When the leaves on a bonsai tree begin to curl, it is a sign that something is wrong with the tree. Identifying the cause of the problem is important for bringing the tree back to health.
Spider mites are a serious problem for bonsai trees, especially those grown indoors. Spider mites appear as red, black or green insects that attack the leaves of a bonsai plant, and they are often found on the undersides of leaves. Mites thrive in dry, dusty atmospheres, so keep the bonsai moist and well-watered. Apply a pesticide to the bonsai to help kill spider mites.
Too much and too little water can be equally damaging for a bonsai tree. Too much water can begin to drown the tree, causing the leaves to curl and yellow, and too little water will dry the tree out, which will also cause the leaves to curl. Use a bonsai potting soil that dries out slowly, as this is healthiest for the tree. Water the bonsai only when the top portion of the potting soil is dry to the touch.
Deciduous bonsai trees will lose their leaves in the fall, just like their full-sized counterparts. The leaves will change color, curl and fall from the tree as the bonsai begins to enter its dormant period. The tree is healthy and should be left alone while it is dormant, apart from regular watering. Leaves will return in the spring.