plants. When plastic heats up it can burn plants, and in cold weather it can hold moisture against plants and causes even more severe freezing damage than if the plant had nothing over it at all. So you can use plastic, but just don't let it touch your plants.

The goal is to trap the warm air around your plants before the temperatures dip and protect the plants from being burnt by frost.

Secure the ends of the material with your rocks or bricks. This will prevent your make-shift tarp from flying away in the wind.

If you have a cut down plastic container, you can place that over smaller plants and push down so it's about an inch deep into the soil. Again, this helps to trap the heat around the plant. You may need to put a rock on top to keep it in place.

The next day, you'll want to remove the coverings by mid-day or your plants may overheat or collect too much condensation (which will subsequently freeze on your plants the following night).

Other Ways to Protect Plants from Frost

Piling mulch around the base of your plants can protect the stems and roots from freezing, but you may still get damage on the leaves of your plants.

Placing plastic jugs of warm water around your plants (under your frost cloth) can help to keep your plants warmer.

A cardboard box can work as a last-minute cloche, but keep in mind that it will absorb moisture. Heavy rain could cause the box to collapse onto your tender plant and damage it.

If you want to learn more about what frost damage is and how to deal with it, here are some resources:

What is Frost Damage?

Got Frost-Damaged Plants? How It Happens, and When and How to Prune