Watering problems

Bonsai soils are specially formulated to dry out slowly, which is healthier for the tree. Watering the tree too frequently can cause it to become stressed and lose leaves, while infrequent watering can dry out the tree and kill it. Water the bonsai tree when the top 1 cm of potting soil is dry to the touch.

Environmental problems

Bonsai trees in unhealthy environments may begin to lose leaves. Direct sunlight can scorch the leaves of a bonsai tree and cause them to fall off, while a total lack of sunlight will slowly kill the tree, beginning with the leaves. Use a grow lamp or ensure that the bonsai receives indirect sunlight throughout the day to keep it healthy.

Pest roblems

Indoor pest problems like mites can cause a bonsai to drop its leaves as the mites destroy them and weaken the tree. Signs of mites include dropped leaves as well as a white film on the leaves before they fall. Treat the bonsai with a gentle spray pesticide according to the directions on the bottle to control mite problems.