Blog: How to Predict a Frost

If you're a gardener who lives with frost, it's important to get your vegetables harvested in time. Here are tips on how to predict the arrival of Jack Frost.

Keep in mind that frost can vary greatly by microclimate. In fact, I had frost in my garden one year and my neighbor down the hill did not! Another year, I harvested all my tomatoes due to a frost warning that turned out to be a false alarm. I ended up having tomatoes ripening on the counter, but better safe than sorry.

How to Predict a Frost

Consider these factors when the radio and TV reports say "frost tonight."

• How warm was it during the day? If the temperature reached 75 degrees F (in the East or North) or 80 degrees F (in the desert Southwest), the chance of the mercury falling below 32 degrees 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 degrees on the evening weather report.
• How is your garden sited? 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? Those plants that are close to the ground have a better chance of being protected by the warmth of the earth.

See average frost dates for your area. Click here for the U.S. Frost Chart [3] and for the Canadian Frost Chart [4] on Almanac.com.

What is the difference between a light frost and a hard freeze?

A light frost occurs when the 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.

Better Safe Than Sorry

If you're a gardener, here are few tips on preparing for frost.

• Harvest basil and other tender herbs before a light frost (when the temperature drops to 32 degrees Fahrenheit). Even if they survive the frost, they don't do well in cold temperatures. The same is true for most annuals.
• Bring geraniums 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 overwinterizing geraniums and putting your garden to bed [5].
• Harvest all tender vegetables before a light frost, including: tomatoes, eggplants, peppers, beans, cucumbers, melons, spinach, okra, squash, and sweet corn. Another option is to protect tender vegetables from light frosts with row covers, old sheets, paper bags, or plastic.
• For plants that can survive a light frost, add a heavy layer of mulch to keep the ground from thawing and 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.

I hope you find these tips helpful. Please share your tips and experiences below!