I believe that my garden is falling victim to frost heave. What, exactly, is frost heave and why does it happen? How do evergreen boughs protect plants from this?
Christopher Eanes, Blairstown, PA
Cross section of frost-heaved soil
Photo/Illustration: Carol Ruzicka
Charles Smith, horticulturist and educator in Great Barrington, Massachusetts, and author of The Weather Resilient Garden, responds: Frost heave happens when a fine-textured soil, such as a silty loam that has plentiful soil moisture, is subjected to freezing temperatures. The pressure created from alternating freezing and thawing lifts the soil, including the plants, up out of the ground. The result looks like a miniature earthquake has hit the garden, causing damage to both the plants and the soil.
For a frost heave to occur, cold air must migrate through the soil layers where there is an area of warmer soil deep beneath the surface and plenty of moisture in the soil. As the cold air sinks into the ground, it freezes the water in the soil into ice particles. The particles come together to form a layer of ice, called an ice lens, along the leading edge of the freezing zone. Additional moisture is also drawn upward from deeper soil layers, desiccating the soil below. This rising moisture freezes, expanding the ice lens even further while creating great pressure, both upward and downward.
The downward pressure damages the soil by compacting it while the upward pressure creates the frost heave that we see in the garden, thus damaging the soil structure by breaking down soil bonds, reducing soil aeration, and creating poor drainage. The soil around the heave often has deep cracks that expose a plant’s roots to the killing cold. All of this causes trouble for plants, but the worst damage occurs when plant roots are lifted, or heaved, out of the soil. Without the protection of the soil, the exposed roots quickly dry out and die in the frigid air.
Frost heave most often occurs in the early spring and sometimes in late fall, when cold temperatures and abundant soil moisture are most often present. Heaves can happen in any type of soil, but some are prone to heaving more than others. Coarse, sandy soils drain so well that the moisture needed to produce heaves is rarely present. Soils that are moisture retentive, such as silt, loam, and clay, are much more prone to heaving.
An effective method to stop frost heave is to insulate the soil. That’s where the evergreen boughs become useful. Placing evergreen boughs over the garden after the ground has frozen in fall will limit heaving by moderating temperature fluctuations and reducing the depth of frost penetration into the soil. Other types of mulches, such as pine bark or wood chips, help manage frost heaves but not as effectively or reliably as evergreen boughs and snow. As a general rule, a foot of snow on top of the ground reduces the depth of frost penetration into the soil by the same amount.
A little known tip about frost heaves is that heaving tends to start in depressions in the soil because they hold more moisture. So the first step is to rake out any low spots in the garden bed. Amending the soil with compost, which improves drainage in most soils, also helps reduce the chance of heaving.