One of the biggest worries of gardeners is the threat of freezing weather and frosts that can harm or kill plants and damage crops. Here’s what you need to know to protect tender plants from freezing temperatures.

What Is a Freeze?

A freeze occurs when temperatures drop below the freezing point of water (32° F or 0° C). When the water inside a plant freezes, it can cause the plant cells to burst, resulting in irreparable damage.

Plants react differently to freezing temperatures:

• Tropical and frost-tender plants: Cannot survive freezing temperatures so they only grow naturally in warmer climates.
• Annual plants: Can’t survive a freeze, but they disperse seeds to replenish their numbers once the weather warms.
• Root-hardy perennials: The foliage is killed back by a freeze, but the roots survive in a dormant state until spring.
• Fully hardy perennials, shrubs, and trees: Enter a dormant state, which decreases vulnerability to freezing temperatures by reducing sap content and conserving water. Spring blooms and early foliage may be damaged by late-spring freezes, but the plants themselves usually recover.

What Is Frost?

Frost occurs on clear, still nights. As the air temperature approaches freezing, the surface temperature of plants can dip below freezing, causing ice crystals to form in the same manner that dew forms on warmer nights. Because temperatures vary just a few feet above the ground, frost can form when your thermometer reads above freezing. Freezing temperatures may or may not be accompanied by frost.

Types of frost include:

• Hoarfrost is the familiar feathery white frost you see on chilly mornings. It results when water in the air is deposited directly in the form of ice crystals.
• Rime happens when water is deposited in liquid form through dew or fog which then freezes. Rime has a glazed appearance.
• Black frost is a term used when frost didn’t form, but plants were nonetheless damaged (and blackened) by freezing temperatures.

Fragile blossoms are threatened by late spring freezes.

Effects of Freezing Temperatures on Plants

For all but the most tender plants, it doesn’t matter whether the conditions produced a frost or a freeze. What’s important is how cold it got and for how long. When temperatures near freezing, a few degrees can make a big difference. To advise gardeners, so they can take proper precautions, different terms are used to describe the severity of a freeze. The chart below explains the various terms that are used:

Temperature	Term(s)	Effect on Plants
Down to 28° F for only a couple of hours	Light Frost, Light Freeze	Usually only harms very tender plants. Ice forms only on the outside of the plant.
25-28° F for several hours	Hard Frost, Killing Frost, Moderate Freeze	Damages foliage and blossoms. Ice forms inside the plant, causing plant cells to burst. Will kill back root-hardy perennials and damage crops.
Below 25° F for several hours	Severe freeze	Causes damage to many plants, mostly through desiccation (drying).

The average first and last frost dates for a given area usually refer to the occurrence of killing frosts. These are most often caused by fronts of arctic air moving in and are more indicative of seasonal change. Research has shown that most crops and plants can recover from brief dips below freezing, but when the temperature reaches 28° F it begins to cause extensive cellular damage and crop loss.

Avoid stimulating tender growth with fertilizer until freezing weather has passed.

How to Protect Tender Plants from Frost or Freeze Damage

If frost is predicted in your area, you may want to take steps to protect vulnerable plants such as:

• Houseplants and tropicals.
• Spring-blooming shrubs and trees such as azalea, rhododendron, and cherry.
• Citrus trees.
• Tender bulbs such as dahlia and elephant ear.
• Warm-season vegetables such as tomato, corn, and pepper.
• Warm-season annuals such as impatiens, petunia, and geranium.

Shrubs can be covered with a blanket to protect them from a late spring frost.

Steps to take when frost or freeze threaten tender plants:

• Bring Indoors: Frost-tender plants in containers should be brought inside during cold weather. Dig up tender bulbs and store them in a cool dry place.
• Water Plants: Water plants thoroughly before a freeze to prevent desiccation and to add insulating water to the soil and plant cells.

• Protect Tender Sprouts: Cover tender plants overnight with an inverted bucket or flower pot, or with a layer of mulch. Be sure to uncover them in the morning when the temperature rises above freezing.
• Cover Shrubs and Trees: Larger plants can be covered with fabric, old bed sheets, burlap, or commercial frost cloths (avoid using plastic). For best results, drape the cover over a frame to keep it from touching the foliage. Fabric covers help to trap heat from the soil, so make sure your cover drapes to the ground. Uncover them in the morning when the temperature rises above freezing.
• Assess Losses: Hardy perennials, trees, and shrubs may recover from a late spring freeze, even if visibly damaged. Their blooms and fruit may be lost for the year, but once they begin actively growing you’ll be able to determine and remove any permanent damage to stems and branches. Frost-tender plants will not recover at all, so avoid planting them until you’re confident that freezing weather has passed.
• Practice Prevention: Choose plants that are hardy for your climate zone, or plant tender plants in containers that can be brought indoors. Avoid applying fertilizer until after the last frost, to prevent a flush of tender growth that can be damaged by the cold.

Though blooms may be lost, hardy plants can recover from a freeze.

Further Information

To find the average frost dates for your area, check out:

• Spring Planting Time
• National Climatic Data Center Freeze and Frost Maps
• Old Farmer’s Almanac U.S. City Frost Chart

Also check out:

• Hardiness and Heat Tolerance – Understanding Your Zone
• Pushing the Limits of Your Planting Zone
• The Gardener’s Guide to Frost by Phillip Harnden (book excerpt)

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