House plants drop leaves for many reasons but most are related to improper care or poor growing conditions.
Either over or under watering will cause leaf drop.
Extremely low humidity will cause sensitive plants, such as gardenia, to drop leaves. Most common house plants will not show leaf drop in response to low humidity only.
Plants is pots that are too small will drop leaves. There may not be enough root room to support all the leaves the plant tries to form so the oldest leaves are dropped off. Because the roots room is inadequate the plant may not be able to absorb enough water and nutrients.
Some leaf drop occurs when plants are subjected to a big change in environment. Such changes occur when plants grown outside for the summer are brought inside for the winter. Greenhouse grown plants may drop leaves if placed in dimly lit house conditions. Leaf drop brought on by a change in environment should last about 3 weeks then stop.
A cause of leaf drop related to environment is that caused by chilling. Tropical plants are sensitive to low, but above freezing, temperatures. Plants on window sills may be exposed to chilling temperatures.
Hot or cold drafts may be a problem to some plant. The prime example of a plant that drop leaves due to exposure to drafts is the poinsettia.
Insects and diseases can cause leaf drop but they are not as common as the previously listed causes.
Some leaf drop on house plants is normal. Older plants should be expected to drop a leaf occasionally. When a plant drops leaves check to see if any of the previously listed causes has occurred. If not, consider how the plants has been taken care in relation to its cultural requirements. It may need more light or fertilizer.