Learn how to predict frost, differentiate between a light frost and a hard freeze, and protect your garden from frost with these tips!
How to Predict a Frost
First, look up the average frost dates for your area to know when frosts are most likely to occur. Use our Frost Dates Calculator to find frost dates for spring and fall.
Keep in mind that frosts can vary greatly by microclimate. In fact, while you may have frost in your garden, your neighbor down the street may see no sign of it!
Consider these factors when the radio and TV reports say “frost tonight.”
- How warm was it during the day? 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 nighttime is slim.
- Is it windy? 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 the ground.
- Is it cloudy? If the Sun sets through a layer of thickening clouds, the clouds will slow radiational cooling and help stave off a frost.
- What is the dew point? As a rule of thumb, don’t worry about a frost if the dew point (the temperature at which water vapor condenses) is above 45°F on the evening weather report.
- How is your garden landscaped? Gardens on slopes or high ground often survive when the coldest air puddles down in the valleys and hollows.
- How far are your plants from the ground? Plants that are close to the ground have a better chance of being protected by the warmth of the earth or the foliage of neighboring plants.
- What’s your local weather forecast? Don’t get blindsided by bad weather!
Difference Between a Light Frost and Hard Freeze
A light frost occurs when the nighttime temperature drops to below 32 degrees Fahrenheit, and refers to the conditions that allow a layer of ice crystals to form when water vapor condenses and freezes without first becoming dew.
A hard freeze is a period of at least four consecutive hours of air temperatures that are below 25 degrees Fahrenheit. Many plants can survive a brief frost, but very few can survive a hard freeze.
Preparing for a Fall Frost
If you’re a gardener, it’s the first fall frost which is most concerning, as it can result in a lot of lost crops. Here are a few fall frost damage prevention tips:
- Harvest basil and other tender herbs before a light frost (when the temperature drops to 32ºF). Even if they survive the frost, they don’t do well in cold temperatures. The same is true for most annuals.
- Bring geraniums and other houseplants (especially tropicals) indoors before the first light frost arrives. Keep them in a sunny window in a relatively moist room; the kitchen is often best. See more about overwintering geraniums and preparing the garden for winter.
- Harvest all tender vegetables and greens before a light frost, including: tomatoes, eggplants, peppers, beans, cucumbers, watermelons, cantaloupe, spinach, okra, squash, and sweet corn. Here are a few tips for ripening green tomatoes specifically.
- Another option is to protect tender vegetables from light frosts with row covers, cold frames, old sheets, paper bags, or clear plastic containers.
- For plants that can survive a light frost, add a heavy layer of mulch to keep the ground from thawing. You can still harvest late into the fall as long as the ground isn’t frozen. These veggies include: beets, broccoli, cabbage, celery, lettuce, parsnips, swiss chard and leafy greens, and arugula.
- Harvest plants that can survive a hard frost last, such as: carrots, garlic, horseradish, leeks, parsnips, radishes, and turnips.
Following these tips should help prevent your garden from taking too much of a hit when the first frost occurs! For even more tips, see Preparing Your Garden for Winter and Fall Chores: Autumn Garden Cleanup.
Be sure to check our more extensive instructions on preparing for frost as well, and let us know in the comments what you do to prepare for frost!