One of the rituals that gardeners endure every winter is to periodically brave bitter temperatures and biting winds to check their plants to see what havoc the weather has wrought. But even the most devoted gardeners aren't likely to give much thought to what's happening beneath their feet as they crunch across ground that has become hard as a rock. If they did, they’d probably be surprised.
The frozen soil is still teeming with life. "When things are looking bleak and you are not comfortable being outside, there are plenty of organisms that have evolved to survive the harsh conditions of winter," said Mary Tiedeman, a soil scientist and graduate student at Florida International University.
Prolific among these organisms are the microscopic ones invisible to the human eye. These include bacteria, amoebas and fungi as well as slightly larger organisms such as nematodes and tardigrades — also known as the water bear — and still larger ones such as earthworms. "One of my favorite examples are giant earthworms several meters long," said Tiedeman, shown right. Other larger organisms that you may not see in your garden — gophers, turtles and some frogs — also depend on the soil for at least certain parts of their life cycle.
One of the fun facts about microscopic organisms is that one teaspoon of healthy soil can have more microscopic organisms than there are people on the planet. There are billions and billions of these organisms in the soil year-round, Tiedeman said. They perform important functions in the garden, and all have developed biological or evolutionary strategies to survive winter. With good gardening practices, home gardeners can help them do that.
Life finds a way
"What people find interesting when talking about gardens and winter is the capacity of living things to survive really harsh conditions," said Tiedeman. People are surprised at the length to which organisms will go to survive.
Some of the microscopic organisms in your garden die, of course. "But even certain fungi or bacteria that that might not survive winter pass on their DNA to the generations to come by leaving spores or reproductive material in the soil," Tiedeman said. "That material will bud and regenerate new organisms once the environment becomes more suitable for growth."
Organisms that are mobile, on the other hand, have developed different wintertime life-preservation strategies. "Earthworms, insect larvae, frogs and other organisms might be able to burrow down deep below the frost layer, the top layer of soil that becomes frozen in winter," Tiedeman said. "Once organisms get down there, some go into hibernation, while others shift to a slower metabolic state and continue their normal functioning."
She is fascinated by a frog species — the ubiquitous wood frog (Rama sylvatica) that is found throughout the continental United States and Canada — that produces a compound similar to antifreeze that enables it to withstand intensely cold temperatures.
The frost layer
Depending on where you live, the frost layer (the depth to which the ground freezes in winter) may be non-existent or it could be several feet deep. As you go from southern to northern latitudes, the expected depth of the frost layer increases as you get into colder climates. "In Georgia in and around Atlanta the range of the frost layer is between five and 10 inches," said Tiedeman. "In central Pennsylvania, it might be 45 inches."
What happens to create the frost layer, said Tiedeman, is that the sun’s rays warm the soil during spring, summer and early fall, allowing it to absorb and store heat energy. When air temperatures eventually cool, there will be more heat energy in the ground than in the air. At this point, heat starts to move from the soil into the atmosphere. Once the surface of the soil dips below 32 degrees Fahrenheit (0 degrees Celsius), water in the ground will start to freeze. "The first layer of the soil to freeze will be right at the surface," said Tiedeman. "Over time, as the air gets colder and colder, the soil will continue to freeze deeper and deeper."
It's important to know the frost layer in your area. Builders for instance, know to install pipes below the frost line as this reduces the risk of frost-related infrastructure damage. Plants have their own infrastructure and, in the case of roots, have adapted their own survival strategy.
"One of the most important of these strategies is to extend their root systems below the frost line," said Tiedeman. "In general, this is a pretty tried and true method. If a root system can extend deeply enough then it has the ability to protect its most vulnerable roots from freezing."
On top of that, plants have developed a strategy to prevent water in the roots above the frost layer from freezing and damaging root cells. As the temperature in the ground gets colder and colder, roots release water from their cells into the surrounding soil. Without this capability, roots could burst in the same way that water-filled pipes burst. "At the first signs of freezing, plants will release water from the roots before that water freezes, expands in the root cells and breaks the cells apart," said Tiedeman.
Another thing happening underfoot and out of sight involves sugars and salts in the water in the roots’ cells. These sugars and salts lower the temperature at which root water will freeze in the same way as oceans don’t freeze at the same temperature as freshwater systems.
How home gardeners can help plants survive winter
If your main thing is growing annual crops and plants, you likely don’t need to worry about a frost layer. But if you have fruit trees or grow food crops such as raspberries, blueberries or anything that you want to survive year after year, a consideration of the frost layer might be appropriate. If you're growing perennials, you're likely already following a climatic zone for the types of plants you're choosing because local nurseries tend to offer only the perennials they know are winter-hardy for their region.
Even so, there are sound practices any home gardener can adopt both in summer and winter to help protect plants from freezes. At the top are two things: adding organic matter to the soil during the growing season to promote root growth, and applying mulch in advance of winter freezes to help insulate roots and keep them from freezing.
"What’s important when you are thinking about promoting root growth is to make sure that the soil has a good structure," said Tiedeman. As a soil scientist, she talks about amending soil to create a granular structure. In homeowner terms, think of that soil as looking like cookie crumbles. Maintaining a healthy soil will help to create conditions that will enable soil organisms to thrive and fulfill the tremendous role they play in maintaining soil health. This in turn indirectly relates to the productivity you might see in your plants as you garden in the warmer months of the year.
"A healthy soil will be loose and not compacted but will kind of crumble as you pick it up," Tiedeman said. "It should also be dark in color and likely have an earthy smell." Granular structure will create lots of air spaces, which enables water to move readily through the soil, ensuring that roots have access to water without the soil becoming too soggy. This will allow the roots to expand both radially and downward. Compacted or dense soil will restrict root growth.
Tiedeman described organic matter as an essential ingredient for all healthy soils and said that it can serve different purposes. One is to add structure to sandy soils and help retain water. Another is to improve the workability of soils high in clay. Organic amendments also act as an insulator because air is a poor conductor for heat. The air pockets within organic matter reduce heat transfer from soil to the atmosphere. "It’s difficult for heat energy to transfer between pore spaces," said Tiedeman. "The same principles can be applied to Styrofoam coolers or jackets filled with goose down, all of which are good insulators. That’s why the pockets of air in rich, organic soil can hold heat in the ground, preventing deep expansion of the frost layer."
When it comes to insulated properties to protect plants during the winter, Tiedeman suggested adding a thick mulch of leaves or woodchips. These can be raked around the base of trees and shrubs or even piled onto the top of vegetable beds. If you add them to vegetable beds, they can be tilled into the soil in the spring. But, in any case, organic matter serves a similar purpose, trapping heat by providing a cushion of air in between their structures.
What happens if you get to winter and realize that, for whatever reason, you haven’t done any of these things and a hard freeze is forecast? "Depending on what your concerns are, it’s never too late to try," said Tiedeman.
The biggest concern midway through winter is if shrubs are suffering from frost heaving. This term refers to ground that goes through cycles of freezing and thawing overnight or within a few days of each other. When that happens moisture in the soil freezes and melts, resulting in a back and forth contraction and expansion of soil water. Over time, this process can literally push poorly rooted plants out of the ground.
If you see plants that have been pushed out of the ground and part of their root ball is exposed, Tiedeman suggested that you reposition the plant by gently pressing its root mass back down, apply top soil to the base of the plant and apply mulch.
If you're trying to push the plant back into the ground, though, you may damage roots as well as compact the soil. Just be sure that you don’t stomp the plant back into the soil. That can cause the plant to have limited access to water and poor gas exchange. Remember that, even in winter, roots need oxygen and they release carbon dioxide, just like animals and humans. If you compact the soil too tightly, you're reducing the ability of the soil to do the job it was intended to do.
Something else Tiedeman said is important for home gardeners to know is that soil microbiologists have identified only around 5 percent of the organisms that are living in the soil. "There are so many more creatures in the soil than the many we already know about," she said. "We know these countless others exist and that they maintain important functions in the soil system, but we don’t know who they are or what they do. That is pretty astonishing!"
Inset photo provided by Mary Tiedeman