Sunburned house plants

The weather has been so warm that I thought it was safe to put my houseplants outside. After only one day outside I noticed that the leaves on some of them had turned white! What happened? Was there a frost?

You have described the symptoms of sunburn in plants. If plants are grown indoors through the winter in a location with bright, but indirect light, they often form "shade leaves". Shade leaves develop under conditions of relatively low light. They efficiently produce food for the plants, but many loose the ability to tolerate exposure to full sunlight. They must be gradually exposed to full sunlight (hardened off). This is done by gradually increasing their time in full sunlight from a few minutes to longer and longer periods.

Some plants will adapt without drastic changes, but some plants will drop their shade leaves and form new sun leaves. Sun leaves have protective mechanisms to protect them from full sunlight. If your plants begin dropping leaves as you change their environmental conditions, this is probably the cause. Leaf drop can also happen when plants are moved indoors in the fall. In the fall, sun leaves may be discarded and new shade leaves formed.

In the case of your injured plants, the leaves have been burned. They will fall and new leaves must be produced. If the plants are healthy, they should do this with little difficulty. While the plants are producing new leaves, they will need moisture. However, since there are far fewer (or no) leaves remaining, the plant will use less water than before. Reduce watering, but don't let the soil dry out completely. As the new leaves develop, water use will increase and irrigation must be gradually increased as leaves are formed.

Reduce fertilizer application during the time of leaf replacement. Don't stop fertilizing, but use a more diluted fertilizer during this time or apply it less often. Nutrients from the fertilizer will be needed for growth, but during redevelopment of leaves, the plant will be using food stored in the plant tissues. Once the leaves have reformed, resume normal fertilization.

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Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms and the NMSU Horticulture Publications page.

Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at desertblooms@nmsu.edu, or at the Desert Blooms Facebook.

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