262 Gilman et al.: Planting Depth and Root Ball Shaving Affects Root Architecture and Performance pruning treatment was applied aſter backfill soil was tamped into place nearly to the top of the root ball. The two root pruning treatments were: 1) no root pruning; and 2) root ball shaved by inserting a sharp, square-tip balling spade to the full extent of the 35 cm long straight blade into the root ball top surface tangent to the trunk 4 to 5 cm inside the periphery (Figure 1). There were a total of 16 treatment com- binations (4 planting depths in containers × 2 land- scape planting depths × 2 root pruning treatments at planting), with three replicate blocks totaling 48 trees. The 48 trees were arranged 3 m apart in rows 3.8 m apart in a randomized complete block design. Water was added to settle backfill soil, and soil was tamped lightly by foot to standardize compaction. No berm or water ring was constructed around the root balls. Trees were mulched in a 1.6 m wide strip up to the trunk along each row immediately aſter planting with a mixture of aged wood chips, bark, and leaves from a local line-clearance contractor, which is common in many North American landscapes. Trees that were planted high were mulched with 5 cm on the root ball and 10 cm outside the ball; in addition to the 10 cm of soil, those planted deeply had 10 cm mulch over the ball and outside the ball. Vegetation between rows was mowed periodically; weeds in mulch were kept in check with three or four annual applications of glyphosate (isopropylamine salt, 41%). Trees were irrigated daily through three Roberts Spot-Spitters (Roberts Irrigation Products, Inc., San Marcos, Idaho, U.S.) positioned at the edge of the root ball directed toward the trunk. Thirty liters of irriga- tion were applied daily through 2008 and then reduced to 15 L through 2010. Fiſteen liters were applied three times weekly through January 2012. Trees were fertilized aſter landscape planting with 400 g of 16-4-8 in November 2008, and March and June 2009, and with 400 g of 20-0-8 in March and May 2010, March and June 2011, and April 2012. Tree Measurements Radius of the root system was measured from trunk to root tips by gentle hand tool excavation of surface soil (top several cm) on the east and west sides (i.e., in the mulch strip) of each tree in May and Novem- ber 2009; this was compared to tree crown radius (trunk to leaf tips) in the same directions to calculate root to crown spread ratio. Trunk diameter (using a diameter tape) and tree height (from mulch surface to topmost bud) were measured annually in September. June 2014 (68 months aſter planting), an air exca- Figure 1. Shaving the root ball at planting (note root pruning spade cutting roots from outer 4 to 5 cm of root ball) after backfill was added to the planting hole. Removed roots and substrate were left in place as shown. Trunk is visible at top of photo, to the right of the pruning spade. vation device was used to expose roots in the top 15 cm in a 50 cm radius 360 degrees around the trunk. Several attributes were measured, including 1) per- cent of trunk circumference with roots (>10 mm diameter) growing tangent to, circling, or embed- ding into trunk; 2) root circling visual rating repre- sented by amount of roots crossing over root collar or main lateral roots—two evaluators calibrated these ratings immediately prior to recording data by viewing every exposed root system and agree- ing on a 1 value for the least (no or only small roots growing over root collar), 2 for some, 3 for abun- dant, 4 for many, and a 5 for the most roots grow- ing over the collar; 3) imprint visual rating on the root system from deflection in 170 L containers— two evaluators calibrated these ratings immediately prior to recording data by viewing every exposed root system and agreeing on a 1 value for the least (no or only small roots deflected by container), 2 for some, 3 for moderate, 4 for strong, and a 5 for most roots deflected by the container (and many of them large); 4) distance from mulch surface to the root collar; 5) diameter of roots >10 mm diameter growing up at more than a 20 degree angle to the horizontal [diameter was measured top-to-bottom and side-to-side, and the mean of these two mea- surements was used as the diameter of a circle to calculate the cross-sectional area (CSA) of each root]. Root circling and imprint ratings were both visually estimated by two observers individually and their ratings averaged prior to statistical analysis. ©2015 International Society of Arboriculture
September 2015
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