The temperature at which a plant dies is not a fixed point but rather a biological threshold determined by the species and its current environment. Plant death from thermal stress involves cellular damage and metabolic collapse, triggered by extremes at either end of the thermometer. The specific lethal temperature varies widely, depending on whether the plant is native to a tropical, temperate, or desert climate.
Lethal Cold: How Freezing and Chilling Damage Plants
Damage from low temperatures occurs through two distinct mechanisms: chilling injury and freezing injury. Chilling injury affects tropical or subtropical plants, such as tomatoes or bananas, at temperatures above freezing, typically between 32°F and 50°F (0°C to 10°C). In this range, the cell membranes of sensitive plants lose fluidity and become rigid, disrupting metabolic processes and the transport of materials. Symptoms include surface pitting, discoloration, and a water-soaked appearance of tissues.
Freezing injury occurs when the temperature drops below 32°F (0°C), caused by the formation of ice crystals within the plant tissues. Water often freezes first outside the cells (extracellular freezing), drawing water out and causing severe cellular dehydration. If the temperature drops too rapidly, ice crystals can form inside the cells (intracellular freezing), mechanically rupturing the cell structure and leading to immediate tissue death. The duration of exposure to below-freezing temperatures is often as significant as the minimum temperature reached.
Lethal Heat: The Danger of High-Temperature Stress
High-temperature stress becomes lethal for many temperate plant species when temperatures exceed 95°F to 105°F (35°C to 40°C). The primary cause of death is the denaturation of proteins, where the complex three-dimensional structures of vital enzymes and structural proteins unravel and lose function. This cellular destruction leads to the aggregation of misfolded proteins and the failure of essential metabolic pathways.
Heat stress also severely compromises the integrity of cellular membranes, causing them to become too fluid, which leads to disorganization and leakage. The breakdown of membranes in organelles like the chloroplasts shuts down photosynthesis, halting the plant’s ability to produce energy. Unlike cold damage, heat death is largely a metabolic and structural catastrophe. Plants attempt to cool themselves through transpiration, but extreme heat drives severe water loss, leading to desiccation.
Factors Modifying Plant Tolerance
The specific thermal death point for a plant is highly variable due to several modifying biological factors. Species genetics is a major factor, as plants have evolved different inherent tolerances based on their native climate. Tropical plants, for example, lack the ability to tolerate chilling temperatures, while temperate plants may survive well below freezing.
Acclimation, the process of a plant slowly adjusting to changing temperatures, significantly shifts lethal thresholds. When exposed to gradually decreasing non-freezing temperatures, many temperate plants undergo cold acclimation. This involves biochemical changes, such as modifying cell membrane composition and accumulating soluble sugars to protect cells from ice damage.
A plant’s hydration status also heavily influences its resistance to thermal extremes. Well-watered plants effectively use evaporative cooling, or transpiration, to lower leaf temperature and resist heat damage. Conversely, a drought-stressed plant closes its stomata to conserve water, making it much more vulnerable to overheating because it loses its primary cooling mechanism.
Practical Steps to Protect Plants from Extremes
Protecting plants from cold weather involves several direct steps to insulate tissues and prevent ice formation. For cold-sensitive container plants, moving them indoors or into a sheltered area like a garage or shed is the most effective measure. In-ground plants benefit from physical barriers, such as covering them with frost cloth, burlap, or an overturned bucket to trap ground heat. Applying a thick layer of mulch around the base of the plant helps insulate the soil and roots, protecting them from deep-penetrating cold.
Mitigating lethal heat stress focuses on preserving the plant’s ability to cool itself and preventing excessive water loss. Deep, thorough watering in the early morning ensures the plant has the necessary moisture for transpiration throughout the hottest part of the day. Providing temporary shade during peak afternoon sun can lower the leaf temperature and reduce the risk of protein denaturation. Watering deeply and less frequently encourages the development of a deeper root system, which is more resilient to surface heat.