The idea that all plant life freezes exactly at \(0^\circ\text{C}\) (\(32^\circ\text{F}\)) is a widespread misconception. While this temperature marks the freezing point of pure water, the biological reality for plant tissue is far more complex. A plant’s vulnerability to freezing is a variable threshold determined by its genetics, internal chemistry, and environmental conditions. Understanding the temperature at which a plant freezes requires looking into the specialized survival mechanisms that dictate how and when ice crystals form.
How Ice Formation Damages Plant Cells
The physical damage to a plant is caused by the formation and growth of ice crystals, not the low temperature itself. Most plant cells contain solutes that allow the water within them to remain liquid, a phenomenon known as supercooling, until temperatures drop below \(0^\circ\text{C}\) (\(32^\circ\text{F}\)). The initial ice usually forms in the apoplast, the spaces outside the living cells, because the water there is purer.
Extracellular ice formation lowers the water potential in the apoplast. This difference causes water to be drawn osmotically out of the cell, leading to cellular dehydration. If this dehydration is too severe or prolonged, the cell membrane structure is irreparably damaged, causing the cell to leak its contents and die.
The second, lethal form of damage is intracellular freezing, which is the formation of ice crystals inside the living cell. This occurs when temperatures drop too rapidly or too low for the cell to safely move its water out. Ice crystals forming within the cytoplasm physically puncture the delicate cell membranes and organelles, destroying the cellular infrastructure.
Strategies Plants Use to Survive Freezing Temperatures
Plants that survive cold winters use a proactive process called cold acclimation or hardening. This process is triggered by gradually decreasing temperatures and shorter day lengths in the autumn, prompting a shift from growth to survival metabolism. Acclimation changes the cell’s composition to tolerate ice formation in the extracellular space.
A primary survival strategy involves increasing the concentration of compatible solutes within the cells, acting like a biological antifreeze. Soluble sugars (such as sucrose and raffinose) and amino acids (like proline) accumulate to depress the cell’s internal freezing point. This elevated solute concentration also helps stabilize proteins and cell membranes against the stresses of dehydration caused by external ice formation.
The plant also manages its water content by reducing the water content of the protoplast, the living part of the cell. This controlled dehydration minimizes the risk of lethal intracellular freezing by ensuring ice formation is confined to the extracellular spaces. The plant also synthesizes specialized protective proteins, such as dehydrins, which bind to cell membranes and macromolecules to stabilize them during water withdrawal and ice presence.
Variables Determining a Plant’s Specific Freezing Point
The specific temperature a plant can tolerate depends on internal genetics and external environmental factors. The most significant factor is the plant’s species and cultivar, which determines the maximum cold hardiness achievable through acclimation. For instance, a tropical houseplant may be damaged near \(4^\circ\text{C}\) (\(40^\circ\text{F}\)), while a fully hardened boreal tree may survive temperatures below \(-40^\circ\text{C}\) (\(-40^\circ\text{F}\)).
The rate at which the temperature drops is also a decisive variable. Slow, gradual cooling allows the plant sufficient time for water to migrate safely out of the cells. Sudden, sharp drops in temperature are more lethal because they promote the rapid, uncontrolled formation of intracellular ice crystals. Actively growing tissues, such as new shoots and buds, are more susceptible to damage than dormant tissues that have completed the hardening process.
The plant’s current health and hydration status plays a direct role in its freezing vulnerability. A well-hydrated plant with high turgor pressure is more prone to damage, as its cells contain more liquid water available to freeze. Conversely, if a plant is too dry, it may suffer desiccation damage regardless of the temperature. Measuring woody stems often reveals two distinct freezing points—one for the dilute extracellular water and a second, lower point for the concentrated cellular water.
Protecting Plants from Frost and Addressing Damage
Protecting sensitive plants involves managing their immediate microclimate to prevent the tissue temperature from dropping below its threshold. A preventative action is watering the soil thoroughly before a forecasted freeze, as moist soil retains and radiates heat more effectively than dry soil. Covering plants with sheets, blankets, or commercial row covers traps heat radiating from the ground, offering several degrees of protection. This technique becomes less effective when air temperatures drop below about \(-2^\circ\text{C}\) (\(28^\circ\text{F}\)).
Potted plants should be moved to a sheltered location, such as a garage or covered porch. Grouping containers together can help create a mutually protective microclimate. Applying a thick layer of mulch around the base of in-ground plants insulates the roots and crown from sudden temperature fluctuations. These interventions focus on prevention, as once ice crystals form, the damage is done.
After a frost event, avoid the temptation to immediately prune or cut back damaged tissue. Frost-damaged leaves and stems typically appear brown, black, or mushy, but they can still provide insulation for the living tissue beneath. Wait until the danger of subsequent frosts has passed and new growth has emerged in the spring before assessing the damage. Prune only the parts of the plant that show no signs of recovery to encourage healthy, new growth.