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Blog: How to Predict a Frost

September 21, 2013

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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.

See average frost dates for your area. Click here for the U.S. Frost Chart and for the Canadian Frost Chart.

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.

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.
  • 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!

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Catherine, our New Media Editor, joined The Old Farmer's Almanac in 2008. She edits content on both this Web site, Almanac.com, and the companion site to The Old Farmer's Almanac for Kids publication, Almanac4kids.com. She also pens the Almanac Companion enewsletters and keeps up with readers on Facebook, Twitter, and Pinterest!

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Comments

Leeks can survive a hard

By Paul W.

Leeks can survive a hard frost? Wow! This my first year planting them and I did not know that. I'm glad to hear it, because I was a little late getting them in and our average first frost is only 5 weeks from now.
Thanks for the information, Catherine.
What kind of mulch can I use on the parsnips?

Paul

Brunswick, ME

Compost, straw or chopped

By Almanac Staff

Compost, straw or chopped leaves are all good mulches for root veggies.

My family folk lore has

By April Devon

My family folk lore has passed down the theory that the day you first hear a cicada, you have 15 weeks until your first frost. we live just south of the Boston area, so the weeks will vary for your locations.

Last year my husband was one day off, and i was a week off. This year i was on the exact day of our first frost and my husband was a week off. Pretty good for folk lore.

For us here in Tampa Fla

By Garden Dog

For us here in Tampa Fla anything close to a frost or freeze will cause damage. Most if not all landscaping is grown in south Fla then shipped up to us. Tropical plants just will not handle cold temps. In a "normal year" we get two maybe three cold nights a winter. Last year we had ten days in a row of hard freeze. Is this our new "normal?"

first HARD freeze was last

By Greg Sherman

first HARD freeze was last night, November 1st temps dipped down to 31

We had our first frost last

By rockinrfarm

We had our first frost last night, Oct. 3, in central Arkansas. May do it again tonight. Very early this year.

My farmer grandfather said

By Sil in Corea

My farmer grandfather said frosts generally come on or after the full moon, which pulls in high air pressure from the north-west.

That's amazing, Sil. I'd love

By Paul W.

That's amazing, Sil. I'd love to hear more of what your grandfather told you about gardening.

As of Sept 30, 2010, we still

By Catherine Boeckmann

As of Sept 30, 2010, we still haven't had frost in my town of Dublin, NH! I will say that I didn't grow regular tomatoes again this year. Instead I grew cherry tomatoes--almost all ripened up! It was a nice long season. Hope you had some gardening successes!

Tomatoes in British Columbia.

By L R 7

Tomatoes in British Columbia. I uprooted my Gardeners Delight cherry tomatoes from my balcony just before our cold snap in September. (Now it's warm again). I picked off the leaves, and hung whole plants upside down to dry indoors. The tomatoes ripen a few at a time making a nice fresh snack. The tiniest will be too small to fry, but provide bright colour contrast in the arrangement! Perhaps they will make a spooky Halloween decoration for my door!

Here in Calais, ME we have

By Sharon Weber

Here in Calais, ME we have had 2 or 3 frosts since late September. However, no hard frost as yet. It was a terrible year in the garden due to above average rain early on and cool temps all summer...no tomatoes to speak of this year. I have never taken to fried green tomatoes, but do like green tomato mincemeat etc..so all is not lost I guess.

We here on the shores of

By Katrina McManus

We here on the shores of Oneida Lake have escaped a frost ,while others inland have had one.The unfrozen lake gives us a bit of insulation since the water holds some warmth.But watch out after it freezes,then it acts like a giant ICE CUBE!

Folklore says if we in LA

By Lynetta Billiot

Folklore says if we in LA have fog in August we can expect frost on that same day in October, we had no fog in August so now I'm not sure! But, usually it is anywhere from mid to late Oct. when we get our first frost. Our garden has long given up it's bounty except for my sweet potatos and I'm told they need to be harvested when the leaves turn yellow or after the first frost. Seems they are much sweeter after it has frosted on them, so I shall wait for frost before harvesting them.

Our first frost in upstate

By Cyndy Cook

Our first frost in upstate New York usually comes anywhere from mid-to late September, and it did so last Friday night (18th). Now we can go and pick apples this coming weekend because the frost will have made them even sweeter! I covered my container tomato plants with plastic because I didn't want to harvest all the fruit just yet, since we're having much warmer weather this week (but not so much sun). I make it so that each container becomes its own little greenhouse. I should be able to prolong their season on the vines into October.

The first frost in Prince

By Al Brooks

The first frost in Prince Albert SK Canada is about the 4Th of Sept. This year the temp has been at a record level and seems more like August than Sept with Temperatures reaching 28 and 29 celsius. As of the 22 of Sept there has been no frost and no temperatures even close and it looks like there will be no frost until Oct. What a great fall.

The first frost in Indiana

By Ann Hicks

The first frost in Indiana usually doesn't arrive until mid or late October. Before that, I snip my herbs, mince them, and put them in plastic containers in the freezer. They make wonderful fresh herb additions to soups and stews! Or you can simply cut down the entire herb plant, hang it upside down in a cool place, and use the herbs dried. I harvest my green tomatoes before the frost and put them in a brown bag on the kitchen counter. They will eventually ripen. However, check them frequently as sometimes they are too green and will never ripen. Then you can use them for fried green tomatoes!

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