How to Predict a Frost (2024)

Frost is one of a gardener’s worst foes! Learn how to predict frost, understand the difference between a frost advisory and a freeze warning, and protect your garden fromfrost.

What Is a Frost,Exactly?

“Frost” refers to thelayer of ice crystals thatform when water vapor on plant matter condenses and freezes without first becoming dew.

• A light frost occurs when the nighttime temperature drops to at or just below32°F(0°C).
• A hard freeze is a period of at least four consecutive hours of air temperatures that are below 28°F (–2°C).

Many plants can survive a brief frost, but very few can survive a hard freeze. (See more about this below.)

Frost Advisoryvs.FreezeWarning

As withother significant weather events, meteorologists will often issue a “warning” or an “advisory,” depending on the likelihood of the event happening and its severity. According to the National Weather Service, the warning terms for frosts and freezes are defined asfollows:

• Frost Advisory: Issued when minimum temperatures are expected to be between 33° and 36°F (0.5° and 2°C). Skies are generally clear andwindslight.

• Freeze Watch: Issued when minimum temperatures are expected to be 32°F (0°C) or less within the next 24 to 36 hours.

• Freeze Warning: Issued when minimum temperatures are imminently expected to be 32°F (0°C) or less.

• Hard Freeze Warning: Issued when minimum temperatures are expected to be 28°F (–2°C)or less.

Of course, the easiest way to predict frost is to let the weatherman do it for you! However, if you want to be able to predict it yourself, readon.

Know Your FrostDates

The first step in predicting frost involves getting to know theaverage frost dates for your area. Put your zip code in ourFrost Dates Calculatorto find frost dates for spring and fall for yourlocation.

Note: These dates are averages, so they canonly tell uswhat is typical not the future. Also, the frost dates are based on a30% probability, meaning that there is a 30% chance of a frost occurring after the given spring frost date or before the fall frost date. (In other words, these dates are NOT absolutes and should only be used as roughguidelines.)

Learn YourMicroclimates

Keep in mind that the occurrence offrostcan vary greatly by microclimate, too. In fact, while you may have frost in your garden,your neighbor across the street may see no sign of it! A microclimate is exactly what it sounds like: a climateon a small scale. For example, if your garden is located at the bottom of a hill where cold air settles, it’s likely to beimpactedby frost earlier than a garden at the top of the hill. Or, if your plants are abutting a rock wall in full sun, they’ll be kept warmerto some extent by the heat given off by therocks.

5 Tips for PredictingFrost

Consider these factors when the radio and TV reports say “frosttonight”:

1. Temperature: How warm was it during the day?It may sound simple, but one of the best ways of determining if a frost is due overnight is to gauge the temperature. If the temperature reached 75ºF (in the East or North) or 80ºF (in the desert Southwest), the chance of the mercury falling below 32ºF at night is slim.See our 5-day forecasts to check your weatherforecast.
2. Is it windy?A windy night is also likely to reduce the likelihood of a frost. A still night allows cold air to pool near the ground; a light breeze stirs things up; a heavy, cold wind sweeps away warm air near theground.
3. Is it cloudy?Observe the sky. If the Sun sets through a layer of thickening clouds, the clouds will slow radiational cooling and help stave off a frost. With clouds, the risk of frost isreduced.
4. Slope: How is your garden landscaped?Gardens on slopes or high ground often survive. However, cold air sinks and willpuddledown into the valleys and hollows. If your home and garden are at thebottom of a slope or in a valley, and there is no wind, then there is higher risk of frost. A landscape with trees can assist in preventing frost. Trees transpire a lot of moisture through theirleaves.
5. What is the dew point?As a rule of thumb, don’t worry about a frost if the dew point (the temperature at which the air is no longer able to ‘hold’ all the moisture within it) is above 45°F on the evening weather report. The more moisture in the air, the less likely a frost.A light watering of the garden a day or two before a frost is predicted can help stop itsettling.

How to Protect PlantsfromFrost

A frost means damage or even death to many vegetable crops. However, there are vegetables which actually benefit from a frost. The flavor of broccoli, for instance, actually improves if the plant has experienced a frost, and carrots get sweeter as the temperaturedrops.Know the frost resistance of different vegetables here.

• Keep an eye on the weather forecast. If it looks like temperatures are going to drop, get ready to protect tenderplants.
• Generally, covering plants to create a temporary pocket of warmer airis the best way to protect them.Make use of season extenderslikerow covers, cold frames, or cloches to protect tender plants, such as seedlings or warm-weather veggies.Row covers or garden fleece canbe used to help create a warmer environment beneath them. You’ll need to use posts, bamboo, or flexible PVC pipingto create space for the plants to grow, then drape landscape fabric or plastic over the frame; weigh down the edges with rocks or bricks or pegs so the covers do not blow away. To protect young plants from frost, use 2-liter soda bottles cut in halfascloches.
• It’s best to have all covers in place well before sunset. Drape loosely to allow for air circulation.Before you cover the plants in late afternoon or early evening, water your plantslightly.
• Remove any covers by mid-morning so that plants can getfull exposure tothe warmingsunlight.

For more frost protection tips, read our main article:Protecting Your Garden from Frost.

LearnMore

Following these tips should help prevent your garden from taking too much of a hit whenfrostoccurs!

For more information, readPreparing Your Garden for Winterand Fall Chores: Autumn Garden Cleanup.

Let us know in the comments what you do to prepare forfrost!