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Why are My Houseplants Yellowing?

FOR IMMEDIATE RELEASE
October 31, 2012

Homeowners can expect a certain amount of yellowing foliage when they bring in their houseplants for winter, said a University of Illinois Extension horticulture educator.

"Those yellowing leaves are the result of the plants adjusting to the lower indoor light levels," said Richard Hentschel. "While they were outdoors, they had all the light they needed and produced foliage based on that. Inside, the light levels are considerably lower, even in the brightest windows."

Moreover, plants outdoors receive light from all directions, either directly or filtered through tree canopies and shade structures. Inside light typically comes from only one direction, and it may not reach the leaves that do not face it.

Yellowing on older, lower leaves is caused by the plant moving the nutrients from a leaf that is less productive to leaves that can make more food. If yellowing is random throughout the plant, it is likely that the reduction in the number of leaves is proportional to the amount of sunlight the plant is receiving.

"A houseplant is more likely to keep leaves that are perfect and healthy. A leaf that is damaged from insect feeding or a foliar disease will be lost," Hentschel said. "While it was outside, even in its damaged state, it was contributing to the plant's overall health because of all the sunlight."

Overwatering is another cause of yellowing. "If plants continue to lose leaves into the winter months, going from green to yellow to brown, keep in mind that once inside they will not be actively growing, and providing water will be much different than outdoors," he said. "Keeping the houseplants on the dry side is safer than too wet."

Make sure that the drainage holes are not closed off by roots circling at the interior base of the pot, which will prevent the soil from draining even if it is a light mix. Root rot can further limit the plant's ability to remain healthy.

When watering, be sure to remove any excess water in the tray so that roots in the very bottom of the pot are not surrounded by water. That excess water can contain high levels of soluble salts from the fertilizer used during the summer that can cause the roots to dehydrate and die.

Gardeners can prevent some of the yellowing by slowing or stopping the feeding program at mid-summer. That will cause the plant to produce less foliage. Prior to bringing them in, move plants from full-sun to shadier locations to begin the transition process.

Once inside, placing houseplants in the appropriate windows will help them to retain their leaves. Remember that the winter sun is not as high in the sky and that south- or west-facing window may provide enough light for plants that would otherwise be in an east or north window. In fact, that north-facing window may not be suitable even for shade-tolerant houseplants.

Careful pruning of the canopy or vines helps reduce stress on the plant and can be done even if the plants have been indoors for a while and are still losing foliage. Group plants with similar needs together to help ensure that they receive proper care.

"Ideally, houseplants should come in before the furnace starts to cycle," Hentschel advised. "That prevents you from rushing to bring them in on the night that the temperature is dropping to freezing and figuring out later where they should be for the rest of the winter."

Source: Richard Hentschel, Extension Educator, Horticulture, hentsche@uiuc.edu