How long does it take for frost to come out of the ground?

Is Ground Frozen Solid: Determining If Soil Is Frozen

No matter how anxious you are to plant your garden, it’s essential that you wait to dig until your soil is ready. Digging in your garden too soon or in the wrong conditions results in two things: frustration for you and poor soil structure. Determining if soil is frozen can make all the difference.

How do you know if the ground is frozen solid? Keep reading to find out how to tell if ground is frozen or not.

How to Avoid Digging in Frozen Soil

Although it may seem as if spring has arrived, it’s important to test the soil for readiness before working your soil or planting your garden. Several very warm days in a row may lead you to believe that the ground is ready to be worked. Be very leery of any early spring digging, especially if you live in a northern climate. Determining if the soil is frozen is paramount to your garden’s success.

How to Tell if Ground is Frozen

Just walking across your soil or patting it with your hand will give away whether it is still frozen or not. Frozen soil is dense and rigid. Frozen soil feels very solid and does not give way under foot. Test your soil first by walking on it or patting it in several locations. If there is no spring or give to the soil, it’s probably still frozen and too cold to work.

It’s best to wait for the ground frozen solid to break up naturally than to try to rush it out of winter dormancy. Soil that is ready for planting is easy to dig and yields to your shovel. If you begin to dig and your shovel seems to be hitting a brick wall, it is evidence that the soil is frozen. Digging frozen soil is hard work and the minute you realize you are working way too hard just to turn up the soil is the time to put the shovel down and exercise some patience.

There is never any sense in getting ahead of the natural sequence of events. Sit back and let the sun do its job; planting time will come soon enough.

In mid-March, with temperatures climbing into the 60s from East Grand Forks, Minn., to Estherville, Iowa, the winter’s frost was rapidly coming out of the ground.

“The first two to four inches of top soil thawed on March 10, and I expect that by the end of the forecasted warm period frost should be out down to eight to 10 inches,” said Jeff Vetsch of the University of Minnesota’s Southern Research and Outreach Center at Waseca.

Vetsch said that the 25-inch frost depth at Waseca measured on March 9 was about average and that it was similar to last year’s maximum frost depth. March 9 was the last day downward movement of frost was measured anywhere in the region.

Frost did not penetrate as deeply at the University of Minnesota’s research station at Lamberton this year as it did last year. This year, on March 9, the frost depth at the Southwest Research and Outreach Center at Lamberton recorded frost at 37 inches on bare ground. Frost on ground with sod on it was at 19 inches at the SWROC on March 9. A year earlier, on March 17, 2014, bare ground frost was at 54 inches.

“Last winter’s frost was unusually deep,” said Bruce Potter, assistant Extension professor for integrated pest management at SWROC. “This winter we did not have the prolonged extreme cold temperatures.”

Based on weather forecasts, Potter said he believes the frost will be out before fieldwork begins in southwestern Minnesota this year. That wasn’t necessarily the case last year.

“In parts of Minnesota last year, the upper soil thawed and dried deep enough to plant corn but there was still some frost lower in the profile,” Potter said.

The Research and Outreach Centers at Lamberton and Waseca are part of a network of weather stations that, among other things, report frost depth to the National Weather Service’s Northwest River Forecast Center. Frost depths are measured using tubes that go 5 to 6 feet into the earth.

In Long Prairie, which is the most northerly frost tube station in the state outside of Duluth and Grand Forks N.D., the frost just kept getting deeper this year. On March 9, Steve and Nancy Potter, who operate the weather station, measured frost at 40 inches. Their frost tube is on soil covered with vegetation and not in an open field.

“In the middle of March of 2014 the frost depth was 11 inches,” Nancy Potter said. “It did not go below 12 inches last year.”

The frost tube at Grand Forks measured frost at 46 inches and Duluth was at 39 inches, on March 9. South of Duluth, in northwestern Wisconsin, frost was at 57 inches. Other frost depth measuring sites in Minnesota include Morris at 30 inches, Montevideo at 42 inches, Blue Earth County at 23 inches and the University of Minnesota at St. Paul at 32 inches.

The River Forecast Center does not have frost tube stations reporting to it in northern Iowa but a tube in the area of Prairie du Chien, Wis., across the Mississippi from Iowa, measured frost at 27 inches. A tube at Sioux Falls, S.D., had frost at 15 inches.

Soil temps

Most areas don’t have frost tubes to measure frost depth so estimating frost depth becomes a guessing game.

“I don’t know of official deep soil temperatures locally,” said Dan Martens, University of Minnesota Extension educator for Stearns, Benton, and Morrison Counties. “Reports from well drillers, plumbers, grave diggers are as good as it gets. And I’d guess it varies widely across the landscape.”

Marten’s colleague Beth Berlin, an Extension educator for horticulture, said she’s heard informal reports of frost as deep as 60 inches or more.

“In regards to the impact on perennials plants, depth of the frost is not as big of a factor as temperatures near the soil surface where the roots for most of the plants are concentrated,” Berlin said. “Due to the lack of snow, it is concerning how hardy the perennials will be. Those who insulated their gardens with mulch such as leaves, straw, or pine needles, will likely have less loss, but for those who were relying on snow cover, plant loss is possible.”

Martens and Vetsch both agree with Berlin that it’s the temperatures near the soil surface that put plants at risk. Vetsch recalls that last year, from March 10 to 22, temperatures reached the mid-30s to mid-40s. That caused alfalfa to break dormancy. Then temperatures plummeted to 10 to 11 F. That damaged the new growth on the alfalfa, Vetsch said.

“Generally if the soil temperature around the crown drops below 15 degrees, there can be damage to the alfalfa crown,” Martens said. “Under those conditions we could have concern for winter wheat and rye as well. Rye is more durable than winter wheat.”

“The frost is down five inches from top and one inch at bottom,” Nancy Potter said of conditions in Long Prairie on March 14. “Only 34 inches to go.”

Visit http://tinyurl.com/lgajfpc to monitor the Northwest River Forecast Center’s frost tube network. Hover your mouse over the dots to see the data that’s hiding there.

How far down does the ground freeze?

I had a very interesting question from my News Editor yesterday.

She wanted to know how far down the ground freezes?

I didn’t ask why she wanted to know, but thought I’d better find out and do some research to keep my boss happy!

Basically, it depends on a few things:

  • How cold the air temperature gets
  • How long the air temperature stays below freezing
  • Whether there is snow cover (and how deep it is)

Long cold spells with no snow cover can cause the ground to freeze to a few feet whereas if there’s persistent snow cover, the ground may only freeze to a few inches deep.

In the Arctic the ground may be permanently frozen for thousands of feet!

I later discovered that the reason my editor wanted to know was because she was concerned about her tortoises who are currently hibernating three feet under the ground in the garden.

I reassured here that, although it’s been very cold and frosty recently (with some low night-time temperatures), the ground is probably not frozen more than an inch or so deep (if at all), because temperatures during the day have risen above freezing thanks to some sunshine.

So hopefully her tortoises, who slept through the coldest December for at least 100 years, will emerge from their sleep, healthy and happy in the spring.

Derek

Overview: Soil and the Spring Thaw

You have all seen highway signs indicating the period of spring thaw and reduced truck loads throughout the northern US and all of Canada — or walked across grass that is just beginning to thaw from the winter making deep footprints or tire marks that would never sink like that during a strong summer rainstorm. You may have noticed decks that moved or gates that stick, then un-stick. Some things sink, some rise and some do both. What is it about the spring thaw that makes landscaping so unpredictable and delicate?

Some cold climate science first, and then we will see how that affects our day to day structures in the north.

The first three to four feet of the crust of the earth receives heat from two sources: the sun and the core of the earth. In most of northern North America, the core of the earth sends up heat at around 7-12deg C (45-54degF) all year round — well above freezing. It is the temperature above the soil that changes with the seasons. As freezing temperatures set in during the beginning of winter, the soil begins to freeze, from the top down. Since there is always heat from the bottom, it takes continuous cold from the top to drive the frost line lower and lower.

If there is no clay in the soil, the freezing moves down rather steadily, with little movement in the soil. When there is a lot of clay in the soil, things change. Clay soil freezes in layers, called ice lenses, drawing water up to the forming lens of ice and actually drying out the soil an inch or more lower down. Only when the soil under the lens is sucked dry will the cold temperatures move deeper into the soil and begin to form a new ice lens. Each lens expands, just like an ice cube in the freezer forms a bump on the top, and pushes the lenses above it even higher — and hence we get frost heaving. For more details check out ICE LENSES.

Although ice lenses can grab onto the sides of posts and even foundations to lift them up, we avoid the more direct formation of ice under structural supports by putting our house foundations as well as fence (Fence post depth ) and deck (Supporting Outdoor Structures ) supports below the local frost depth.

Now let’s go to the spring thaw. We still have a small constant heat coming up from below trying always to thaw the ground, but the real thaw comes more quickly from the top down. Imagine this block of ice that is sitting there just below the top of the ground. Depending on the depth of frost in your locality it might be as shallow as 6 inches in Vancouver to as deep as 4 feet in much of the rest of Canada. No rainwater or snow melt can percolate through this block of ice to flow into the water table. But the rain and the melt keep coming. That means that the little layer of thawed out soil on the top is quickly completely saturated. This is the marshy soupy landscaping that you should simply stay off of for a couple of weeks to avoid damaging it.

If there is a fair amount of clay in this soil — it will expand and paving stones and other surface landscaping will probably heave temporarily upward, until this soil begins to loose its excess water. It is best to let this soil drain to a normal moisture content before undertaking any repairs, like that jammed gate, because clay soils will shrink as they loose water and the problem may just go away. If you walk on paving stones that are on a mushy foundation, you will force some of that clay out to the sides, and when it shrinks back to normal, the paving stone will drop lower than normal — if you stayed off of it until the thaw was well advanced, this doesn’t happen and the heaved stone will just go back to where it came from.

All saturated soils are too fluid and basically unstable. We actually have to wait until that hidden block of ice under this area finally thaws out, letting the water flow downwards and drain the surface before working on it or even trying to access winter damage. Even the highway trucks in our northern climate need to carry lighter loads until the frost is gone allowing the water to drain and the soil to return to its normal strength.

This frozen landscape that can’t percolate off surface water is also the cause of most flooded basements. In this case, simple landscaping to run surface water away from the house will handle most of the spring basement flooding problem: Yard Drainage.

Air frost and ground frost – what’s the difference?

Today, most of us woke up to a cold, frosty start with scrapers at the ready to remove the ice from car windscreens – never a pleasant chore first thing in the morning.

Considering that it’s mid-November, we’ve seen relatively few frosts so far, although that is likely to change into next week, as cold Arctic air spreads down from the north.

In order for frost to form, there are a few vital ingredients – chilly air, clear skies and light winds. These combined provide the perfect frosty recipe.

What causes frost to form?

Chilly air is the obvious ingredient, because if the air is inherently warm, then it doesn’t matter how little cloud or how light the winds are, it won’t cool sufficiently.

Clear skies allow the heat from the ground to escape into space. This process is particularly effective in autumn and winter because the nights are longer than the days. This results in more hours of the day when heat can escape and thus more cooling.

Light winds are crucial to avoid mixing the air at the close to the ground with the air just above. As the air cools from the ground up, any mixing from above will bring relatively warmer air towards the ground – stopping the temperature from falling further.

So that’s how frost forms, but did you know that there are notable differences between an air frost and a ground frost?

Ground frost

A ground frost occurs when ice forms on the ground, objects or trees, where the surfaces have a temperature of freezing or below, causing water to freeze.

Weather stations measure the ground temperature with a thermometer that’s 5cm from the ground. When the temperature at this point hits 0C, then a ground frost has been recorded.

However, because the ground cools quicker than the air around a metre above, it is possible for a ground frost to occur without an air frost.

There’s also something called a grass frost, which is where natural surfaces such as grass freeze when man-made surfaces such as tarmac and concrete don’t. This type of frost is of most interest to gardeners.

Air frost

An air frost occurs when the depth of air above the ground has increased to the level at which weather stations measure air temperature – which is 1.25 metres in the UK.

So, effectively the layer of cold air near the ground hitting 0C or below has become thicker and thicker as more heat energy is lost to space and the temperature falls.

Hoar frost

This is another type of frost that we typically associate with a frosty morning and has a white appearance.

A white frost, consisting of little blobs of ice, occurs when dew has formed and then subsequently frozen when the temperature hits 0C.

A feathery frost, consisting of feathers and needles, occurs when the temperature has already reached 0C and then dew forms.

Don’t forget, you can keep track of the next frosty night on the Channel 4 Weather website. You can also send me your frosty weather pictures on Twitter – @liamdutton

Frozen Ground and the Frost Line: How and Why it Freezes

How Deep Does the Ground Freeze in Winter?

Ground frost occurs when the ground contains water, and the temperature of the ground goes below 0° C (32° F). More than half of all the land in the Northern Hemisphere freezes and thaws every year, and is called seasonally frozen ground. One-fourth of the land in the Northern Hemisphere has an underground layer that stays frozen all year long. If the ground remains frozen for at least 2 years in a row it is called permafrost.

What causes ground frost?

When ground is frozen solid, the water between the rocks, soil, and pebbles, and even inside the rocks, has frozen and becomes pore ice. So officially, the ground freezes when the water in the ground becomes ice.

Frost Depth

Frost Depth (or the frost line) is the deepest point to which ground water will freeze. Frost depths vary depending upon the frost line in each location and can have a great impact on many construction practices. For example, any crews digging to access utility lines or preparing ground for a concrete pour will need to be aware of their local frost depth.

When ground water freezes its volume expands by 9%. For this reason, pressure sensitive structures, such as water and sewer lines, need to be buried below the frost depth to avoid ruptures. When water turns into ice, it can expand with great force and cause the ground to swell. In areas with a cold winter season ground frost can damage roads. For example, water turning to ice under roads sometime creates frost heave. The expanding ice pushes up the road and creates a hump, which later, after a thaw, will create potholes and sunken sections in a roadway.

The frost line varies depending on the length of time the air is cold. The longer the cold period, the deeper the ground will freeze. But the depth of frozen ground is limited, because Earth is warm deep inside.

What Affects the Frost Line?

Most of Earth’s heat comes from the Sun (Figure 1). The ground stores a lot of the Sun’s heat and reflects the rest into the air. Snow and ice are light colored and reflect more heat away. Ocean water and bare ground reflect less heat, instead absorbing it. This transfer of heat between the ground and the air is called the surface energy flux.

Figure 1. This diagram shows how the Earth’s atmosphere and the ground reflects and absorbs the Sun’s energy.

—Credit: NASA Atmospheric Science Data Center

Heat is also coming from the inside of the Earth. The Earth’s core is very hot, and its heat moves towards the surface. Heat from volcanoes, rivers, lakes, and other sources can also spread through the ground. This heat keeps some areas unfrozen, even though surface temperatures are low.

In general, deeper permafrost is very old. One researcher found that the deepest part of the permafrost underneath Prudhoe Bay, Alaska, is more than 500,000 years old.

The Temperature Gradient

When the temperature of the ground drops below 0° C (32° F), it freezes; however, the ground temperature can be different from the temperature of the air above it. This temperature gradient means that layers deep within the ground may be colder or warmer than layers near the surface.

The top layer of ground may respond to conditions on the surface, but the layers below may not change as quickly. On a warm summer day, the surface of the ground absorbs heat and becomes hotter than the air. But the temperature a few feet underground may be much lower than the air. It is the opposite in the winter; the surface of the ground cools, but the layer deep underground may stay warmer than the surface. The upper layer of ground stops heat from moving between the cold air and the deeper layers of the ground, insulating itself.

How does the local landscape affect ground frost?

Ground frost is affected by more than just temperature swings, seasonal changes, and location. Snow, soil, plants, and other aspects of the local landscape also affect frozen ground.

Snow

A thick layer of snow acts like a blanket so that heat does not leave the ground. Only a thin layer of ground will freeze under a thick layer of snow.

Soil type

Some soils freeze more easily than others. Light-colored soils freeze sooner and stay frozen longer than dark soils. Light-colored soils and rocks reflect sunlight, keeping the ground cooler. Loose soils like sand have more space for water and ice forms more easily. Dense soils with small particles do not have as much space for water. Clay, for example, does not freeze as easily as sand.

Peat

Peat forms when dead plants do not fully decompose. The ground under peat is usually colder than ground not covered by a peat layer. In the winter, peat freezes and allows heat to leave the ground. Because the heat escapes, more frozen ground and permafrost form.

Plants

In the summer, plants keep the soil underneath them cooler because they block some sunlight from reaching the ground. Evergreen trees especially keep the ground cooler. Evergreen trees do not lose their leaves in the winter. This means that the trees block sunlight from warming the ground. Plus, their branches block snow from reaching the ground underneath. The bare ground loses heat more easily. Permafrost often forms under evergreen trees.

Slopes

Hillsides and mountain slopes can affect frozen ground and permafrost. If a slope gets more sunlight because of the way it faces, the ground will be warmer and will be less likely to freeze. In the Northern Hemisphere, slopes that face south, towards the Sun, get more sunlight than shady slopes that face north. The opposite is true in the Southern Hemisphere.

Steep slopes are likely to contain frozen ground. The steepness of the slope affects how much sunlight it gets. Steep slopes do not get as much direct sunlight, so they are colder. Steep slopes do not hold snow cover very well, so the bare ground loses more heat. Wind direction also affects whether frozen ground forms. If a slope faces into the wind, the ground will lose more heat. Plus, the wind will blow snow away making the ground even colder.

Lakes and rivers

Lakes and rivers are sources of heat in cold places. The water is warmer than the surrounding air and can keep the ground beneath it warmer in the winter. Lakes and rivers might not have frozen ground under them. Or, they might have a thicker active layer compared to nearby land.

Powerblanket Ground Frost Solutions

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National Snow and Ice Data Center

Figure 1. In 1967, the Green Bay Packers and the Dallas Cowboys had a hard time getting their footing when they played a championship football game on frozen ground. (High-resolution image not available)
—Credit: Pro Football Hall of Fame

What is the ground like when it freezes? What conditions cause it to freeze, and to stay frozen? A very famous New Year’s Eve football game helps illustrate the answers.

On December 31, 1967, the Green Bay Packers and the Dallas Cowboys competed for the National Football League Championship in Green Bay, Wisconsin. Temperatures that day were cold, as low as -25° Celsius (-13º Fahrenheit). The players noticed that the football field under their feet became hard, so their cleated shoes could not dig into the normally soft soil. The players slipped and struggled to stay on their feet. Why? The ground had frozen solid. The athletes were experiencing frozen ground firsthand. The historic game came to be known as the Ice Bowl (Figure 1).

Why does the ground freeze?

What does it mean to say that the ground has frozen solid? It means that water between the rocks, soil, and pebbles, and even inside the rocks, has frozen. This frozen water is called pore ice. The ground freezes when the water in the ground becomes ice, as it did during the Ice Bowl.

The ground thaws when the pore ice melts. Note that when talking about frozen ground thawing, scientists do not use the word “melt.” That term refers to a solid becoming a liquid. When frozen soil thaws, it is still a solid.

Figure 2. This giant iceberg floats in the ocean near Antarctica because ice is less dense than liquid water.
—Credit: Rob Bauer and Ted Scambos, NSIDC

How does water change to ice inside the ground?

Water, like all types of matter, freezes at a specific temperature. The freezing point for water is 0 degrees Celsius (32 degrees Fahrenheit). When the temperature of water falls to 0 degrees Celsius and below, it begins to change to ice. As it freezes, it releases heat to its surroundings.

However, in some ways water is not like other types of matter. The same amount of water fills more space once it has turned to ice. Scientists use the word density, saying that liquid water is more dense than ice. Because ice is not as dense as water, it floats. You can see this when you put ice cubes in a glass of water: they float. The same is true of icebergs, which float in the ocean (Figure 2).

What does the density of water have to do with frozen ground?

When water turns into ice, it can expand with great force. Ice forming in the soil pushes on the ground, causing it to swell.

People who live in areas with a cold winter season know that frozen ground can damage roads. For example, water turning to ice under roads sometime creates frost heave. The expanding ice pushes up the road and creates a hump. When water freezes and then melts, it helps create potholes and sunken sections in a roadway.

Sometimes, in very cold places, a layer of pure ice forms under the soil. This layer is called segregated ice—it is not mixed in with the soil (Figure 3). Segregated ice can be a few meters (up to 10 feet) thick. Segregated ice forms when pore ice attracts water, which freezes and attracts even more water. This effect is called cryosuction. Cryosuction makes the frozen layer grow, and the growing layer expands the soil even more. Cryosuction can make permanently frozen ground expand by 50 percent.

Figure 3. The thin, grayish layers near the knees of the person in the photograph are segregated ice. Above the ice are layers of sediment.
—Credit: Natural Resources Canada

Are all of the layers underground the same temperature?

When the temperature of the ground drops below 0° Celsius (32° Fahrenheit), it freezes. However, the ground temperature can be different from the temperature of the air above it. Layers deep within the ground may be colder or warmer than layers near the surface of the ground.

The top layer of ground may respond to conditions on the surface, but the layers below may not change as quickly. On a warm summer day, the surface of the ground can absorb heat and become hotter than the air. But the temperature a meter (a few feet) underground may be much lower than the air. In the winter, the opposite happens. The surface of the ground cools, but the layer deep underground may stay warmer than the surface. The upper layer of ground stops heat from moving between the cold air and the deeper layers of the ground. As a result, the ground insulates itself.

The ground is not the only thing that insulates itself from the air. For example, imagine a lake on a hot summer day. The first few feet of the lake will be warm. But closer to the bottom of the lake, the water will be much cooler. The Sun’s heat has less effect on the water deeper below the surface.

This layering of temperatures is called a temperature gradient. The summer temperature gradient in a cold place like Fairbanks, Alaska, might look like this: the air temperature is above freezing, the ground surface is above freezing, but deeper layers of the ground are permanently frozen.

Figure 4. This diagram shows how the Earth’s atmosphere and the ground reflects and absorbs the Sun’s energy.
—Credit: NASA Atmospheric Science Data Center

The type of soil in an area also affects how the ground will store heat. Loose soils like sand have more space for water. In loose soils with large particles, ice forms more easily. Dense soils with small particles do not have as much space for water. Clay, for example, does not freeze as easily as sand.

How deep can the ground freeze?

How deep the ground will freeze can depend a lot on the length of time that the air is cold. The longer the cold period, the deeper the ground will freeze. But the depth of frozen ground is limited, because Earth is warm deep inside.

Most of Earth’s heat comes from the Sun (Figure 4). The ground stores a lot of the Sun’s heat and reflects the rest into the air. Snow and ice are light colored and reflect more heat away. Ocean water and bare ground reflect less heat, instead absorbing it. This transfer of heat between the ground and the air is called the surface energy flux.

Heat is also coming from the inside of the Earth. The Earth’s core is very hot, and its heat moves towards the surface (Figure 5). This movement of heat to the surface is called the geothermal heat flux. The geothermal heat flux can stop the ground from freezing. Even in very cold areas, the ground can only freeze so far before the geothermal heat flux stops it.

Figure 5. Deep inside, the Earth is hot. The mantle and liquid outer core are molten rock. The inner core is solid, but it too is hot. This heat moves through Earth’s layers to the surface.
—Credit: Lawrence Livermore National Laboratory

Heat from volcanoes, rivers, lakes, and other sources can also spread through the ground. This heat keeps some areas unfrozen, even though surface temperatures are low.

In general, deeper permafrost is older. One researcher found that the deepest part of the permafrost underneath Prudhoe Bay, Alaska, has been frozen for more than 500,000 years. Frozen ground underneath the ocean is called subsea permafrost.

What are the types of frozen ground?

Frozen ground is can be either seasonally frozen ground or permafrost. Seasonally frozen ground freezes in the winter and thaws in the summer. More than half of the land in the Northern Hemisphere has some seasonally frozen ground.

Permafrost is a type of frozen ground that stays at or below 0° Celsius (32° Fahrenheit) for at least two years. Permafrost does not have to contain water or ice. As long as the temperature of the ground stays below freezing, it is still considered frozen ground, even if it is completely dry. If permafrost begins to warm significantly, it thaws.

These two types of frozen ground can occur separately, or together. A layer of ground that freezes and thaws every year may sit on top of permafrost. This is called the active layer. The active layer is seasonally frozen ground, and is not part of the permafrost. Permafrost begins where the seasonally frozen ground ends.

What are the types of permafrost?

Scientists classify permafrost into two main types:

Continuous permafrost

Continuous permafrost exists under almost the entire land surface in an area. Areas with continuous permafrost often have permafrost layers more than 100 meters (330 feet) thick. The deepest permafrost ever found is in Siberia, a region in northern Russia. One area in Siberia has a permafrost layer that extends down 1,650 meters (5,413 feet).

Discontinuous permafrost

Discontinuous permafrost exists under a large portion of a particular area or only in a few specific places. Alpine permafrost is discontinuous permafrost that exists on the tops of mountains where the ground stays very cold. In areas with discontinuous permafrost, the permafrost layer may extend as deep as ten meters (thirty-three feet) underground. Taliks are sections of unfrozen ground within permafrost.

Discontinuous permafrost can be isolated or sporadic. It is called isolated if less than ten percent of the surface has permafrost under it. Sporadic means ten to fifty percent of the surface has permafrost under it.

The most important factor in determining when to plant any vegetable in your garden is the “LAST FROST DATE” in the spring, and the “FIRST FROST DATE” in the fall for your area. These dates for a given area are based on historical weather data from that area collected over a 30 year period and compiled by the National Climatic Data Center from over 5,800 Weather Monitoring Stations throughout the United States.

FREEZE vs. FROST
For each Weather Monitoring Station, a FREEZE DAY is any day in the year that the temperature reaches 32°F or below. A FROST DAY is any day in the year that the temperature reaches 36°F or below.

So why worry about FROST DAYS more so than FREEZE DAYS? Well, a Freeze is what will kill many plants. But, Weather Monitoring Stations are typically mounted four to six feet above the ground. During clear, calm, and cold nights the temperature at ground level, where your garden is, can become much colder and even freeze. So we’re just trying to play it a little safer by focusing on FROST days.

SPRING
In the spring, as temperatures start to warm, the LAST day of the year that a FREEZE DAY occurs is considered the LAST FREEZE DATE for that year. As the temperatures continue to warm, although no more FREEZE DAYS may occur, the LAST day of the year that a FROST DAY occurs is considered the LAST FROST DATE for that year. Usually about a week or two after the LAST FREEZE DATE.

The LAST FROST DATE for your area is the day of the year, based on these 30 year averages, that there is only a 10% chance that there will be a FROST on that day. So, at least statistically, you should be safe to plant ON or AFTER this date.

FALL
In the fall, as temperatures start to cool, the FIRST day of the year that a FROST DAY occurs is considered the FIRST FROST DATE for that year. As the temperatures continue to cool, usually about a week or two later, the FIRST FREEZE DAY of the year will occur.

The FIRST FROST DATE for your area is the day of the year, based on these 30 year averages, that there is a 90% chance that there will be a FROST on that date. So again, at least statistically, you will want to have completed your harvest ON or BEFORE this day.

REMEMBER
Remember, although statistically accurate, you are really just playing the odds with these dates. It’s kind of like going to Las Vegas, but instead of gambling with your money, you put your tomato plants up on the blackjack table. (Editors Note: For your own safety, DO NOT attempt this in any gaming establishment.) Before planting you should ALWAYS check your local weather to see if a frost or freeze is in the forecast.

NEW AND IMPROVED
The National Climatic Data Center has recently released an entirely new data set, this one collected between 1981 and 2010. With this data, we’ve created a new and improved way for finding the FREEZE and FROST DATES for your specific area.

Enter your 5-DIGIT ZIP CODE in the calculator, then click on the FIND IT button. The FREEZE and FROST DATES for your area will then be displayed in the RESULTS section. If you want to look up another location, Click the RESET button, and start over.

I hope you find this information helpful, and I wish you Happy (freeze damage free) Planting.

Data Source: National Climatic Data Center

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