How to Protect Plants From Frost in the Fall and Spring

An unexpected frost in spring or fall can wreak havoc on your gardening plans by zapping new buds and blooms in spring, spoiling late-season fruits, or killing off the last of your flowers. But cooler temperatures can also benefit some plants—especially in fall, when they boost the flavors in autumn crops and prepare ornamental trees and shrubs for their required dormant period.

Knowing which plants to protect and how to do it—with straw, hardwood mulch, blankets, and other easy techniques—allows you to help your plants survive and thrive in any season.

Frost vs. Freeze

A frost and a freeze are two different weather events, though they pose similar dangers to your plants. “A frost occurs when temperatures are typically between 33 and 36 degrees Fahrenheit,” says Amy Poston Lentz, horticulture specialist and Master Gardener coordinator at Colorado State University Extension. “Even though air temperatures are above the freezing mark of 32 degrees, ice crystals will still form on the surfaces of leaves, which are at the freezing point.” This means watching the weather forecast for overnight temperatures just above freezing—not only those below.

When air temperatures drop below 32 degrees, that’s a freeze. “This is when you will see the most damage to a plant, as cell walls break due to the pressure of expansion of water turning into ice crystals inside plant tissues,” says Lentz.

The Fall Frost

While the first temperature drop of late summer or early fall might take you by surprise, it’s usually less important to protect ornamental plants and vegetables during this season, since many of them are ending their growing season or preparing for dormancy.

Trees and Perennials

Trees and ornamental perennial plants actually benefit from experiencing temperatures that signal the shifting seasons. “Trees, shrubs, and woody ground covers, like lilacs, roses, and English ivy, are examples of plants that do not need covering to protect them from a fall frost,” says Lentz. “In fact, they need those changes in temperature in the fall to help them prepare for the winter by going dormant.”

As for your annuals? By the time fall temperatures have cooled enough for a light frost, your annuals are already shutting down for the year. They may bounce back after a single frost, but you don’t need to stress about extending their growing season. “If annuals are looking spectacular and [you] want to keep them going past a frost, [you] can cover them,” says Pamela J. Bennett, horticulture educator for Ohio State University Extension in Clark County. “However, plants like impatiens and begonias are very sensitive to frost and will be damaged. Typically, gardeners are ready to end the season, and once annuals get hit by frost they pull them from the ground.”

Vegetables and Fruit

The fall-harvest produce in your vegetable garden can withstand—and often improve with—a drop in temperatures. “Cool-season vegetables can handle light frosts in the fall, even becoming more sweet in flavor with crops like carrots, beets, parsnips, kale and turnips,” says Lentz.

The last of your summer fruits and vegetables, though—including peppers, tomatoes, melons, and cucumbers—can use a frost cover in the fall. “Many warm-season vegetables can be damaged by temperatures that dip to freezing or below, showing up as blackened leaves and spots on fruits,” says Lentz.

The Spring Frost

Most of your plants are more susceptible to damage during an unexpected spring frost. “Spring frosts affect plants differently than fall frosts, because many plants are waking up from dormancy with tender tissues that develop into new leaves and flower buds,” says Lentz.

New Seedlings

Vegetable, fruit, and flower seedlings should often be planted after your last frost date to give them the best chance at survival. "Spring frost is a totally different animal and has a lot of components," says Bennett, noting that the temperatures before a cold snap can play into your plants' ability to survive one. "If it’s really warm for several days and the temperatures drop all of a sudden, plant material might be nice and tender and very susceptible to frost," she says.

Trees and Perennials

Dormant trees, shrubs, and other perennials risk losing the season's fruit or flowers during a spring frost. "If the flower buds are killed off by frost, they will not develop to produce a fruit," says Lentz. "Most ornamental trees and shrubs will be okay with a light frost, however they can be protected if there is concern of spring flowers being nipped by frost."

How to Protect Plants From Frost at Any Point in the Year

Depending on your plant's size, location, type, and hardiness, you can prepare it for cold temperatures using one of these methods.

Mulch

In your vegetable garden, opt for a thick layer of straw or ground-up leaves to protect early-season fruits, like strawberries, in spring, and cool-season crops, like carrots and beets, in the fall.

Hardwood mulch takes longer to break down, so it's not a good choice for vegetable gardens, say the experts, but a 2- to 4-inch-thick layer can be helpful for ornamental trees and shrubs on the edge of their hardiness zone. "Perennials don’t need mulch unless they are marginally hardy in the area," says Bennett. Cover the crown of the plant with mulch, and remove it before growth picks back up in the spring.

Cloth

Fabric covers are your best option for light frosts, says Lentz, as long as they don't get wet. Fabrics designed for use as plant coverings, like Reemay, allow light to get through while enclosing warm air, adding up to 4 degrees to the temperature inside, says Lentz. "This can also be re-used for insect control later in the season for non-fruiting crops like lettuce,” she says.

Specialty fabrics aren't required, though: "Old sheets and blankets work great," says Bennett. "You can use wire coat hangers, cut in pieces and bent to secure the material, or buy U-clips sometimes used with sod or landscape fabric."

Cloth blankets or row covers laid over a raised frame—so they don't touch the plants below—are a good option for protecting young seedlings in spring, says Lentz. "This will trap in the radiant heat from the soil and vegetation and hopefully provide enough heat to keep the plants protected until the temperatures return to above freezing,” she says.

Plastic

Plastic isn't the best choice for protecting your plants; Lentz notes that it requires building a structure over your garden to suspend the plastic away from your foliage. "If the plastic comes into contact with the plants, they will freeze,” she says. "Things can heat up very quickly underneath once the sun rises and potentially can harm the plants underneath. The tunnel will need to be opened up and ventilated or the plastic removed when temperatures rise in the morning."

Water

While the water itself doesn't necessarily protect the plant from a frost, it does help your plant resist damage from cold-season risks. "Don’t let them go dry into the winter,” says Bennett. "Water them thoroughly if we have a dry fall. Proper planting and keeping plants healthy going into the winter is the best way to protect plants. If they go into winter in stress, they are possibly more susceptible to damage."

Individual Plants

To protect individual plants—instead of your entire garden—use a small hot cap: This can be a wax paper dome, an upside down jar or container, or a glass cloche. "They can be effective with light frosts, but only if they are able to cover the entire plant with minimal touching of the foliage," says Lentz.

Plants in Containers

Small plants in containers need to be protected from cold temperatures, too; you don't want the soil to freeze. "Depending on the type of container, you can bury it or mulch completely to cover the container, place straw bales around them, or wrap them," says Bennett. "I move all of my tender succulents into my garage, where it stays around 40 degrees. They never freeze, but they never grow all that great—when I take them out in the spring, they resume growth and look great."