Seeing a crop through to harvest can pose a challenge when frost is in the forecast.

When water vapor condenses and freezes instead of forming dew, we see ice crystals on outdoor surfaces.

This is called hoar, light, or white frost. It forms when the air is moist and the ground temperature drops to 32°F or lower.

And because the ground temperature can be cooler than the air temperature, it may form on the soil even when the thermometer reads above freezing.

Damage occurs when ice crystals form inside the tissue of vulnerable plants, causing it to split open and leaching away essential nutrients.

Conversely, when both the air and ground temperatures fall below freezing and the air is dry, freezing without visible frost takes place. This is called hard or black frost, or a freeze.

A freeze dries out plant tissue – in effect, burning it.

In this article you will learn about vegetable hardiness, what to do when a freeze is predicted, and how to evaluate damage.

Understanding Hardiness

Vegetables vary, ranging from able to withstand a freeze to being unable to tolerate even a light frost.

Degrees of hardiness in plants are described as:

Let’s take a closer look at each category:

Hardy

When a vegetable species is referred to as “hardy,” according to James Myers, plant breeder and researcher at Oregon State University this means it can withstand air temperatures below 28°F.

Hardy crops thrive in cool weather with three to six hours of sun per day.

Planting time is in early spring or late summer. They react adversely to heat, often bolting – or going to seed – if the temperature spikes.

Examples of hardy vegetables include:

They can overwinter in all but the most extreme weather conditions.

Half-Hardy

Some vegetables are classified as “half-hardy.” They can tolerate one or several light frosts and temperatures in the range of 28 to 32°F.

Half-hardy crops also do best in cool weather, however, they need a minimum of six hours of sun each day, also known as “full sun.”

Planting time is in late spring, just before the last frost date, or during the summer, with enough time allowed for maturity in early fall.

Examples of half-hardy vegetables are:

Like hardy plants, half-hardy species may bolt during a heatwave.

Tender

Then there are the more fragile crops we refer to as “tender.”

They require temperatures above 32°F and may tolerate a very light frost.

Tender vegetables need eight hours of sun per day to flourish, and cannot be planted outside until the last average frost date in spring has passed.

Examples of tender crops are:

Summer planting must take place early enough to allow for maturity before the first average frost date in fall.

Very Tender

Finally, there are “very tender” vegetables that require eight to 12 hours of sunlight per day.

They can’t tolerate any degree of ice crystal formation, and must be planted after all risk of frost has passed.

Examples of very tender vegetables include:

A second summer crop must mature before the first average fall frost date.

Preparing for a Frost Event

As the first frost date approaches, continue watering, taking care to aim your hose at the soil level near the roots, and not over the foliage.

Watering plants during the day before a freeze is predicted is beneficial.

It allows your crops to absorb moisture, enabling them to retain daytime warmth and generate heat through the evaporative process of transpiration.

Harvest mature crops, as well as those that continue to ripen after picking, like peppers and tomatoes.

Collect available seeds to save for next year.