Some causes of leaf drop in a schefflera plant include too much or too little light, overwatering, changes in temperature and humidity, and viral diseases. A plant infected with a viral disease causes the leaves to develop discolorations, turn yellow and eventually drop. A schefflera with this disease needs to be disposed of immediately as it has no cure and can spread to other plants.
While a schefflera plant can survive with access to direct sunlight, the soil needs to be moist and well watered. The plant thrives best under indirect sunlight, as too much light can scorch the leaves and cause them to turn yellow and fall off. A lack of light also causes the leaves to yellow and drop off. This normally happens when potted indoor schefflera are placed far from natural light or at a north-facing windows. Moving the plant to a south-, east- or west-facing window or adding fluorescent lights can prevent leaf drop.
Overwatering a schefflera plant or allowing it to sit in soggy soil can cause leaf drop. When watering the plant, soak the soil until it seeps from the drainage holes, and then empty the drainage tray to keep the roots from sitting in the water. Poorly drained potting soil can also cause the roots to sit in water. Replace the soil with loose potting media, and work in compost or other organic matter to improve the drainage.