University of Maryland Extension

Excess Light - Sunburn

excess light orchid

Excess Sunlight

Some plants grow best in partial sun or shade. When shade-loving plants are exposed to full sun, leaf scorch or sunburn can occur. Strong sun and heat cause the breakdown of chlorophyll in the leaf. Damage appears as pale, bleached or faded areas. These areas eventually become brown and brittle. Symptoms are more severe when strong sun is combined with dry soil conditions.

sunburn avocado

Sunburn may also occur on houseplants that are adapted to low light conditions and moved directly outdoors without a "hardening off" period. To avoid sun scorch on houseplants, place plants in full sun for 2 hours per day initially and gradually increase the length of sun exposure.
Houseplants can be classified according to their light requirements: low, medium or high. Important aspects of indoor light are intensity, duration, and quality. Light intensity depends on the distance of the light source from the plant. The brightest light is found in a south facing window, bay window or sun room. The duration is the length of time the plant receives light. Most flowering houseplants are indifferent to duration. However, some houseplants such as Poinsettia, Kalanchoe and Christmas cactus only flower when day length is short, 11 hours or less. Plants need some period of darkness to develop properly; so do not expose them to more than 16 hours of light. Artificial lighting can improve the quality of light the plant receives. Red, far-red and blue wavelengths are most important for plant development. Incandescent plus fluorescent light sources or horticultural grow lights best mimic natural sunlight.

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