Leaf drop on a favorite houseplant is a frustrating problem because it can be hard to diagnose the causes and correct the situation. It's also possible that it is not a problem at all—leaf drop is a normal condition of growth for many plants, in which lower leaves die and fall off gradually as part of the life cycle. If, however, you suddenly lose a lot of leaves at once, or if you start losing healthy green leaves, then you might have one of the following problems.
This is the most common cause of leaf drop, but it can be the hardest to correct. Shock is most often caused by a sudden change in conditions, such as when a houseplant has been enjoying outdoor conditions and is then brought indoors as cold weather approaches. The opposite is also true: an indoor plant brought outdoor for the summer may also experience shock. Shock is usually a response to dramatic changes in temperature, humidity, light levels, or watering habits. Newly acquired plants, for instance, often go into shock as they transition from the perfect conditions of a greenhouse to less-than-ideal home conditions. The same is true for newly repotted or divided plants.
Sadly, there's not much you can do about shock, other than hope the plant survives. In most cases the shock is a temporary condition; as the plant adjusts to new conditions, its health will return. If you are transitioning a plant to outdoor conditions, do so gradually—giving the plant increasing long visits to the outdoors until it is acclimated to the conditions. Do the same when bringing the plant indoors for the winter—start in fall with short visits indoors to get it accustomed to the change.
Many indoor houseplants are tropical species, and when they are grown in the dry indoor conditions found in northern winter climates, they will react by dropping leaves. This is a natural response since the plant is attempting to conserve its moisture loss by reducing the number of leaves that are transpiring moisture. This can be seen as a form of shock, but because dry winter conditions are slow to develop, it may occur quite gradually. Resting the pot on a tray of pebbles kept constantly wet may help with humidity levels. You can also mist the leaves regularly to prevent them from drying out.
Plants that are in high-traffic areas or are frequently brushed will sometimes drop leaves inexplicably. Pets and children rubbing plants can cause leaf drop. Try moving the plant to a different location, or elevate to a height where it will be safe from contact.
Certain pests, such as mealybugs, spider mites, and scale, can cause leaf drop. Check fallen leaves carefully for telltale signs of infestation. If you see pests, treat the plant and the leaf-drop should stop. Insecticidal soap is a good low-impact pesticide to use on indoor pests.
If your plant experiences this leaf-drop in winter, along with low humidity levels, consider whether the plant is getting enough light. Sunlight levels are low in the sky and the light is indirect in the winter months, even for plants that sit directly in front of a window. If your plant starts to drop leaves, look for a sunnier spot for it, or augment its light level by providing artificial light.
Cold Temperatures (or Warm Temperatures)
If the plant is exposed to cold drafts, many tropical plants will begin to drop healthy leaves. Conversely, a plant that finds itself exposed to the heat of a radiator or heat duct when the furnace begins to run in fall and winter may drop leaves because it is too warm. Keep your plants away from extremes of heat and cold.
Too Little Water (or Too Much Water)
Keeping soil moisture levels can be tricky with indoor plants. Leaf drop can occur either because the soil is too wet or too dry. In general, don't water until the top inch or so of potting soil feels dry, and when you do water, give the plant a thorough soaking.
If the leaf drop is preceded by leaves turning yellow or pale green first, it's possible your plant is reacting to insufficient fertilizer. Try increasing its feeding schedule to see if that helps.