Known for their tropical habitats, orchids (Orchidaceae) enjoy warm and humid conditions to produce their stunning blossoms on long, outstretched stems. According to Brooklyn Botanic Garden, this huge flowering family prefers U.S. Department of Agriculture plant hardiness zones 3 through 10 for terrestrial species such as ladyslipper orchids (Paphiopedilum spp.) and warmer zones 10 and 11 for or moth orchids (Phalaenopsis spp.). Most orchid species do not grow better in sunlight but prefer partial shade.
Indirect sunlight exposure is critical for indoor orchids to generate flower stems and blossoms. Choose any window that does not face north for your orchid. North-facing locations do not provide enough sunlight for maximum photosynthesis and flower development. The plant should not be directly on the windowsill -- place it several feet in from the glass to avoid direct sunlight. Use sheer curtains on the window to avoid any accidental sun rays from striking the orchid throughout the day. Alternatively, you can raise your indoor orchids under fluorescent lights ran for approximately 12 hours each day. The lights should be no closer than 6 inches above the foliage to prevent heat stress.
If you live in warmer regions with mild winters, you can create an outdoor oasis for your orchids under a tree canopy. Choose a tree that has some light penetration rather than a dense canopy with deep shade. Many orchids are epiphytic -- they grow on tree limbs to access the flowing rainwater on the bark. Using cotton string, carefully attach your orchid to a tree limb. You should observe your orchid throughout the next year to verify that direct sunlight does not burn the leaves through open foliage gaps in the tree canopy. Sunlight heat radiating onto the ground and reflecting into the tree canopy provides a sheltered area for healthy orchid growth.
Your orchid leaves reflect their sunlight stress level through color changes. Dense shading hinders photosynthesis so leaves respond with more chlorophyll to increase energy production -- your leaves change from a brilliant green to a darkened hue. In contrast, direct sunlight scorches leaves. They appear yellow to beige in color as sunlight damage progresses. Moving your orchid to a new location alleviates sunlight stress, but the plant needs to recover with new leaf growth; your flowering may be reduced or fail altogether for one or more flowering periods.
Orchid leaves should have the same ambient temperature as their surrounding environment. If they feel hot, they are exposed to too much sunlight. In fact, orchids need some temperature swings between day and night to signal flowering periods. In general, your orchid needs to be exposed to a minimum 10-degree temperature difference from night to day for normal flowering. Outdoor orchids easily achieve this difference, but indoor specimens may need to be moved to a cooler location to stimulate flowering. Once flower buds emerge, your orchid requires normal indirect sunlight to photosynthesize and support the growing blossoms.