When does root development in a container reach the point of being root bound? The answer is not simple. Root bound begins at that point where the growth rate of the tree begins to decline due to root intermingling and congestion, a decrease in soil oxygen and available space for further root development It continues to the point where tree growth cannot be stimulated to proceed at even modest rates, regardless of fertilizer, water, light, or temperature conditions. The greater the decline in plant growth and vigor due to root congestion and decreased root activity, the more difficult establishment of the tree will be when transplanted into a larger container and especially in the landscape. Being root bound is, therefore, not a specific point or condition but rather a progressively undesirable condition from the ideal state of root development to the very undesirable point where the tree should be thrown away. A key point to remember is that once a tree's normal growth is restricted, for any reason, the tree will never catch up with trees that have not been restricted, assuming all other factors are equal. The ideal point when transplanting should be done to avoid any root bound stress, is as soon as the root development has progressed so that it will hold the potting soil mix together, no more. This is MUCH sooner than would normally be anticipated from observing the root system. Once container grown trees gets root bound beyond a certain difficult to define point, it becomes a liability, not an asset. When a tree reaches the point of OPTIMUM root growth it should be shifted into a larger container. If not, shortly thereafter a point is reached where its value and performance decreases very rapidly. The emphasis on white root tips, visible when the container is removed, cannot be over emphasized. The white root tips play a key role in establishing the tree when it is transplanted or put into the landscape. (Carl E. Whitcomb, Ph.D.) - Millet