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Percent roots harvested on field grown trees

There is a huge misunderstanding in the popular and scientific literature on the percent roots harvested on field-grown nursery trees. Dr. Gary Watson performed the first work in this area in 1982 (Watson and Himelick 1982) and this work has been quoted in publications for the past 20 years. There have been at least three additions and refinements of this original work in the past 15 years (Gilman 1988; Gilman and Beeson 1996b). Unfortunately, this additional information is rarely included in popular publications, web sites, or pamphets written on the subject of tree planting. This has led to misinformation and confusion about the percent roots harvested when a typical field-grown nursery tree is harvested. No longer can we simply say "most of the roots are lost from field-grown trees when they are dug from the nursery".

Table 1. There are several ways to express the percentage of the root system lost when field-grown trees are dug from a nursery.

1) One is to calculate the soil volume in a typical root ball (using ANSI Z60.1 standards) and compare this to the total theoretical root system volume on the entire tree (assuming roots extend a distance equal to three times the dripline and roots are 18 inches deep). This is what Watson and Himelick did and they found that from 2 to 5% of the total theoretical root volume was havested with a field-grown tree.
2) A second way to calculate the percentage of roots harvested in a field-grown tree is to measure root length harvested in the root ball and compare this with the amount left behind in the nursery. Gilman did this in 1988 and found for honeylocust, green ash and poplar that from 5 to 8% of the root system length was harvested on a field-grown tree.
3) A third method is to measure the surface area of the roots in the root ball and compare this with the surface area left behind in the nursery. Harris and Gilman did this in 1993 and found that 55% of the root surface area was harvested in the root ball, leaving 45% in the nursery.
A fourth method that has been used is root weight. Gilman and Beeson did this in 1996 and found that up to 82% of the root weight was harvested with a typical root ball.

So, what percentage of the root system is harvested in a typical root ball? The answer to this question appears to depend on what question you ask. If you ask what percentage of root weight is harvested, the answer is up to 82%. If you ask what percentage of the root length is harvested, the answer is more like 5 to 8%. There is a further complicating factor-root pruning in the nursery. None of the above mentioned studies were performed on root pruned trees and root pruning increases the amount of roots in the root ball. This is why survival of root pruned trees after transplanting is so much greater than on trees that were not root pruned (Gilman 2001, Gilman et al. 2002).