How do plants know when to drop their leaves?

All leaves have a definite life span and are dropped following receipt of internal or environmental signals. The process is called abscission and is facilitated by the formation of an abscission zone at the base of the petiole. Plants that drop all of their leaves within a short time, resulting in a temporarily bare, leafless plant, are called deciduous plants. Those that drop leaves a few at a time throughout the life of the plant are called evergreen plants (because they appear to be fully leafed at all times).
Hormones trigger the formation of the abscission layer. The dropping off of the leaf is aided by anatomical changes in the abscission zone where two tissue zones differentiate; the one nearest the stem accumulates suberin in the cell walls — blocking the flow of materials — while cells of the separation layer on the blade side simply disintegrate. The suberized zone left on the stem after the leaf falls is called the leaf scar; visible within it are the bundle scars, the remnants of the vascular strands.