Freezing conditions are generally harmful to plants, but the extent of the damage varies significantly based on the plant’s hardiness and the specific circumstances of the freeze. Ice formation is the primary mechanism that causes injury, making it a serious threat to actively growing or non-acclimated plant tissues. Understanding how water behaves at freezing temperatures is the first step in protecting your garden from a devastating cold snap.
How Freezing Temperatures Harm Plant Cells
Freezing water causes damage at the cellular level through two distinct mechanisms: physical rupture and dehydration. When temperatures drop below freezing, water often forms ice crystals in the intercellular spaces outside the plant cells. This water is purer and freezes at a higher temperature than the concentrated sap inside the cell.
As this extracellular ice forms, it drastically lowers the water potential, drawing water out of the plant cells through osmosis. This process, known as freeze-induced desiccation, causes the cell to shrink. This results in a concentration of solutes that can become toxic, effectively “freeze-drying” the tissue. The cell membranes are damaged when they shrink past a tolerable limit, leading to permanent injury.
The second, often lethal, form of damage is intracellular freezing, where ice crystals form directly inside the cell’s protoplasm. These crystals physically expand as they solidify, which can puncture and rupture the cell wall and plasma membrane. This mechanical damage causes the cell’s contents to leak out, resulting in immediate tissue death. The severity of this damage depends largely on the speed of the temperature drop; a rapid plunge prevents the cells from dehydrating sufficiently to protect themselves.
Identifying Different Forms of Cold Injury
Cold injury to plants manifests in different ways depending on how the freezing event occurred. A common type is frost damage, often associated with a radiational freeze on a clear, calm night. During this event, the plant surface loses heat faster than the surrounding air, dropping tissue temperature to 32 degrees Fahrenheit or below, even if the air temperature is slightly above freezing. This typically results in superficial damage to new, tender growth, causing leaves and flowers to look blackened or water-soaked.
A more severe and physically destructive form is ice encasement, also known as glaze ice, which occurs when freezing rain covers the entire plant. While a thin layer of ice can sometimes offer slight insulation to dormant plants, a thick coating is highly destructive to active growth. The physical weight of the ice can snap branches and split the trunks of woody plants, especially evergreens. A dense ice layer also blocks sunlight and impedes gas exchange, potentially leading to suffocation of the encased tissues.
Protecting Plants Before a Freeze Event
Proactive steps taken before a freeze warning can significantly reduce the potential for damage. One of the most effective preparations is to thoroughly water the soil around your plants a day or two before the expected cold snap. Moist soil holds and radiates heat more efficiently than dry soil, helping to insulate the root zone from the cold air above.
Physical Barriers
For sensitive plants, providing a physical barrier is a practical defense against cold air and frost formation. Sheets, blankets, or specialized frost cloth should be draped over the plant and extended all the way to the ground to trap the residual warmth radiating from the soil. Avoid letting plastic materials directly touch the foliage, as this contact can worsen freeze damage.
Protecting Potted Plants
Potted plants are especially vulnerable because their roots are exposed to cold air on all sides. Move containerized plants to a sheltered location, such as a garage, shed, or close to a heat-retaining structure like a south-facing wall, to offer substantial protection. For plants that cannot be moved, grouping them tightly together and adding a thick layer of mulch can help moderate the soil temperature.
Hardening Off
In regions where cold snaps are common, gradually exposing plants to cooler temperatures in the fall is a process called hardening off. This acclimatization allows plants to build up natural defenses. It increases the concentration of internal solutes, which lowers the freezing point of the cell sap and enhances tolerance to desiccation.
Assessing Damage and Aiding Recovery
After a freeze event, the first rule of recovery is patience; resist the urge to immediately prune damaged foliage. Freeze-damaged leaves and stems act as a natural insulating layer, protecting underlying healthy tissue from subsequent cold weather. Pruning too soon can also stimulate new, vulnerable growth at the wrong time of year.
The Scratch Test
To determine the extent of damage, particularly on trees and shrubs, perform a simple viability check called the scratch test. Gently scrape a small area of the bark on a stem, starting at the tips and moving toward the base. A bright green color just beneath the outer layer indicates that the cambium tissue is alive. Conversely, brown or black tissue signifies that the stem section is dead.
If tissue appears mushy, slimy, or waterlogged, remove it quickly to prevent fungal or bacterial infections. Once the danger of frost has fully passed, begin removing clearly dead material, cutting back to the nearest healthy green tissue or bud. Avoid heavy fertilization until new growth is well-established, but ensure the plant receives adequate moisture, as frozen soil can temporarily prevent water from reaching the roots.