Cacti are succulent plants uniquely adapted to survive in challenging environments. They have evolved morphological features, such as water-storing stems and spines, to thrive where water is scarce. Their unique form of photosynthesis, Crassulacean Acid Metabolism (CAM), allows them to conserve moisture by opening their stomata only at night. This resilience is not limitless, and temperature extremes—both low and high—represent the primary threats to their survival.
Understanding Cold Stress and Freezing Points
Cold temperatures pose a significant threat to cacti because of their high internal water content. When stem tissue temperature drops near or below the freezing point (32°F or 0°C), ice crystals form in the extracellular space. This process draws water out of the plant cells, causing cellular dehydration and leading to the rupture of cell walls as ice expands. For most desert species, temperatures sustained below 32°F are often lethal, while tropical varieties show damage as high as 45°F to 50°F.
The visible signs of cold damage become apparent as the tissue thaws. Initially, the affected area appears as pale patches that quickly turn black or dark brown. The tissue loses firmness and becomes soft or mushy, indicating that the internal cell structure has collapsed. In less severe cases, the plant may exhibit orange or yellowish scabbing, especially when cold temperatures combine with high humidity.
Immediate action during a cold snap involves providing a temporary cover, such as a frost blanket, or relocating potted plants indoors. After exposure, any soft or mushy tissue will not recover and should be pruned away using a sterile blade. Allowing the cut to dry and callous over prevents rot from spreading into the healthy material. Withholding water during the cold period is also a protective measure, as a drier plant is less susceptible to internal freezing damage.
Managing Extreme Heat and Thermal Stress
While cacti are known for heat tolerance, prolonged exposure to excessive warmth causes thermal stress, especially when paired with intense sunlight. High temperatures can cause proteins and enzyme systems within the plant cells to denature, or lose their functional structure. Temperatures sustained above a plant’s thermal limit lead to the melting of lipid membranes within the chloroplasts, disrupting photosynthesis. For many species, the sustained air temperature threshold for physiological stress is often between 105°F and 120°F, though the internal tissue temperature limit is much higher.
A specific form of heat stress is sun scald, or photodamage, which occurs when a cactus is exposed to intense solar radiation. This damage is most common on unacclimated plants, such as those recently moved from a greenhouse or shaded indoor location. Symptoms include yellowing or bleaching of the skin on the sun-facing side, which can turn into permanent, scarred white or brown patches. In severe cases, the plant may shrivel or collapse due to the inability to manage water loss.
Protection from excessive heat involves providing filtered light during the hottest part of the day, typically the afternoon. Utilizing a shade cloth (rated between 50% and 90% shade) significantly reduces the amount of radiation reaching the plant surface. Small plants can be temporarily shielded with simple barriers or relocated to a spot receiving morning sun only. For plants moved to a brighter location, a slow, gradual increase in sun exposure over several weeks allows the plant to build its natural defenses.
Variables That Change the Tolerance Limits
The exact temperature tolerance is not a fixed number and varies dramatically based on the species and its current physiological state. Highly cold-hardy varieties, such as certain Opuntia species, can survive tissue temperatures as low as -11°F (-24°C). In contrast, many tropical or epiphytic cacti are damaged by temperatures below 50°F (10°C). High-temperature tolerance can range from 131°F (55°C) to 158°F (70°C) for the stem tissue of some desert dwellers.
Acclimation is a fundamental factor that shifts the survival threshold for heat and cold. Plants gradually exposed to cooler temperatures synthesize cryoprotectants like sugars, which act as natural antifreeze by lowering the cellular freezing point. Conversely, exposure to increasing warmth stimulates the production of heat-shock proteins, which help stabilize and protect other proteins from thermal damage. This gradual conditioning allows a cactus to tolerate extremes that would instantly kill an unacclimated specimen.
Environmental factors like humidity and the plant’s life stage also modulate temperature stress. High humidity, especially when combined with cold, increases the risk of cold-related damage and fungal issues. Larger and older stem segments often demonstrate greater tolerance to both low and high temperatures than younger growth. Containerized cacti are more vulnerable, as their root zones are exposed to greater temperature fluctuations than those planted in the ground.