Under or over-watering, mineral deficiencies, cold temperatures and stress from a move can all cause a bonsai tree's leaves to turn yellow and fall off. The different species of bonsai trees are best suited for different climates, so it is best to choose a species that will work in the current climate.
One of the most common causes of yellowing leaves is under or over-watering. Bonsai trees require various amounts of water, depending on the type of tree. Water the tree in the morning and moisten the soil and mist the leaves. Too much water can lead to root rot, which will also cause the leaves to fall off. Ensure the pot offers suitable drainage for the tree.
Mineral deficiencies in the soil are another common cause of falling leaves from bonsai trees. A liquid fertilizer can be mixed with water and added every 2 to 4 weeks to ensure the tree is receiving the right amount of nutrients. Bonsai trees grow best outdoors, but they do not fare well in cold temperatures. The ideal climate for a bonsai tree depends on the species of tree it is.
Moving the tree to another location or re-potting it can cause stress to the tree, which can also cause the leaves to fall off. Ensure the tree is receiving enough water and is in a proper climate to reduce the stress to the tree.