|Peace Lily, Spathiphyllum sp., with a yellow sun leaf|
I take most of my houseplants outside every summer. They seem to enjoy a few months on the patio, growing a multitude of lush leaves in the more direct sunlight. Then in the fall as the nights cool off, I bring them back in the house. After the transition, I notice several of the leaves become yellow and dry up. I'd like to think they're mourning the passing of another summer, but really they are going through a normal physiologic process to streamline their metabolism for the lower light conditions inside the house.
According to Ward Upham, Extension Horticulturalist for Kansas State University, plants growing in the intense light conditions that would occur outdoors or in a greenhouse actually produce leaves that are structurally different than leaves produced in low light conditions. The leaves exposed to more light, or "sun leaves" are usually smaller and thicker and contain less chlorophyll than the low light "shade leaves." Chlorophyll is the chemical plants use to convert sunlight into energy. The shade leaves, in contrast, are fewer in number and larger. If a plant is gradually moved from an area of high light to low light, the sun leaves do have some capacity to convert themselves into shade leaves. However, if the transition is sudden, the sun leaves will yellow and fall off and will be replaced by new shade leaves.
To avoid this, when bringing houseplants in for the winter, place them in the sunniest window possible then over the course of a couple of weeks, slowly move them to their final winter location. If, however, your house is like mine and the sunniest indoor location is still no match for the outdoors, yellow leaves can be trimmed away, making room for the shade leaves.
Once houseplants have acclimated to life indoors, their growth will slow considerably. This slower phase of life means that during the winter, houseplants need less frequent watering and fertilization. Excessive watering fills the normal air spaces in potting soil, keeping the plant's roots from getting enough oxygen. This can lead to root rot and an unhealthy plant. The best time to water is when the soil is dry about one inch deep. This can be determined by testing the soil with you fingers or garden centers sell inexpensive soil moisture monitors that can make watering more scientific. When watering houseplants, apply enough so that a small amount of water runs out of the bottom of the pot. This helps flush out salts that tend to accumulate in potting soil. But don't let the pot sit in standing water for days since this can also lead to root rot. If the pot is still sitting in a tray of water after 24 hours or so, empty it out. Another option would be to place pebbles in the drip tray then set the pot on top of that. That keeps the roots out of the water while also increasing the humidity around the plant.
Most houseplants are tropical in origin and Colorado is much drier than the tropics. Keeping a humidifier near houseplants or misting the plants frequently will give them a more tropical microclimate. Over fertilization in the winter months will cause houseplants to become leggy and weak. As a rule of thumb, apply half the usual amount of fertilizer to flowering plants. Foliage plants only need about 1/4 the amount of fertilizer required in the summer.