Orchids, native to tropical and subtropical regions, are highly sensitive to temperature variations. Because they originate in climates with stable warmth, temperature is the most influential factor determining their survival and growth. Defining “too cold” is not a single number but a threshold that depends entirely on the specific type of orchid being grown. When temperatures drop below this plant-specific minimum, the orchid’s metabolism becomes severely impacted, leading to physiological stress and potential damage. This article defines the minimum safe temperatures for different orchid groups and provides practical steps for protecting them from the cold.
Categorizing Cold Tolerance in Orchids
Orchid varieties are generally grouped into three categories based on their native habitat and the minimum safe temperature they require, typically measured as the lowest acceptable night temperature. Exposing any orchid to temperatures even slightly below its designated minimum, especially for extended periods, constitutes “too cold” and can initiate chilling injury.
Warm-growing orchids, which originate from tropical lowlands near sea level, generally require night temperatures to remain above 60°F (15.6°C) to 65°F (18.3°C). Exposure below this range can quickly stress the plant and inhibit proper growth. Common Phalaenopsis and Vanda hybrids fall into this category and may drop leaves or show damage if the temperature dips below 60°F.
Intermediate-growing orchids are naturally found at higher elevations in tropical areas or in subtropical zones, where the temperature fluctuation is more pronounced. These varieties thrive with night temperatures between 55°F (12.8°C) and 60°F (15.6°C). Many Cattleya and Oncidium alliance members fit this profile, but prolonged exposure to temperatures in the low 50s can still be detrimental.
Cool-growing orchids hail from high-altitude cloud forests, accepting night temperatures as low as 50°F (10°C) to 55°F (12.8°C). Some, like certain Cymbidium species, can tolerate brief dips into the low 40s (4.5°C) with minimal damage. Crossing the 50°F threshold will cause stress, and freezing temperatures below 32°F (0°C) result in fatal damage due to ice crystal formation within the cells.
Identifying Symptoms of Cold Exposure
When an orchid is subjected to temperatures below its tolerance level, symptoms of chilling injury often manifest days after the exposure. A common sign is the appearance of water-soaked patches on the leaves, caused by cell rupture. These translucent areas quickly turn black or dark brown as the tissue dies, creating a necrotic spot.
The plant may also exhibit general limpness or wilting in its leaves and pseudobulbs, indicating disrupted water transport. For orchids in bloom or bud, a sudden temperature drop can trigger “bud blast,” causing flowers or developing buds to drop prematurely. In severe freeze damage cases, pseudobulbs may become squishy and brown, or the roots may turn soft and mushy.
If the cold exposure was brief and damage is confined to a few leaf spots, the orchid may recover, but severe, widespread damage is often fatal. Recovery depends heavily on the post-exposure environment, as intense light or low humidity following chilling stress can worsen the damage. Weakened plants are also more susceptible to secondary infections from bacteria or fungi, which spread rapidly through compromised tissue.
Strategies for Protecting Orchids from Low Temperatures
Protecting orchids from cold requires proactive planning and swift action to ensure temperatures remain above the minimum threshold. For indoor care, correct placement is the first defense against chilling injury. Orchids should be kept away from windows, especially at night, because cold air radiating from the glass can be several degrees lower than the ambient room temperature, and drafts from leaky exterior doors must be avoided.
Closing blinds or drawing curtains at night creates an insulating layer between the plant and cold glass. To maintain ideal temperatures and humidity in heated indoor environments, growers can place orchids on humidity trays or run a humidifier, as dry air from central heating exacerbates cold stress. Plants situated near a vent or radiator should be moved, as sudden blasts of hot or cold air cause harmful temperature swings.
For orchids grown outdoors, the most proactive step is bringing them inside before the predicted temperature drops below their minimum tolerance. If temporary outdoor protection is necessary for a short cold snap, covering the plants with a frost cloth, blanket, or burlap provides a few degrees of insulation. Avoid watering plants late in the day during cold weather, as moisture in the potting medium and on the leaves can cool the plant further and increase the risk of freezing.
In the event of an unexpected, severe cold snap, emergency steps include moving the most sensitive plants to a warmer, interior room, such as a bathroom or heated basement. If a large collection cannot be moved completely, grouping plants together in a sheltered location and covering them temporarily with plastic sheeting or blankets helps trap residual heat. The goal during an emergency is simply survival, and even sub-optimal light conditions can be tolerated for a few days until the danger of freezing passes.