Cold weather presents a significant challenge to garden plants, causing stress that can lead to severe damage or death. This stress is categorized into chilling injury and freezing injury. Chilling stress occurs above freezing, typically between 0°C and 15°C, and primarily affects tropical or subtropical plants like tomatoes and peppers. Freezing stress occurs below 0°C, causing ice formation within the tissues of virtually any species. Understanding these internal processes and the natural biological defenses plants employ is key to mitigating the effects of cold in your garden.
Physiological Mechanisms of Cold Injury
Damage from sub-freezing temperatures is a failure of the plant’s cellular structure due to the physics of water. When the temperature falls below 0°C, ice crystals typically form first in the extracellular space between the plant cells. This extracellular ice decreases the water potential, drawing water out of the cell’s protoplast through osmosis. This results in cellular dehydration, causing the cell to shrink and sometimes collapse, which is a major cause of freezing injury.
If the temperature drops too rapidly, or if the plant is not acclimated, ice crystals can form directly inside the cell (intracellular freezing). This is almost always lethal because the sharp ice crystals physically puncture and rupture cell membranes and organelles. Damaged cell membranes lose their ability to regulate substance flow, leading to electrolyte leakage and irreversible cell destruction.
Chilling injury, which occurs above freezing, involves a mechanism centered on the plant’s membranes. Cell membranes are composed of lipids that must maintain a fluid state for correct function, allowing for transport and metabolic activity. For chilling-sensitive species, temperatures between 0°C and 15°C cause these lipids to lose fluidity and solidify into a rigid gel state. This loss of membrane flexibility disrupts vital metabolic processes, leading to physiological disorders and eventual cell death.
Observable Signs of Cold Damage
After a cold event, gardeners can observe distinct visual symptoms indicating cellular damage. One common sign is the immediate appearance of water-soaked tissue, which quickly turns brown or black as damaged cells die and leak their contents. This necrotic tissue is often seen first on the margins of leaves or the tips of new growth, the most exposed parts of the plant.
Wilting and drooping leaves are common indicators of cold shock or freeze injury. This is caused by the loss of turgor pressure as water is drawn out during extracellular freezing or due to membrane damage preventing water retention. In woody plants, a rapid temperature drop can cause the bark to split or crack, particularly on young tree trunks. This happens when the outer bark contracts faster than the inner wood, leaving the tree vulnerable to pests and disease.
Natural Plant Adaptation to Cold
Many perennial and winter-hardy species have evolved biological strategies to survive cold periods, known as cold acclimation or hardening. This process is initiated by environmental cues like shorter daylight hours and cooling temperatures, leading the plant into dormancy. Dormancy is a period of suspended growth and metabolic activity that conserves energy and allows for internal biochemical changes.
A central component of acclimation is the accumulation of compatible solutes, such as sugars (like sucrose) and amino acids (like proline), within the cell’s cytoplasm. These solutes act as cryoprotectants, increasing the concentration of dissolved substances inside the cell. This lowers the freezing point of the internal water, allowing it to remain liquid at sub-zero temperatures. The resulting osmotic effect helps retain water inside the cell, counteracting the dehydrating pull of extracellular ice formation.
Plants also modify their cell membranes during hardening to maintain functionality. They increase the proportion of unsaturated fatty acids in the membrane lipids, preventing them from solidifying into a rigid gel state at low temperatures. This preserved fluidity ensures the membrane remains flexible and permeable, allowing the plant to continue essential transport and signaling processes. Some plants even produce specialized antifreeze proteins, which bind to extracellular ice crystals to prevent them from growing to a harmful size.
Protective Measures Gardeners Can Take
Gardeners can intervene with practical steps to supplement a plant’s natural cold tolerance and minimize damage. Physical coverings are effective short-term solutions for tender plants, using materials like frost fabric, blankets, or burlap draped over a frame to avoid direct contact with foliage. These covers trap heat radiating from the soil, providing insulation that protects against light frost and cold wind.
Proper soil management is an important strategy, particularly before a deep freeze. Watering the garden thoroughly a day before a freeze is beneficial because wet soil absorbs and stores more solar energy than dry soil. This moisture releases heat more slowly overnight, radiating warmth upward and slightly elevating the air temperature around the plants. Applying a thick layer of organic mulch, such as straw or wood chips, helps insulate the soil and protect shallow root systems from freezing.
For long-term protection, strategic planting and the use of microclimates can significantly reduce cold exposure. Locating sensitive plants near structures like brick walls or fences is helpful because these surfaces absorb solar heat during the day and release it slowly at night. Container plants should be grouped tightly or moved into a sheltered area, as their exposed roots are more susceptible to freezing than those planted directly in the ground.