Tropical orchids are more susceptible to injury due to cold than the temperate orchids. These injuries caused by exposure to low temperatures, affect all parts as well as at all stages of development of tropical orchids; except dormant and underground pseudobulbs. Extent of injury depends upon the duration of exposure to the chilling temperatures. Short periods of exposure may produce only temporary damage. However, prolonged exposure to cold weather can lead to permanent injury, which can cause death of tissue and in extreme cases, the orchid.
Symptoms of cold damage are not unique, but common with other diseases and stresses. These symptoms are not immediately visible but become evident only after internal tissues and functions get affected. Sometimes the symptoms appear much later, once the plant is shifted to warmer temperature and higher light intensity.
Some of the symptoms are:
• Orchids exposed to extreme cold show stunted growth and decreased vigour
• Appearance of surface lesions on affected parts
• Plant tissue appearing water soaked, even if sparingly watered.
• Discoloured leaves; turning brown or yellow
• Plants show a decreased resistance to disease and pests
Prevention and Cure:
• Use of protective structures is advised such as jute cloth/sacks, burlap sacks, plastic sheets; taking care that the plant is able to breathe. You can also cover the roots with extra growth medium to prevent freezing of roots.
• Orchids, especially the tropical ones, should be acclimatized to decreasing temperatures to increase their resistance to cold.
• Avoid over watering and keep an eye on weather forecast. Remember to reduce the watering frequency, if a dip in temperature is predicted.
• If an orchid gets exposed to low temperatures and shows signs of cold injury, it should then be kept in high humidity but with a lower sunlight intensity.
• Cold injuries do not spread; more sensitive parts are more easily affected. It is best to remove the damaged part/tissue.
However, remember prevention is better than cure.