House plants in particular are prone to dropping their leaves in situations where they are not completely happy with their environment. A number of factors can cause this problem, which usually is only a temporary situation. Most plants will soon return to normal good health when the crisis is over. Ironically, insects and disease are rather uncommon causes of leaf drop. Instead, environmental factors and improper maintenance techniques tend be the more common reasons why plants drop their leaves.
Moisture Fluctuations and Air Humidity
One reason that plants drop their leaves is the moisture level of the soil or the surrounding air. Know your plant well beforehand so you will know how much water it requires and try to keep the soil at those levels at all times. Drought periods, followed by heavy watering, followed by more drought, can make the plant unstable and cause it to drop its leaves. Typically it will be the older leaves that drop first so the newest ones can stay in place. The amount of moisture in the air should not be allowed to vary to any extremes either. If a plant thrives on a high humidity atmosphere during the summer and that abruptly disappears in the winter, the plant will be unstable again.
Root Binding and Hard Packed Soil
Failing to repot your plant when it grows too big for the first one can cause serious problems with the plant. When this happens the roots can no longer function as they should and stop delivering the proper amounts of moisture to the whole plant, so parts of it die and drop off. Root binding tends to make the soil clump up and become hard, which causes a lack of oxygen to the plant's roots. This results in the oldest leaves turning yellow and dropping off. Repotting the plant will correct the problem.
Changes in Environment and Chilling
Moving a plant indoors when it has been outside all summer can shock the plant, causing a temporary dropping of leaves. This is due to the sudden change in temperature, sunlight and humidity levels. Moving a plant from one room to another can create the same disturbance. It is possible that the second room is not as cool or as warm as the first one, or perhaps does not receive the same amount of light. Separating the roots to create two plants, no matter how gently it is done, usually results in some level of plant shock and a temporary loss of leaves.
Excessive Fertilizing and Gas Fumes
It is very easy to over-fertilize a plant. While it usually does no harm to under-fertilize the plant, using excessive fertilizers can make the plant unstable, causing it to drop some of its leaves. If a furnace, space heater or gas stove develops a leak, it is dangerous to people and animals but it is also damaging to plant life in the home. It can make a plant go into shock and it will be unable to get enough oxygen. Without proper oxygenation, the leaves will quickly turn yellow and drop off.