Prevalence of Deep Roots

Deep root systems are receiving much more attention recently, but the problem is not new. In the 1980’s, deep roots were reported on New York City trees and attributed to nursery practices prior to planting in the landscape. Near the same time, in a study of declining sugar maples in Wisconsin, diseases, such as the Phythothora, were found to occur frequently on the base of the trunk and buttress roots of trees with roots that were too deep, similar to the tree in the photo. (photo by: T. Smiley)

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Surveys of recently planted trees have yielded mixed results.

• Data from a 1989 study in a Chicago suburb documented that about one-third of street trees had structural roots more than 3 inches below the soil surface (current industry consensus of “too deep threshold” in average situations).
• Data from a park site in Chicago planted in 2004 showed that the root systems of almost twice as many trees, nearly two-thirds, were more than 3 inches deep.
• Surveys of recently planted trees on the East Coast showed that the average soil depth over the root system was less than 3 inches in Charlotte and Cape Cod, but slightly over 6 inches in Long Island. (See Smiley [pdf]) (photo by: T. Smiley)
• In a Minnesota study of street trees 3-9 inches dbh, the percentage of trees with the structural roots over 3 inches varied somewhat by species, but overall was quite high (see table) (See Giblin [pdf])

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Since the root depth measurements were taken after the trees were planted in all cases, we cannot be certain if the roots were too deep in the root ball, or if mistakes were made in the planting process, or both.

Average depth of structural roots in the field

• The average depth of structural roots in Tennessee nurseries was 4.2 inches. Individual species ranged from 2-7 inches (See Airhardt [pdf]) (photo by: D. Airhardt)
• In Ohio nurseries, the average depth of structural roots from seven sources was 2.4 inches. Only one source was over 3 inches deep. (See Rathjens [pdf])

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Average depth of structural roots in harvested root balls

• In root balls from Indiana nurseries, the average depth of the structural roots was 3.9 inches. (McKenzie and Holt Purdue, unpublished)
• Roots averaged 3.8 inches deep in root balls from Charlotte, North Carolina sources (McKenzie and Holt, Purdue unpublished)
• In a study of brokers from Ohio, root depth in root balls averaged 3.4 inches. Of the eight sources, half averaged over 3 inches deep. (See Rathjens [pdf]) (photo by: R. Rathjens)

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