Arboriculture & Urban Forestry 41(3): May 2015 container wall. For this reason, Lindström et al. (2005) tested a stabilized or reinforced substrate that could be easily removed from the container, intact, before all permanent roots were formed from the tap root and deflected by container walls. Stabilized substrates were developed to bind the root ball together to facilitate easy liſting and transfer to larger containers without loss of sub- strate. However, few trees are grown in this sub- strate (personal observation). In contrast, Coutts et al. (1990) found that some taxa are stabilized aſter planting by adventitious roots that develop aſter out-planting from propagation containers. There is good evidence in the forestry (Salo- nius et al. 2000; Ortega et al. 2006) and horticul- ture (Harris et al. 1971; Marshall and Gilman 1998; Amoroso et al. 2010; Gilman et al. 2010a) literature that root architecture is more impor- tant than root mass to the health and anchorage of trees as they become established. Straight lateral roots radiating from the trunk are associated with well-anchored trees (Lindgren and Örlander 1978; Lindström and Rune 1999; Ortega et al. 2006; Gil- man and Weise 2012). The objective of the present study was to determine impact of retention time in 11 and 57 L nursery containers on subsequent root architecture in the root ball on finished trees in 170 L containers. The taxa chosen for study were selected due to their popularity in many parts of temperate North America and elsewhere. MATERIALS AND METHODS In February 2007, 80 uniform rooted cutting liners of Magnolia grandiflora L. Miss Chloe® rooted in square 7.3 cm across × 14 cm deep smooth- sided containers (Anderson Band AB39, Stuewe and Son, Inc., Tangent, Oregon, U.S.), and Acer rubrum L. ‘Florida Flame’ and Ulmus parvifolia Jacq. Allée® rooted in circular (5.1 cm top diam- eter, 13 cm tall ribbed containers, 38 Groovetube, Growing Systems, Inc., Milwaukee, Wisconsin, U.S.) propagation containers were potted into 11 L round, black, solid-walled nursery contain- ers (Table 1). Magnolia roots originated primar- ily 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 13 mm below substrate surface by removing an appropriate 147 amount of substrate from the top of liner root ball. This depth was found to be the ideal liner plant- ing depth, for the one taxa tested (Gilman and Har- chick 2008). The plot was located on woven black ground cloth in USDA hardiness zone 8b (mean low temperature -10°C) in Gainesville, Florida, U.S. Table 1. Retention time in 11, 57, and 170 L containers. Container sizez (L) Retention time treatment A B C D z 11 Retention time in container (months) 4 7 9 57 8 12 10 11 14 170 20 15 12 6 Container dimensions as follows: 11 L = 27 cm top diameter, 25 cm tall; 57 L = 44 cm top diameter, 38 cm tall; 170 L = 75 cm top diameter, 48 cm tall, round, solid-walled, black plastic containers (Nursery Supplies, Inc., Chambersburg, Pennsylvania, U.S.). Trees in 11 L containers were grouped by taxa and spaced pot-to-pot (i.e., touching one another) except for a 0.5 m walk row every four rows. Trees were irrigated two or three times daily, totaling 3.8 L through one Roberts (Rob- erts Irrigation Products, Inc. San Marcos, Idaho, U.S.) Spot-Spitter per container until autumn 2007 when irrigation frequency and volume was reduced for the dormant season. Trees in 11 L containers were randomly chosen for shifting into 57 L round, black, solid-walled nursery con- tainers (Table 1); they were spaced 1.8 m apart and irrigated three times daily (weather dictat- ing) in the growing season with a maximum of 15 L through two Roberts Spot-Spitters. Trees were shifted into 170 L round, black, solid-walled nursery containers (Table 1) in place, remain- ing 1.8 m apart. Irrigation occurred two to three times daily in the growing season, with a maxi- mum of 45 L through three Roberts Spot-Spitters, until October 2009 when volume was dropped to 15 L daily or less frequently as weather dictated. Elm and maple shoots were pruned, and trunks staked, to develop one leader and to shorten and remove large lower branches, creating a 1.5 m trunk clear of branches to mimic standard prac- tice. Magnolias were pruned only to maintain a central dominant leader. All trees were secured to a wire trellis system for stability in wind. Trees were retained in 11 and 57 L contain- ers before shifting to the larger size for four combinations of time (Table 1). Root balls were ©2015 International Society of Arboriculture
May 2015
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