The majority of houseplants used for decoration are tropical or subtropical species that will not thrive outdoors in temperate climates. They have specific requirements for light, water, heat and feeding and any variation from these requirements will cause problems for the plant. Yellowing leaves are a sign of a problem.
Except cacti and succulents, houseplants cannot tolerate extreme temperature fluctuation. According to the University of Missouri Extension, a daytime temperature between 65 and 75 F is ideal, with a maximum drop of 10 degrees at night. Sudden temperature fluctuation of 20 degrees can cause leaves to yellow rapidly and fall off. If it is impossible to alter the room conditions, then move the plant to a different area in the house where temperature fluctuation is less extreme.
Almost all houseplants require at least 12 hours of daily daylight, but the intensity of light varies considerably between species. Some plants, like agapanthus, require intensive light and thrive on south-facing windowsills. Others such as sansevieria require shady conditions. If a light-loving plant is placed in shade, the lower leaves will turn yellow, dry up and fall off. Check the light requirements for the plant and move it accordingly.
Every plant has its own requirements in terms of frequency and quantity of water. Overwatering and not watering enough are both equally harmful. In "The House Plant Expert", Dr. D.G. Hessayon informs us that "more plants die through overwatering than any other single cause." Waterlogged compost prevents air reaching the roots and the roots themselves rot, depriving the plant of its life source. Leaves turn yellow, wilt and may become soft and rot.
Central heating creates dry atmospheres. According to the University of Missouri Extension, except for cacti and desert plants, houseplants require around 60 percent humidity. Too little humidity causes leaves to yellow and wilt.
Double potting the plant with a layer of moist peat between the pots or standing plants of a tray of moist gravel is effective while some plants thrive in the moister air of bathrooms, shower rooms and kitchens.
Plants may be sensitive to minerals and salts that occur in domestic water. Per University of Missouri Extension, rectifying salt build-up often rescues ailing plants. Plants such as carnivorous plants, gardenias, orchids and azaleas hate lime, so tap water from hard water areas should be avoided. Excess calcium in lime-intolerant plants causes upper leaves to turn yellow but remain firm.
Age and Viruses
Occasionally, an older leaf at the base of the plant will turn yellow and fall off as part of a natural process. If several leaves turn yellow at the same time, the most likely cause is overwatering or cold drafts.
Viral infections are usually caused by insects or were present when the plant was purchased. Leaves become pale green or yellow in patches and the plant fails to thrive. There is no cure and the plant should be disposed of.