The concept of “too cold” for a plant is highly variable, depending on the species’ genetic origin and the environmental context of the temperature drop. Plants do not share a single, universal temperature threshold for damage, making protection a nuanced process. The severity of the injury depends not just on the lowest temperature reached, but also on the duration of the cold period and the plant’s physiological state beforehand.
The Two Primary Mechanisms of Cold Injury
Plant damage from low temperatures occurs through two distinct biological pathways: chilling injury and freezing injury. Chilling injury affects plants that evolved in warm climates, such as tropical and subtropical varieties. This damage occurs at temperatures well above the freezing point, generally between 32°F and 50°F. Exposure to this non-freezing cold disrupts cellular machinery by causing cell membranes to stiffen. This dysfunction impairs the transport of necessary substances and inactivates enzymes required for photosynthesis and respiration. Symptoms often appear as water-soaked spots, surface pitting, or browning, potentially leading to physiological collapse.
Freezing injury, commonly called frost damage, occurs when temperatures fall below 32°F, causing water within the plant tissues to turn into ice. This is an immediate and physically destructive form of injury. Ice crystals typically form first in the intercellular spaces, drawing water out of the cells and causing severe cellular dehydration. If the temperature drops low enough, ice can form inside the cell itself, leading to the physical rupture of the cell walls. Damage from freezing is usually irreversible, resulting in blackened, wilted, or mushy tissue.
Specific Temperature Thresholds for Plant Survival
The temperature that causes damage is directly linked to a plant’s evolutionary history, resulting in three general categories of cold tolerance. Tropical and warm-season plants are the most susceptible, often suffering chilling injury below 50°F. Sensitive plants like basil, tomatoes, and many houseplants require protection when overnight lows consistently dip below 55°F. Brief exposure to temperatures between 32°F and 50°F can lead to reduced growth and discoloration in these varieties.
Tender annuals and vegetables form a second category, typically killed by a light frost. These plants, including petunias, impatiens, and peppers, will not survive when the temperature reaches 32°F or slightly below. Their delicate foliage and shallow root systems offer little protection from ice formation. A temperature drop to 28°F is considered a hard frost and is sufficient to end the growing season for nearly all tender crops.
Hardy annuals, perennials, and many cool-season vegetables possess greater cold tolerance. Plants such as pansies, kale, and cabbage can often survive a light frost, showing minimal damage down to approximately 28°F. Perennials and woody shrubs prepare for winter by entering dormancy, allowing them to withstand much colder temperatures without significant harm.
Environmental and Physiological Factors Modifying Cold Tolerance
Temperature thresholds are not absolute and can be significantly altered by a plant’s physiological state and local environmental conditions. The most important physiological factor is cold acclimation, or hardening, observed mainly in temperate plants. Acclimation occurs when a plant is exposed to gradually decreasing, non-freezing temperatures over days or weeks. This slow transition triggers biochemical changes, including the accumulation of soluble sugars, which effectively lower the freezing point of the cell sap.
A sudden, sharp drop in temperature is far more damaging than a gradual one, preventing the plant from activating protective mechanisms. The duration of the cold is also significant; a brief overnight dip to 30°F is less harmful than a prolonged period at the same temperature. Extended cold allows more heat to escape from the soil, increasing the likelihood of damage to root systems and lower tissues.
Local environmental factors, or microclimates, further modify survival chances. Wind exposure increases evaporative cooling on leaf surfaces, making plant tissue colder than the surrounding air. Conversely, placing sensitive plants near a stone or concrete wall offers protection, as these structures absorb heat during the day and slowly release it overnight. Moist soil also retains and radiates heat more effectively than dry soil, creating a warmer buffer zone around the roots.
Immediate Steps to Protect Vulnerable Plants
When cold temperatures are forecast, gardeners can take immediate, proactive steps to mitigate damage.
Managing Soil Moisture
Watering the garden thoroughly before a freeze is highly recommended. Moist soil has a higher heat capacity than dry soil. This damp soil absorbs more solar energy during the day and releases that stored heat slowly throughout the night, warming the air directly above the ground and around the plant base.
Using Physical Barriers
Physical barriers provide the most direct defense against frost formation on foliage. Use old blankets, burlap, bedsheets, or specialized row covers to completely drape over vulnerable plants. For maximum effect, the covering material should extend all the way to the ground, trapping the residual heat radiating from the soil. It is beneficial to use stakes or hoops to ensure the material does not directly touch the plant’s leaves, as the cover itself can become cold and transfer that temperature to the foliage.
Protecting Containers and Roots
Container plants offer the easiest solution, as they can simply be moved to a protected location. Bringing pots into a garage, shed, or placing them directly against the warm, south-facing wall of a house provides immediate thermal protection. For plants too large to move, consider adding a thick layer of straw or mulch around the base to insulate the root zone from the cold air. Remember to remove all covers the following morning once temperatures rise above freezing to prevent the plants from overheating in the sun.