32 Gilman et al.: Effect of Container Type and Root Pruning on Root Architecture example, Gilman and Harchick (2008) reported that nearly all trees produced in one air root pruning container—without mechanical root pruning—produced enough circling roots to grade them as culls according to Florida Grades and Standards for Nursery Plants (Anonymous 2015). The first stage (Gilman et al. 2010a) of the current study showed that certain #3 (stan- dard size designation, Anonymous 2014) con- tainers induced circling roots; whereas, others induced a more descending root system. A combination of roots growing horizontally and downward into landscape soil may result in the most stable trees (Gilman and Harchick 2014). Root pruning can also be used to manage root growth in containers. Root pruning of seedlings reduced root defects (Harris et al. 1971) and pro- duced more symmetrically distributed lateral roots years aſter planting (Krasowski 2003). One study showed that light cutting of circling roots of shrubs enhanced the amount of roots grow- ing into substrate outside of the original root ball (Blanusa et al. 2007). However, nearly all studies were performed on small seedlings, not landscape-sized trees. Preliminary work showed that shaving the periphery and bottom of the root ball when #3 containers were shiſted to #15 containers was an efficient method of nearly eliminating root defects on seven temperate and tropical tree species (Gilman et al. 2010b). Less understood is the impact on root architecture from growing in containers with walls constructed of perforated plastic and other porous materials. This project reports on the continuation of a study published (Gilman et al. 2010a) on trees produced in eight types of #3 containers. The objective of this study was to measure impacts of container type, root pruning during shiſting to a larger container, and cardinal direction (north half versus south half of root ball) on root architecture in identical types of #15 and #45 containers. A cultivar of red maple (Acer rubrum L. ‘Florida Flame’) was chosen for the study because red maple is a common shade tree grown throughout much of North America, and trees in the Acer genus develop root defects readily (Tate 1980). ‘Florida Flame’ is propagated by rooting current-year shoots removed from parent trees; this procedure removes genetic variability among individuals, com- pared to a cultivar graſted to seedling root stock. ©2016 International Society of Arboriculture MATERIAL AND METHODS In April 2008, 384 uniform rooted cuttings approxi- mately 15 cm tall of Acer rubrum L. ‘Florida Flame’ rooted in circular containers (5.1 cm top diameter, 13 cm tall ribbed containers, 38 Groove Tube, Grow- ing Systems, Inc., Milwaukee, Wisconsin, U.S.) were planted into eight different #3 (approximately 11 L) container types (Table 1). The point where the top- most root emerged from the stem (root collar) was placed 13 mm below substrate surface by removing an appropriate amount of substrate and associated roots from the top of the liner root ball where needed. Substrate was 20: 60: 20 (New Florida peat: pine bark: sand, by volume; Florida Potting Soil, Inc., Orlando, Florida, U.S.) for RT, RB, FN, CR, and JP (Table 1), and 50: 40: 10 (New Florida peat: pine bark: sand, by volume) for AP, SP, and SS. New Flor- ida peat is a compost of Florida peat and hardwood fines (Florida Potting Soils, Inc., Davenport, Florida, U.S.). Substrates were recommended by the con- tainer manufacturers and are considered an integral part of the growing systems. Substrate volumes in #3 containers were standardized, except for JP, which was 15% smaller (Gilman et al. 2010a). Despite conformance with industry standards (Anony- release; Harrells, Inc., Lakeland, Florida, U.S.) was incorporated into substrate prior to shiſting at (0.011 g/cm3 O5 : 10 K2 ), and no other fertilizer was applied. Trees in #3 containers were placed on woven black ground cloth in Gainesville, Florida, U.S. (USDA Hardiness Zone 8b) spaced pot-to-pot (i.e., touching one another) except for a 0.5 m wide walk space every four rows. Containers were spaced so tree trunks were 0.4 m apart in July 2008 to allow growth of branches along the lower trunk. Trees were arranged in a randomized complete block design with one container of each type in each block. Each was irrigated two to three times totaling 3.8 L daily through one Roberts (Roberts Irrigation Products, Inc., San Marcos, Idaho, U.S.) Spot-Spitter per container until autumn 2008 when irrigation frequency and volume were reduced for the dor- mant season. Trees were staked in May 2008 to hold mous 2014), dimensions and volumes for #15 and #45 containers varied considerably by manufac- turer (Table 1), so substrate volumes could not be standardized. Each was filled with substrate to near the top of the side wall when shiſting into the con- tainer. Fertilizer (18 N: 5 P2 O, controlled
January 2016
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