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With 4 feet of snow dumped on South Dakota early this month, those of us in the northern portions of the United States can all be assured that Old Man Winter is on his way. Being prepared for freezing temperatures will help your garden survive damage from the cold and a frequent affliction in late fall and early winter: frost heave.
Frost or Freeze?
The United States Department of Agriculture defines the differences between frost and freeze, and the varying degrees of damage from either one.
Water vapor freezing on the surface at a temperature of 32˚F or below is considered frost, the formation of tiny ice crystals when the water vapor condenses. Ground level temperature is colder than the air temperature above it. Frost can damage tender plants, but usually not kill them.
A freeze happens when surface temperatures of 32˚F or colder prevail for days. A hard freeze is defined as 25˚F or below, and with both types of freeze, vegetation will be damaged unless protected. Another term--killing freeze--depends on hardiness of plants and their exposure.
What is Frost Heave?
The continuous alternation between freezing and thawing can affect many solid structures such as pavement, concrete, and building foundations, but those solid inanimate objects will not die from what is known as frost heave. In the garden, frost heave can quickly destroy plants and small shrubs if steps are not taken to prevent this winter damage.
For frost heaving to occur, the soil must have the ability to conduct water, an affinity for water, and high saturation; i.e., moisture retentive soils such as silt, loam, and clay. Additionally, a supply of water must be available and, lastly, freezing temperatures must be present.
How and When Does Frost Heave Happen?
Alternative freezing and thawing creates pressure which lifts the soil upward, usually taking the plant with it. Cold air penetrates down through the soil to the area of warmer soil and moisture, freezing it and forming a layer known as an "ice lens." The cold air from above presses down while the frozen soil beneath the plant pushes up. A frost heave reduces soil aeration and creates poor drainage, which adversely affects the plant. As it happens over and over, the roots can be exposed to the freezing air which will either break them or desiccate them, or both. Late fall and early spring are the times when frost heave is most likely to happen, as temperatures are very cold and soil moisture is abundant. Low-lying areas in the garden are the most susceptible since they tend to gather and hold more moisture. Plantings at the base of a slope or in tiered gardens should be monitored closely as cold weather approaches.
Forewarned is Forearmed!
Based on the United States National Arboretum zone map, determine your hardiness zone, then check the table below to see when you could expect the earliest frost, as well as the last frost of the season.
ZONE FIRST FROST DATE LAST FROST DATE
1*
July 15
June 15*
2
August 15
May 15
3
September 15
May 15
4
September 15
May 15
5
October 15
April 15
6
October 15
April 15
7
October 15
April 15
8
November 15
March 15
9
December 15
February 15
10
December 15
January 31
11
No Frost
No Frost
*Year-round potential for frost
What Can You Do?
The most recommended method of protecting your plants from frost heave is to insulate the soil with evergreen boughs. If a plant already shows signs of heaving, press it firmly back into the ground, then cover the soil around the root area with leaves, followed by boughs of evergreen. Other types of mulch can work, but not as effectively as the boughs, or even snow. One source stated that a foot of snow on top of the ground reduces the depth of frost penetration by the same amount. 1
1 Smith, Charles. The Weather Resilient Garden (Storey Publishing, 2004)
About Toni Leland
Toni Leland has been writing for over 20 years. She has written a biweekly gardening newspaper column as a spokesman for the Ohio State University Master Gardener program, and writes for Grit magazine and Romance Writers Report. She has been a gardener all her life, working soil all over the world. In her day job, she scripts and produces educational DVDs about caring for Miniature Horses, as well as writing and editing books about this unique height breed.
Posted by doone (from Ocala, FL) on January 21, 2009 at 07:33 PM:
We are getting a hard freeze in Florida tonight (19-22degrees). Do we have to leave a sprinkler system run on our strawberry plants all night long? Is there anything else that can be done to save them. Thanks
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Subject: How late to mulch
Posted by lizpadjen (from Preston, MN) on November 24, 2008 at 08:18 AM:
I am very confused about the protection from frost heave. I have read that it is good to wait to mulch until after a good freeze. How hard a freeze? Is it ok to wait as long as temperatures don't get too warm after the frosts? Once there is a frost what temperatures are dangerous? I live in zone 4.
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Posted by joeyramone (from Schenectady, NY) on June 13, 2009 at 08:44 AM:
I'd like the same answers as Liz... I've heard that some perennials shouldn't be mulched. Is that true???
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Subject: Frost heave
Posted by Hemophobic (from Kannapolis, NC) on November 21, 2008 at 07:58 AM:
Toni: Your article about frost heave was most interesting, although generally in NC we don't have to worry about that. However, this may be one of those winters when we have more northern-like temps. We've already had several days of below freezing weather. This morning the northern half of NC is getting snow, not just in the mountains, and I just planted 3 expensive roses yesterday when it was 61. Now we're in for frigid temps and windy conditions.
Thank you for sharing this info with us.
Angie
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Posted by darius (from So.Appalachian Mtns, VA) on November 21, 2008 at 04:47 PM:
Toni, why are evergreen boughs better than a thick mulch of leaves or straw?
Thanks for the article.
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Posted by carrielamont (from Milton, MA) on November 21, 2008 at 06:31 PM:
Toni, I was wondering the same thing! I have no ready access to evergreen boughs, but we've got our leaves, the neighbor's leaves, last year's leaves, the rest of the neighborhood's leaves ... leaves we have. Thank you for reminding me of this now instead of me having to hit myself over the head in April!
This message was edited Nov 21, 2008 5:32 PM
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Posted by tonileland (from Nashport, OH) on November 21, 2008 at 08:38 PM:
My references stated that the evergreen boughs provide protection and keep the soil temperature from changing too quickly. There was no elaboration on this, but the author did say that mulch or pine bark would work, but not as well because of its density. I should probably dig a little deeper and see if I can find a more definitive reason. :o)
I think a significant layer of leaves surrounding the base of the plant would work just fine.
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Posted by darius (from So.Appalachian Mtns, VA) on November 22, 2008 at 09:31 AM:
Okay, I got it, just had to think it through ... the evergreen boughs have 'air pockets' or spaces between the branches, thus they don't 'pack' densely like chips. So, straw would offer a similar protection. Leaves might work as well, or might not, depending on shape and if they have been run over with a mulching mower.
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Posted by tonileland (from Nashport, OH) on November 22, 2008 at 12:55 PM:
Darius, I'm thinking with the leaves thing that the larger ones (hence, more fluffy) would be better than mulched. We have industrial quantities of oak here, with leaves reaching 7 inches long. When they gather in a corner or against the fence, they provide a pretty good layer of insulation.
Certainly something to think about if one doesn't have evergreens handy, or doesn't want to cut them.
Wouldn't straw blow around?
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Posted by darius (from So.Appalachian Mtns, VA) on November 22, 2008 at 03:23 PM:
Straw will blow a bit in strong winds, but the stalks tend to intertwine so most of it stays in place.
