Established trees that have been growing in the ground have roots that reach out far beyond the branches or drip line. These long branches are used by the tree to anchor and support it. However most of the small feeder roots, which bring in food and nutrients to the tree, are likely to be found growing off the main roots at some distance from the tree itself.
Root systems are vital to the health and longevity of trees. All plants need water, oxygen, and nutrients. These are most readily available near the soil surface where precipitation infiltrates the soil and oxygen from the atmosphere diffuses into the porous soil. Most roots, therefore, especially the important, tiny, absorbing roots, proliferate near the soil surface.
The majority of a large tree’s roots are in the upper 18"-24" of soil. When space is available, roots can spread two to three times further than the branches. Tree roots are often associated with situations that cause damage to structures, pavements, and utilities. In almost every case, roots are not the cause of the problem.
tree is dug for moving and transplanting,
generally the root ball taken is only the circumference of the drip line, sometimes less. To encourage the development of feeder roots closer to the drip line, root pruning is done. Root pruning is also sometimes used to maintain a dwarfed size and to encourage flowering of a fruit tree or slow to bloom vine, such as wisteria.
When moving a tree, you must sever the long, large roots that anchor the tree to the ground. These roots are not the roots that bring nutrients to the tree. The smaller, feeder roots, that bring nutrients to the tree, usually grow some way from the tree, off the main roots.
When you move the tree, you usually only move the area near the root ball, which does not include most of the feeder roots. To cause new feeder roots to grow, you cut the roots all the way around the tree. You need to prune the tree roots early enough to give the tree time to recover from the root pruning to grow a new supply of feeder roots that will support the tree when moved. The time required depends up the size and species.