Every plant species has an optimal thermal range for growth and reproduction, and temperatures outside this zone induce stress that can lead to damage or death. The designation of a “bad temperature” is highly dependent on the plant’s genetic origin. For example, tropical species are sensitive to temperatures well above freezing, while temperate-zone plants can tolerate significant cold. When plants are pushed outside their native thermal limits, their internal processes begin to fail, leading to physiological disruptions that range from temporary wilting to irreversible cellular collapse. Understanding how these thermal extremes cause harm allows for more effective intervention and protection.
Mechanisms of Damage from Extreme Cold
Low temperatures injure plants through two primary mechanisms: chilling injury and freezing injury. Chilling injury affects susceptible species, mainly those of tropical or subtropical origin, at temperatures between 0°C and 15°C. At this range, cellular membranes lose their fluid nature, becoming rigid and impairing the transport of water and nutrients. This disruption of membrane function can lead to symptoms like surface pitting, water-soaked spots, or internal browning, even though no ice crystals have formed inside the tissue.
Freezing injury occurs below 0°C and is more physically destructive, affecting nearly all plant types. The most common form of damage is extracellular freezing, where ice crystals form in the spaces between cells. As water freezes outside the cell, it draws liquid water out of the plant cells in a process similar to dehydration. This desiccation causes the cells to shrink and collapse, leading to tissue death.
The formation of ice crystals inside the cell (intracellular freezing) is fatal and is more likely during a rapid drop in temperature. Visual symptoms of severe cold damage include foliage that appears blackened, mushy, or water-soaked, indicating ruptured cell walls. Woody plants may exhibit stem splitting or discolored tissue beneath the bark, appearing brown or black instead of the healthy green color.
Mechanisms of Damage from Extreme Heat
When air temperatures rise above approximately 30°C to 35°C, plants experience heat stress primarily through protein denaturation and water imbalance. High temperatures cause proteins, including metabolic enzymes, to lose their stable three-dimensional structure. This loss leads to enzyme malfunction and a general metabolic shutdown, severely impairing processes like photosynthesis.
Heat also induces a water management crisis by increasing the rate of transpiration, the plant’s natural cooling mechanism. To conserve moisture, plants close their stomata, which prevents the intake of carbon dioxide necessary for photosynthesis. This reaction forces the plant to slow its growth and metabolism to survive the heat.
The visible signs of heat damage are direct consequences of this physiological distress. Persistent wilting, even when the soil is moist, indicates that roots cannot absorb water fast enough to keep pace with loss through the leaves. Leaves may curl or roll up to reduce their surface area exposed to the sun. Fruit and stems can develop sunscald, appearing as white, watery, or blistered patches on the sun-exposed side. Severe heat stress can cause leaf scorch, or trigger the plant to abort flowers and fruit to conserve resources.
Practical Strategies for Protection and Recovery
Protection Strategies
Proactive steps can mitigate both heat and cold damage before it occurs. To protect against cold, apply a thick layer of organic mulch around the base of plants to insulate the soil and safeguard the root zone. Before a predicted freeze, deeply water the soil, as moist soil holds and radiates more heat than dry soil. For tender plants, covering them with frost cloth or containers traps radiant heat, but the covering must be removed once temperatures rise above freezing.
Protection against heat stress centers on managing water and sun exposure. Deep watering in the early morning ensures the plant is fully hydrated before the midday sun and encourages deep root growth. Temporary shade cloths or moving container plants to a sheltered location during the hottest part of the day can lower the plant’s temperature and reduce scorch risk. Avoid applying fertilizer during periods of extreme heat or cold, as this stimulates vulnerable new growth.
Recovery Strategies
Assessing a plant’s recoverability requires patience, as the full extent of damage may not be apparent for several days or weeks. For woody plants damaged by cold, gently scrape the bark; healthy tissue will be green, while dead tissue will be brown or black. Avoid immediate, heavy pruning, as the dead or scorched foliage can still provide insulation or shade to underlying healthy tissue. Only remove mushy, soft, or completely dead material, and wait until new growth begins to prune back to healthy wood.