Arboriculture & Urban Forestry 36(3): May 2010 scribed. Liners of Cathedral Oak® live oak planted deeply into #3 Accelerator® (Nursery Supplies Inc., Chambersburg, PA) con- tainers generated some circling roots over the root flare which reduced their quality (Gilman et al. 2006). However, quality was reduced most when #3 trees were planted deeply into #15 sized containers because trees apparently lost their capacity to gener- ate adventitious roots along the buried portion of the trunk. Fare (2005) presented photographs showing that red maple planted deeply into #3 smooth sided containers had many roots grow- ing over the root flare compared to trees planted shallower. The objective of this study was to measure stem girdling root formation, root length over root flare, depth of the flare, and trunk growth resulting from different planting depths when shifting from one nursery container size to the next for pro- ducing 6 cm caliper (trunk diameter) landscape sized trees. MATERIALS AND METHODS In June 2005, 104 stem cuttings of Magnolia grandiflora MISS CHLOE® sided containers, and Acer rubrum L. ‘Florida Flame’ and Ul- mus parvifolia Allée® 130 mm deep smooth-sided cones were planted into #3 (27 cm across top and bottom, 20 cm deep) Air-Pot™ rooted in square 6.9 cm across × 14 cm deep smooth- rooted in circular 55 mm diameter × cylindrical black plastic containers (Caledonian Tree Company, Ltd., Scotland). The plot was located in USDA hardiness zone 8b in Gaines- ville, FL. Magnolia roots originated primarily near the end of the cutting; whereas, roots on maples and elms emerged near the end and from along the buried stem. The point where the top- most root emerged from stem was placed either 13 mm (shal- low) or 64 mm (deep) below #3 substrate surface by removing an appropriate amount of substrate from top of liner root ball. Trees in #3 containers were spaced pot-to-pot (i.e., touch- ing one another) except for a 1.8 m walk row every four rows. Trees were irrigated 2 or 3 times daily totaling 3.8 L through one Roberts (Roberts Irrigation Products, Inc. San Marcos, ID) Spot-Spitter per container until autumn 2005, when irrigation fre- quency and volume was reduced. In late January 2006, all trees were shifted into #15 (47 cm across top and bottom, 30 cm deep) containers, whereby half the trees of all species × #3 planting depths were positioned shallow (#3 substrate surface even with #15 substrate surface) or deep (#3 substrate surface 64 mm below #15 substrate surface). A total of four planting depth combina- tions resulted from the two planting sessions: 1) 13 mm deep into #3, 0 mm deep into #15; 2) 64 mm deep into #3, 0 mm deep into #15; 3) 13 mm deep into #3, 64 mm deep into #15; and 4) 64 mm deep into #3, 64 mm deep into #15. No roots were pruned or me- chanically manipulated in any way at any time during the study. Trees in #15 containers were spaced 2.4 m apart and irri- gated three times daily in the growing season (weather dictat- ing), with a maximum 15 L through two Roberts Spot-Spitters. In February 2007, all trees were shifted into #45 (75 cm across top, 61 cm across bottom, 48 cm deep) round smooth-sided black plastic containers (Nursery Supplies, Inc., Chambersburg, PA) with #15 substrate surface even with #45 substrate surface on all trees. Trees remained 2.4 m apart and were irrigated two to three times daily in the growing season with a maximum 45 L through three Roberts Spot-Spitters until October, when volume was dropped to 15 L daily or less frequently as weather dictated. Elm and maple shoots were pruned and trunks staked in 2006 133 and 2007 to develop one leader, and to shorten and remove large lower branches creating a 1.5 m trunk clear of branches. Mag- nolias were pruned only to maintain a central dominant lead- er. Trees were secured to a trellis system for stability in wind. Substrate was 50:40:10 (New Florida peat: pine bark: sand, volume). New Florida peat is a compost of Florida peat and hardwood bark fines (Florida Potting Soil, Inc., Orlan- do, FL). Fertilizer (18-5-10 controlled release, Harrells Inc., Lakeland FL) was incorporated into substrate prior to plant- ing at 10.74 kg/m3 , and no other fertilizer was applied. Weeds were periodically pulled from container substrate. Except- ing some elms, trees did not root out of pots and into ground. In September 2007, root flare (point where main roots emerged from trunk) was evaluated as visible at the substrate surface or not visible on all trees. Trees were graded as cull or not according to Florida Grades and Standards for Nursery Stock (Anonymous 1998), prior to any substrate removal. A cull rat- ing was given to trees with one or more roots larger than one- tenth trunk diameter (measured 15 cm from ground) that together circled more than one-third around the circumference of trunk. Five blocks of each cultivar (20 trees total for each cultivar) were randomly selected for removing the top 15 cm of substrate from root ball using a strong stream of air from an AirKnife® . Trees were graded as cull or not following removal of substrate. The outside edge of the #15 container was used as a form to paint a circle centered on the trunk on #45 substrate surface. Roots greater than 5 mm diameter in the painted circle that grew over the root flare were removed and their length measured. Trunk circumference was measured at substrate surface just above any swelling associated with the flare as was length of any root seg- ment greater than 5 mm diameter that rested against the trunk over the flare [these were designated stem girdling roots (SGRs)]. SGR length was totaled and the percent of trunk circumference with SGRs was calculated; if more than one root was touching at the same point only one was measured. This provided the per- centage of trunk circumference with a SGR in contact with the trunk. The study authors measured the distance between substrate surface and the part of the root flare 8 cm from the trunk closest to substrate surface. The largest diameter three main roots emerg- ing directly from trunk were rated as deflected by the liner and/or #3 container wall or not deflected. Caliper (trunk diameter 15 cm from ground) and tree height were measured annually on all trees. Cultivars were arranged in their own randomized complete block design with four trees, one from each planting depth combi- nation, in each of 26 blocks. Cultivar blocks were adjacent to one another. Analysis of variance in the GLM procedure within SAS (SAS Institute, Cary, NC) was used to evaluate impact of main ef- fects and interactions on measured parameters. The two main ef- fects were planting depth into #3 containers and planting depth into #15. Each main effect had two levels, shallow and deep planting. Percent trees with visible root flare at substrate surface, or those evaluated as culls prior to and after substrate removal were ana- lyzed with two-way ANOVA using GENMOD procedure in SAS. RESULTS Crown Growth Caliper of finished trees in #45 containers was significantly larg- er for both magnolia and maple planted shallow (13 mm below ©2010 International Society of Arboriculture
May 2010
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