Frost or Freeze?
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 DATE1*
2 August 15 May 15
3 September 15 May 15
4 September 15 May 15
5 October 15
6 October 15
7 October 15
8 November 15
9 December 15
10 December 15
11 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)