At what temperature does it frost? Early spring brings the chance for a killing freeze; learn the frost temperature for plants.
Spring is the transition season from winter to summer. Temperatures rise, but cold outbreaks remain possible, especially in early spring. As for weather patterns, spring is the transition period from the winter storms and fronts to the more tranquil, high-pressure-dominated summer weather. Summer precipitation is mainly convective, with showers and thunderstorms developing in the warm, humid air. Spring can bring severe weather, strong thunderstorms, and even tornadoes as still-potent winter fronts interact with the increasingly warm, unstable air ahead of them. The majority of tornadoes in the U.S. occur in spring. Some areas can see spring flooding as melting snowpacks swell streams and rivers.
When does spring begin? Traditionally, the first day of spring is said to occur on the vernal equinox, which is around March 20 for the Northern Hemisphere. This is the astronomical spring start based on the relationship between the Earth and the sun. On this day, Earth is perpendicular to the sun, and everyone gets at least 12 hours of day and night. After this, the North Pole tilts more toward the sun. The days get longer, and the sun’s direct rays heat the ground more. This is why it gets warmer. Meteorologists consider March 1 to be the start of the three months of spring. It’s easier to bookkeep that way and usually relates well to weather conditions.
When it comes to nature, the start of spring – when perennial vegetation reemerges from winter dormancy – follows no set date. Although perennial plants respond to day length, which increases at about the same rate every year, temperature is the significant factor in determining when plants reemerge, and that’s anything but consistent from year to year. Warm temperatures in late winter can be enough to start things growing. It’s just the opposite for prolonged coldness into spring. Temperatures depend on the particular weather pattern we’re in, and that varies from year to year. Of course, this also varies by latitude. Spring arrives later the farther north you live.
As for spring planting, soil temperatures are critical for seed germination, and the best timing depends on what you’re planting. Planting too early can risk killing the seeds or slow development. Planting too late can expose cool-season vegetables to the excessively warm temperatures of summer. Typically, information on when to plant seeds is readily available on seed packages or in seed catalogs, through local agricultural agencies or extension services, or online. To determine soil temperature, you can purchase a relatively inexpensive soil thermometer at garden stores or farm supply shops and online. There are also various websites that display current soil temperatures.
Once a plant emerges from the soil or leaves develop on trees, air temperatures are critical, especially at or below freezing. The reemergence is due to water circulating throughout the plant parts aboveground. New leaves and flowers are highly vulnerable if this internal water freezes. Flowers die and won’t come back until next spring. This means no fruit on the fruit trees. New leaves can grow to replace the dead, but this can take weeks.
With all of this uncertainty, there’s help for those who look forward to spring planting each year. The National Centers for Environmental Information (NCEI) has a page dedicated to spring freeze. This will give you a range of dates for when your area has experienced the last spring freezes in previous years. Such information is also usually available at local extension services. Similar products are generated in Canada and available in provincial agricultural offices.
At What Temperature Does it Frost?
For current weather information, visit your local National Weather Service (NWS) office. The NWS uses two terms relating to potentially damaging low temperatures in spring: “frost” and “freeze,” and they refer to two different situations. Frost consists of white ice crystals that form on an exposed outside surface. The temperature of the surface must be below freezing, and it must also hit the dew point for condensation to occur. For drier air, the temperature can drop below freezing without frost particles developing. The occurrence of frost on a plant’s surface will do no harm. The same is true for snow.
Frost Temperature for Plants
When the NWS issues a “Frost Advisory,” it means “widespread frost formation is expected over an extensive area.” Official air temperatures may only be in the mid-30s degrees F, but the temperatures near the surface can drop below freezing. These are typically “radiational cooling” events, meaning that even though air temperatures can be above freezing, surfaces can radiate heat out into space, lowering their temperature even more. Such conditions are usually associated with clear skies and light or calm winds. Most commonly, these occur a day or two after a cold front has passed. They also can vary significantly with local topography and elevation. With little wind to mix the air, denser cold air sinks to lower elevations. Valleys can be colder than hilltops by 10 degrees or more. We can have the so-called “frost pockets.”
A “freeze” event is “when the surface air temperature is expected to be 32 degrees or below over a widespread area for a significant period of time.” Often, there’s wind and even cloud cover, and no visible frost may occur. This can happen immediately after a strong, cold frontal passage. Temperatures decrease with height, and higher elevations become colder. Wind chill isn’t a concern since plants aren’t susceptible to it. The NWS will use the term “killing” if significant plant damage is expected, and a “hard freeze” means temperatures of 28 degrees or less. A “freeze watch” is issued when “there is a potential for significant freezing temperatures within the next 24 to 26 hours.” “Freeze Warnings” are issued if the event is expected within the next 24 hours. Below-freezing temperatures cause damage with or without visible frost. The lower the temperature and the longer it stays below freezing, the greater the risk of damage.
There are things you can do to protect plants from the cold, but primarily for frost conditions, not freeze situations. Check out “How to Warm Soil and Protect Seedlings from Frost.”
The Enviromental Protection Agency (EPA) noted, “In recent years, the final spring frost has been occurring earlier than at any point since 1895.” This has helped produce an average increase in the growing season in the contiguous 48 states “by more than two weeks since the beginning of the 20th century.” Unfortunately, this can increase the threat of a “killing freeze.” Cold spells can still occur, even if the overall weather pattern is warm. It only takes one cold morning to do the damage. And it doesn’t have to be record-breaking cold, depending on how much plants have developed.
For more than 30 years, Ed Brotak taught thousands of college students about weather, and he’s helped many pursue careers in meteorology. He lives in Asheville, North Carolina, with his wife, Liz.
Originally published in the March/ April 2025 issue of Grit magazine and regularly vetted for accuracy.
