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Why is frost heave happening to my garden?

By Melanie Mathieson
Gardening Guru

Frost heave happens when a soil, such as a garden soil is subjected to on going freezing and thawing temperatures. The pressure created from alternating freezing and thawing lifts the soil and creates pockets within the soil. If this occurs repeatedly, soon the soil will heave upwards out of the ground sometimes taking plants with it. The result leaves humps of soil and possibly plants laying on the soil surface, with exposed roots. This obviously can cause further damage to both the plants and the soil.
For frost heaving 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. This happens most often in our region when the soils are not fully saturated with water prior to freezing in the winter. It can also happen when we get a thick layer of insulating snow over the ground in early winter before we have had ongoing freezing temperatures that freezes the ground completely. If frost penetrates the ground deep and in a consistent manner at the beginning of winter then frost heaving usually doesn’t occur.
When soils are not thoroughly frozen and pockets of air lay beneath the soil surface, cold air can sink into the ground. When this happens any moisture particles become frozen creating ice particles within the soil. These 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 (drying out) 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. The worst damage occurs when plant roots are lifted, or heaved, out of the soil because without the protection of the soil, 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 temperatures fluctuate between seasonably cold and unseasonably warm over very short periods. An effective method to stop frost heave is to insulate the soil. Insulating the soil helps to moderate temperature fluctuations and reduce rapid thawing of the ground in the spring. Usually a heavy snow cover provides adequate insulation for hibernating plants. Gardeners who mulch their soil year round with mulches, such as pine bark or wood chips often do not have problems with frost heaves, especially in a winter with average snowfalls and temperatures. For plants that are extra susceptible to frost heave, such as Pincushion Flower (Scabiosa), Coral Bells (Heuchera), Shasta Daisy (Leucanthemum), Blanket Flower (Gaillardia), Elephant ears (Bergenia), Coreopsis (Coreopsis), Seathrift (Armeria), Garden Mums (Chrysanthemum) and Painted Daisy (Tanacetum), you should add extra protection with a layer of thick mulch you apply for the winter season only. You always remove this layer of mulch in the spring with the danger of frost has passed. Straw is an great mulch for this purpose as offers both great insulating properties and is relatively easy to remove in the spring. You want to apply straw that has been processed with as little weeds as possible so you do not end up with some unwanted plants next year. The best straw for the garden, is flax straw, hopefully you have a source for this somewhere in the District. The next best form of winter mulch is fall leaves. Pine boughs, bark mulch or wood chops can also be used.
Another way to help prevent frost heave is by raking out any low spots in your gardens that may be present. Frost heaves tend to start in depressions in the soil because they hold more moisture on the surface of the soil and trap pockets of colder air. .A good time to level out these spots is in the spring and again during fall as you are both preparing for and cleaning up the garden. You should also amend the soil with compost to further improve the soil’s drainage, which lessens the chance of heaving as well. Well-drained soils will also warm faster in spring. Plants should also be chosen for their suitability to cold temperatures such as deciduous trees and shrubs, bulbs, or perennials that are cold hardy.
One last tip is that it is advisable to avoid planting perennials (particularly those on this list) really late in the growing season (mid to late October in our region) as some fall planted perennials will not have sufficient time to establish root systems that resist the expanding and contracting of our soils. This is why fall-planted garden mums often do not survive the winter and slow rooting spring bulbs get heaves out of their holes.