44 Pittenger and Hodel: Evaluation of Circular Root Barriers Table 5. Effects of root barriers on mean numbers of small, medium, and large roots of Liquidambar styraciflua present within 15 cm (6 in) of the soil surface in five radial distance zones from the trunk 6 years after planting, Riverside, CA, 1998. z Zone 1 (3 cm < radius £ 18 cm) Barrier Check #5 container Zone 2 (18 cm < radius £ 30 cm) Zone 3 (30 cm < radius £ 60 cm) Zone 4 (60 cm < radius £ 120 cm) Zone 5 (120 cm < radius £ 180 cm) Small Medium Large Small Medium Large Small Medium Large Small Medium Large Small Medium Large 3.7 a 3.1 a 4.0 a 7.0 a 4.6 a 4.2 a 11.9 a 6.6 a 2.3 a 14.6 4.8 a 0.4 6.3 1.7 a 0.1 4.0 a 1.4 b 0.3 b 6.8 a 1.6 bc 0.0 b 9.0 a 2.2 bc 0.1 b 8.4 0.7 b 0.0 5.3 0.4 b 0.0 Polyethylene sleeve — — — 3.9 ab 2.0 b 0.5 b 9.4 a 3.8 ab 0.8 b 12.9 2.7 ab 0.2 6.4 0.5 b 0.0 #15 container DeepRoot ® Least significant difference y — — — 1.5 bc 0.0 c 0.0 b 3.1 b 0.1 c 0.0 b 5.4 0.5 b 0.0 2.5 0.1 b 0.0 — — — — — — 2.1 b 0.6 c 0.0 b 7.0 0.4 b 0.0 4.2 0.1 b 0.0 2.0 1.5 0.9 3.8 1.8 ( P = 0.05) diameter £ 5 cm (diameter £ 2 in). y z Root diameter size classes: small, 1.25 cm £ diameter < 2.5 cm (0.5 in £ diameter < 1 in); medium, 2.5 cm £ diameter < 5 cm (1 in £ diameter < 2 in); large, NS = not significant; means within columns followed by the same letter are not significantly different. barriers, with the #15 container treatment having the most dra- matic height reduction. Trees growing in the #15 container and DeepRoot barrier treatments also produced the fewest total roots in the excavated area, which may be in part responsible for the reduced growth exhibited by trees in these barriers. Trees whose growth was reduced did not express diminution in any other aes- thetic characteristics. DISCUSSION Our findings provide confirmation of similar results from the shorter-term studies reviewed here, document the size of larger roots that develop near the surface 6 years after planting, and narrowly delineate the distance outside a barrier where surface roots will likely appear several years after installation. The results demonstrate circular physical root barriers of various dimensions and formed from of a variety of low-cost materials are about as effective as commercial root barriers and can nearly eliminate large surface tree roots to a 180 cm radius of the trunk for at Table 6. Mean cumulative trunk caliper increase in Liquidambar s tyraciflua and Ficus microcarpa 6 years after planting in five circular root barrier treatments, Riverside, CA, 1998. Barrier treatment Check 12 mil polyethylene 5 gallon DeepRoot ® 15 gallon Least significant difference ( P = 0.05) Species treatment Liquidambar Ficus Least significant difference ( P = 0.05) 2 × 5 factorial statistical effects z Species (S) Barrier (B) S × B z mm 160.0 a 156.0 ab 161.8 a 159.7 a 143.9 b 12.6 149.1 a 162.8 b 8.0 *** ** NS NS, *, **, *** = not significant, and significant at P = 0.05, 0.01, and 0.001, respectively; means followed by the same letter are not significantly different. ©2009 International Society of Arboriculture Table 7. Mean cumulative height increase in Liquidambar s tyraciflua and Ficus microcarpa 6 years after planting in five circular root barrier treatments, Riverside, CA, 1998. Barrier treatment Check 12 mil polyethylene 5 gallon DeepRoot ® 15 gallon 2 × 5 factorial statistical effects z Species (S) Barrier (B) S × B z Ficus (cm) 379.6 396.5 445.3 455.4 393.2 Least significant difference ( P = 0.05) NS Liquidambar (cm) 496.4 a 437.6 a 451.8 ab 402.9 b 329.0 b 75.1 NS ** *** NS, *, **, *** = not significant, and significant at P = 0.05, 0.01, and 0.001, respectively; means followed by the same letter are not significantly different. least 6 years after trees are planted. For container-grown trees, simply removing the bottom of a tree’s nursery container and planting it directly in the landscape can effectively reduce the number of large surface roots that develop. Our findings agree with Costello et al. (1997) in that trees produce fewer surface roots 120 to 180 cm from the trunk. Thus, simply keeping pave- ment and other hardscape at least 120 cm away from trees might be equally effective as a root barrier in reducing the possibility of surface root problems. However, 6 years after barriers and trees were installed, smaller roots had returned in reduced numbers to the surface soil just beyond the barriers, and we found many roots 1.25 cm or greater in diameter less than 2.5 cm growing in the upper 15 cm of the soil just beyond barriers and within 18 to 60 cm of the tree trunk. No barrier treatment reduced the number of small roots (those greater than 1.25 and less than 2.5 cm in diameter in this study) of either species beyond 60 cm radius from the trunk. It is reason- able that in many sites, roots in this diameter class would continue growing in diameter and could potentially cause damage to over- laying or adjacent pavement and infrastructure in the future. The results underscore conclusions of Costello et al. (1997) that root barriers only delay problems from surface roots and of Gilman (1996) that reduced root numbers do not necessarily mean less root damage. 1.2 4.3 3.0 1.2 NS 3.2 NS NS 1.1 NS
January 2009
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