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A mature shrub with a root ball weighs 100 pounds per square foot. A shrub more than 3 feet tall has to be moved with a root ball. Root balls should be 12 to 24 inches deep.
Two seasons before you transplant, prune the root system. To encourage lateral roots near the trunk, fall transplants should be root pruned in spring before new buds appear; spring transplants should be root pruned in autumn after the leaves fall.
To root prune, cut a narrow trench the size of the root ball and 24 inches deep.
When digging a root-pruning trench, for every 1 inch of trunk diameter, allow 15 to 18 inches of distance from the trunk.
When you’ve finished root pruning, fill the trench with water and replace soil and topsoil.