Asked how long his legs were, tall Abraham Lincoln is quoted as saying "Long enough to reach the ground." Most shrubs and trees reflect that characterization; roots need to be long enough to hold a bush in the ground and reach the nutrients and water that will help it grow. A healthy root system is critical to any plant and particularly important to the support of woody stems, abundant leaves and new growth occurring at a distance from nourishing roots.
Factors Affecting Root Depth -- Beginnings
Nursery plants give some idea of the proportion of roots to shrub. Often roots will fill a 1-gallon tub when the growing shrub is less than the tub's height. Shrub roots in nursery pots seldom amount to less than one-third the height of the young plant, serving as a reminder that a well-developed root system is critical to plant growth.
Several soil factors have to be present for shrub roots to establish and grow well. Soil should be free of large rocks and other debris in the soil that may impede growth. Hard packed or high-clay-content soil make it hard for roots to grow and seek needed water and nutrients. Soil also may also need amendments, like peat moss or rotted compost or sand, to provide the correct pH balance and nutrients needed for healthy shrub growth.
Water -- Insufficient
Shrubs that do not receive enough water will send out roots to search, beginning at the surface of the ground. Long dry spells or predictably dry seasons customarily send shrub roots deeper down into the ground to find water. Erratic or frequent, but brief, watering, encourages the spread of surface or near surface roots, often to the detriment of the shrub. These roots are more susceptible to withering in ongoing heat and drought, damaging plant growth.
Water -- Too Much
Shrubs can make great contributions to erosion control, but may not be strong enough to do the job alone. Overwhelmed with flooding water, shrub roots can drown. Accompanying deep-rooted shrubs with middle-rooted shrubs and other kinds of plants can provide a thirstier overall response to water-caused erosion, along with a better network of all kinds of soil-holding roots.
Defining "Deep" by Digging
Much of what we know about the depth of shrub roots is either not quantified or regarded as already known to gardeners. Therefore, the bulk of our knowledge about shrub roots is based on digging them out. The bulk of mature shrubs display roots approximately a quarter to one-third the length/depth of the whole plant's height. There are, of course, exceptions: cultivated ironwood has shallow roots, while the roots of any thorny barberry bush slated for removal seem to be of infinite depth.
Defining "Deep" by Transplanting
Both agriculturalists and nurserymen advise against burying shrub roots too deep. A classic shrub transplant hole is close to the depth of the root ball, but twice its diameter. Roots extend both down and out, reaching generally farther down into the ground with growth, but room needs to be made for spread as well as depth.
The University of Maine Extension advises, for example, that anyone moving a middle-sized tree or large shrub plan to dig a trench from 15 to 24 inches deep to get under the bulk of mature roots. The University of Idaho Extension provides a formula for determining the size of in ground root balls using trunk diameters. Both point out, however, that a root ball a foot or more in diameter can weigh several hundred pounds, leaving transplanting to professional equipment and workers.
A Predictable Source of Deep Roots
Native plant roots tend to go deep. While a native plant's root system will not always outdistance that of a cultivated peer, chances are high that root strength has allowed a native to persist in spite of many depredations. Seek out a native when deep roots are of particular concern.