According to Bonsai Primer, common causes of falling bonsai leaves include natural leaf shedding, inadequate light and excessive watering. Inadequate lighting is a particular problem with indoor bonsai. Leaves have a life span and eventually die. When large numbers of a bonsai's leaves suddenly die and fall off, the tree has a serious and potentially fatal problem that requires prompt intervention.
Other causes of dropping bonsai leaves are prolonged cool temperatures and inadequate watering. Saggy, limp and underwatered leaves recover quickly when watered. Excessive water and poor drainage present greater risks to the tree. Once the roots and trunk begin to rot, most bonsai die.
Watering problems are easier to prevent than to cure. Fuku Bonsai recommends submerging the roots and anchor rock for thirty minutes per week. After watering, the tree needs to drain on a bed of clean, dry gravel. This is an excellent opportunity to examine the tree for root rot and fungus. Chronic overwatering turns the naturally white root tips black. This reveals that the tree has begun to rot.
According to Garden Guides, most bonsai problems develop gradually and respond well to early treatment. If a root inspection reveals black tips, they must be immediately removed with sterile scissors or clippers. The same tools are effective for removing dead or damaged branches and leaves.