Like humans, plants can also get sunburned when exposed to hot sun. It happens when the plants are overexposed to direct sunlight than the required amount. There is no specific temperature point to define the start of sun burn in orchids. It depends on the type of orchids, humidity and the time of exposure. Usually orchids thrive well when given few hours of indirect sunlight.
The symptoms of sunburn include: yellow dark spots in leaves either unevenly or near the tip of the plant. If enough care is not taken immediately the whole plant including stem may get sunburned. Please note that spotting / yellowing of leaf / bulb shriveling is always not a sign of sunburn. It could also be due to aging or plant’s response to physiological stress (water, fertilizer).
When the long awaited beautiful blooms get sunburned its quite disappointing to lose the flowers as soon they have bloomed. Flowers with thin petals tend to burn easily than leaves when exposed to hot sun. The flowers turn into brown, wrinkle and after few days they wither. Unopened flower buds may also shed or they may bloom only partially. Once the flower is sunburned, it’s difficult to save the flower or flower buds. If brown patch or light wrinkles are seen on the flowers, it should be immediately moved out and placed in a place away from direct sunlight.
Photo details : Here is a photo of one of my orchids which showed signs of sunburn. Just before this picture was taken most of the flowers withered rapidly, all the new growth stopped, new flower stalks stopped growing and the orchid developed brown spotting. The orchid was originally placed in a glass window sill and suffered when we had a heat wave. However the plant suffered less damage.
Pseudo bulbs and Roots
Over exposure to sun can cause the bulbs to dehydrate, which may cause shriveling of bulbs. Any new growth on the plants may be stopped. The roots on the surface may turn brown and dry out. If the roots are not healthy re-potting the plant might be a good idea. In extreme cases there is a chance of less survival of the plant.
Plants placed near windowsill are also prone to sunburn due to the heating effect of glass. It is necessary to provide a better spot for the plants during hot weather to avoid burning.
If you suspect that your orchid shows signs of sunburn, move it away from direct sunlight. If the leaves are burned severely (whole leaf has turned into brown) the leaves can be removed, mild spotted leaf can be kept intact. If the flowers show signs of burning move the plant away to a better place. If flowers start to wither the flower stalks can be removed. Any new stalks with flower buds can be left intact as it may have some chance of success. Don’t fertilize the damaged orchids until they show some signs of improvement. A sunburned orchid will take some time to recover, until then no over watering and no frequent fertilizing.
Please let me know in comments your experiences.