Frost or Freeze — What’s the Difference?

Frost or freeze? Weather reports often refer to such things as a “light frost” or a “hard freeze.” So what’s the difference between a frost and a freeze. And, more important, what temperature is “light” and what temperature is “hard”?

Light Frost

Frost occurs when the temperature hovers around freezing and the air is humid. The moisture in the air condenses and freezes, forming visible ice crystals on plants and other surfaces. Damage may occur to the tips of plants, but usually not to the lower parts near the ground.

As the soil cools in the fall, heat from the ground rises and cold air sinks. Frost is therefore more likely to occur on lower land than on hillsides. Our Tennessee farm is all hills and “hollers,” and we sometimes see frost in the “hollers” but not on the hills.

Generally a light frost will happen when the nighttime temperature dips below 32°F. But if the ground is already cold, frost can occur at a temperature as high as 36°F. Frost is generally short-term and occurs overnight or early in the morning.

So one difference between a frost and a freeze is that a frost creates a visible white coating of ice crystals, while a freeze does not. Another difference is that, for a freeze to occur, the temperature must reach below freezing. When it does so, and the air is dry, a freeze occurs without producing frost.

Light Freeze

A light freeze happens when the temperature remains just below freezing for a short time. The temperature range of a light freeze is 32°F to 29°F. A light freeze kills plants designated as tender, and may cause some damage to other plants. When frost accompanies a light freeze, it might be called a light frost.

Vegetables most likely to be damaged or destroyed by a light freeze include beans, cucumbers, eggplants, melons, peppers, squash, sweet corn, and tomatoes.

Moderate Freeze

A moderate, or medium, freeze occurs at temperatures between 28°F and 25°F. A moderate freeze damages most plants to some extent. For a killing freeze to occur, the earth has to lose enough heat for the ground itself to freeze.

Vegetables that will likely survive a moderate freeze include beets, carrots, cauliflower, celery, Chinese cabbage, lettuce, parsnips, peas, Swiss chard, and mache.

Hard Freeze

A hard freeze is also called a heavy freeze, severe freeze, or killing freeze. Sometimes it’s called a hard frost. But, as you know, it’s not really a frost unless it creates a veneer of visible ice crystals.

A hard freeze occurs under one of two conditions. It happens when the temperature remains below 28°F for four or more consecutive hours. It also happens when the temperature reaches 24°F or below for any amount of time.

A hard freeze causes serious damage to most plants, because it usually occurs after the soil has lost enough heat to start freezing. A long, hard freeze — one that causes crunchy footsteps — can kill even hardy perennials.

Vegetables that might survive a hard freeze include broccoli, Brussels sprouts, cabbage, kale, kohlrabi, parsley, radishes, spinach, and turnips.

Temperature Hardiness

Garden seeds and plants often come with information on their cold hardiness. Sometimes hardiness is in terms of temperature, but too often it refers to vague descriptions like “frost hardy” or, even more vague, “half hardy.” I use the above information to help me translate seed-jargon vagueness into usable data.

In my database of the plants my husband and I grow in our garden I have a field for hardiness temperature. During cold weather, we monitor temperatures using a digital thermometer with a outdoor remote sensor and max-min memory.

Armed with information on the hardiness temperatures of our garden plants, plus current temperature conditions, we have a pretty good idea which crops we need to harvest and which ones we can leave in the garden a little longer. A few things, like Siberian kale and Eskimo carrots, we are able to harvest throughout winter and into spring.

If you would like a jump start in determining the temperature hardiness of the plants or seeds you grow, this Virginia market gardener offers a good starting point. Her winter temperatures are colder than ours here in Tennessee, and she grows a wider variety of vegetables than we do.