The temperature at which plants freeze does not have a single, universal answer. While 32°F (0°C) is the point where water turns to ice, most plant tissues contain dissolved solutes that lower this freezing point slightly (freezing point depression). The actual temperature causing irreversible damage is highly variable, depending on the plant species, tissue, and preparation for cold weather. Plant freezing is a complex biological process dictated by how ice formation interacts with cellular structures.
The Mechanism of Freezing Injury
Freezing injury to plants is primarily a consequence of dehydration, not the mechanical piercing of cells by ice crystals. As the temperature drops below freezing, ice typically forms in the extracellular spaces, which are the gaps between plant cells. The water in these spaces is purer and contains fewer dissolved substances, causing it to freeze first.
Once ice crystals form outside the cells, the water potential in the extracellular space significantly decreases. This creates an imbalance with the liquid water inside the cell, which is rich in solutes. To equalize this pressure gradient, water moves out of the cell through the semipermeable membrane to join the external ice. This loss of water causes the cell to shrink and dehydrate, which ultimately leads to cellular death and tissue damage.
If the temperature drops too rapidly, the cell does not have enough time to transport water outside, and ice may form intracellularly (inside the cell’s protoplasm). This rapid formation of ice is almost always immediately fatal, causing physical disruption of the internal cellular machinery. Therefore, freezing-tolerant plants manage the slow, controlled formation of ice outside their cells and tolerate severe dehydration.
Factors Determining the Critical Temperature
The temperature threshold for damage is largely determined by a plant’s ability to undergo cold acclimation, also known as hardening. This biological process involves exposure to gradually decreasing, non-freezing temperatures (often between 32°F and 50°F or 0°C and 10°C), which triggers physiological changes.
During acclimation, the plant increases the concentration of soluble sugars, proteins, and other osmolytes within its cells. This accumulation lowers the intracellular freezing point, allowing the liquid water inside the cell to remain unfrozen at lower temperatures, a process called supercooling. The plasma membrane also changes its lipid composition, making it more flexible and resilient to the mechanical stress caused by freeze-induced dehydration. A successfully acclimated plant can withstand temperatures far below what would kill a non-acclimated plant of the same species.
The physical state and age of the plant tissue also introduce great variability into the critical temperature. Actively growing tissue, such as new shoots, flowers, and tender foliage, has a high water content and minimal solute accumulation, making it the most vulnerable to freezing injury. Flower buds and open blossoms on fruit trees are notoriously sensitive, with critical damage temperatures often ranging from 22°F to 30°F (-5.5°C to -1°C).
In contrast, dormant woody stems and hardened roots are the most tolerant, having shed most of their free water and completed the acclimation process. The rate at which the temperature drops is also a significant factor in the outcome. A slow, gradual freeze allows for controlled extracellular ice formation and water movement, while a sudden, severe drop can induce lethal intracellular freezing, even in otherwise hardy plants.
Preparing for and Mitigating Freeze Damage
Understanding the dehydration mechanism of freezing injury provides clear, actionable strategies for protecting plants. One of the most effective pre-freeze measures is ensuring the soil is adequately watered before the cold event. Moist soil holds significantly more heat than dry soil, and this stored thermal energy radiates upward during the night, slightly raising the air temperature immediately surrounding the plant crown.
Covering plants is a common and highly effective method for protecting against a radiation freeze, which occurs on calm, clear nights. Covers such as blankets, sheets, or commercial frost fabric trap the heat radiating from the soil surface, preventing its escape. For maximum effect, the cover should extend all the way to the ground to completely seal in the thermal energy. Care should also be taken to ensure the material does not directly contact the foliage.
Potted plants are especially vulnerable because their roots are exposed above ground and lack the insulating protection of the earth. Moving container plants to a sheltered location, like a garage or against a warm building wall, or grouping them tightly together can significantly reduce heat loss. Covers must be placed before sunset to trap the daytime heat and should be removed shortly after sunrise once temperatures rise above freezing to prevent the plants from overheating.