Spring Frost Strategies: How to Protect Your Garden From Unexpected Cold Snaps

The last frost date for an area is the key milestone in any gardening season, but Mother Nature doesn’t always play by the rules. Be prepared.

Choose winter-hardy vegetables that don't mind the cold, such as cabbage, broccoli, carrots, radishes, or spinach. Iuliia Karnaushenko/Shutterstock

Frost comes in not just one, but two distinct forms.

The most obvious is when a cold front moves into the area, dropping temperatures significantly. It’s 65 degrees F and sunny one day and then 32 degrees F the next, threatening vulnerable new plantings and newly budding fruit trees.

The second type is called a “radiation frost” and occurs on calm, clear nights with dropping temperatures and a cloudless sky. Radiation frost is most common at the beginning and end of the growing season.

Radiation frost is typically just a few degrees lower than ideal growing temps, so a gardener’s solution can be as simple as what our great-grandmothers did: Cover the backyard garden with old sheets and blankets. Remove the coverings during the day so the soil can absorb the sun’s warmth, then tuck the plants in each night.

Ideally, the coverings shouldn’t touch the plants, but sometimes that’s not realistic, so do the best you can. However, don’t let the fabric get wet or covered by damp, thick dew, as this can cause colder temperatures to be held directly adjacent to the plants. You can either prop the sheet above the plant to keep them from touching or swap the damp sheet with a dry one the next morning.

This advice—and the following tips—can be used to get a jump on the growing season.

Cold-Hardy Warriors

One of the easiest ways to deal with the threat of frost is to choose plants that don’t mind—and, in fact, may even thrive in—temperatures down to 28 or 32 degrees F. Some examples are broccoli, cabbage, carrots, chives, lettuce, leeks, peas, radish, spinach, and Swiss chard.

Since most winter-hardy vegetables don’t need pollinators, there’s no need to plant flowers to attract them. Still, it’s nice to add early, cheerful colors to the garden. Calendula (edible pot marigold), narcissus, tulips, grape hyacinths, and gently named but incredibly durable pansies are some popular choices that won’t blink at some short-term frost.

Best and Worst Locations

Hot air rises. Cold air drops. This is the science behind ceiling-mounted air conditioning vents, which are designed to disperse the air evenly along the ceiling first, then let it drop to cool the room.

The physics also apply to outdoor weather. Avoid frost pockets, which are depressions in the garden that cold air can drain into but not get back out of. They can be as small as a single plant or as large as an entire section of the yard. These spots can have significantly lower temperatures than the surrounding soil.

Instead, plant new seedlings in more frost-resistant areas. These include the higher spots in the garden or even on slopes. Nearby structures can also be a big boon, including house walls and fences.

South- and west-facing walls and fences will hold heat the longest, particularly if they’re painted a dark color. They will also help protect against damaging winds. Other suitable areas for planting include near hedges, shrubbery, benches, and other garden furniture or structures.

Extra Plant Muscle

Although cold-hardy seedlings can be planted directly in the ground, warm-weather crops are another matter. If the last frost date has passed but the weather is still less than ideal when it’s time to plant tomatoes, peppers, zucchini, cucumber, and similar veggies, it’s important to harden them off for two weeks.

To harden seedlings, place them outside for two to three hours during the warmest part of the day (typically noon to 5 p.m.). Gradually increase their hours outside over the course of two weeks. Water them lightly, and bring them in each night. Once they’re up to 10 to 12 hours, leave them out for 24 hours for the last few days and then plant them.

When Frost Surprises

In addition to sheets and blankets, budget-friendly protection for unexpected frost includes inverted clay pots, plastic pots, and milk jugs or two-liter bottles with the bottoms cut off. All are good for small plants and seedlings. Also consider buckets, cardboard boxes, old curtains and towels, burlap sacks, drop cloths, and tarps.

Be sure to remove them during the day, particularly nonbreathable drop cloths and tarps, so as not to smother the plants. Put them back on before dusk or before the temperature is expected to drop.

Another trick commonly used for roses, young shrubs, and tender perennials is to mound the plants with mulch or homemade compost. Be sure to pull it back after the danger has passed so it doesn’t lead to bugs or disease at the plant’s base.

Heat Sink

Fill plastic bottles with water, screw on the lids, and set them outside to soak up the heat—preferably in direct sunlight. They can be milk or water jugs, juice bottles, or two-liter soda bottles. Don’t use glass bottles, as they can break if the water freezes.

Typically, bigger is better—in fact, some gardeners use 55-gallon drums to warm unheated greenhouses—but you can also use numerous small water bottles for the same effect.

The idea is to turn them into efficient heat sinks, which will share their warmth throughout the night. One bottle or several can be placed next to plants underneath their cover to act as a no-power-needed heat source.

Covering plants with an old sheet or blanket is one of the easiest ways to protect them from frost damage. Svyatoslav Balan/Shutterstock

Grower’s Tips

Save old window screens to use them as windbreaks around the garden. They’re much cheaper and easier to set up than even DIY burlap screens.

Get Out the Watering Can

Water the morning before a frost. You may ask: Won’t this make them colder? In fact, the opposite is true. Wet soil holds heat better and keeps the roots warmer than dry soil does. Add a thick layer of your favorite mulch over the newly watered soil to act as insulation.

Form a Heat Island

Ideally, container plants would be brought inside before a frost, but that’s not always practical. The second option is to cluster them together to share warmth and create a “heat island.” Cover and add water bottle heat sinks throughout for additional protection.

Put the Pruning Shears Down

Frost-damaged leaves and branches are unsightly, but leave them alone until the temperatures stop dipping below 40 degrees F. Pruning can stimulate vulnerable new growth that can be killed in the next frost. After the temperatures rise, trim the minimum required to remove any damaged ends. The general rule is to trim less rather than more; many plants will show an amazing recovery.