Arboriculture & Urban Forestry 42(4): July 2016 wanted to determine if root pruning could prove just as effective when performed in dormancy aſter the hurricane season ended in November. There is limited information on the impact of season of root pruning on root ball quality and transplanting. Mechanical root pruning strategies have been tested to improve root system quality in containers by reducing occurrence of circling roots. Early work showed that manual root pruning of tree seedlings raised in propagation containers reduced root defects (Harris et al. 1971a; Harris et al. 1971b) and produced more symmetrically distributed lat- eral roots aſter reforestation planting (Krasowski 2003). Krasowski and Owens (2000) found that despite a smaller root ball at planting, root systems of mechanically pruned Picea glauca (Moench) Voss seedlings produced more root growth in field soil than control treatments. Gilman et al. (1996) showed that shallow (2 cm deep) vertical slicing (oſten called scoring) from top to bottom on larger (11 L) root balls of Burford holly (Ilex cornuta ‘Burfordii’), when planting to field soil, resulted in a redistribution of roots, not an increase in roots compared with non-pruned controls. Scoring 11 L container root balls on Q. virginiana as they were shiſted into 57 L containers resulted in an improve- ment in root system quality (Gilman et al. 2009). Despite these experiences, there were no reports of employing root ball scoring on container lin- ers used for establishing a field nursery and how that might influence landscape transplanting or anchorage. Researchers also wanted to test obser- vations made by nursery operators about root ball attributes and transplant ability of field-grown trees established as larger liner containers (57 L) instead of the more traditional 3 and 11 L containers. The specific objectives of the current study were to determine the effects of tree liner container size, root ball slicing when planting into field soil, and season of field root pruning on root system qual- ity of field-grown nursery stock, as well as survival, growth, and anchorage aſter landscape planting. MATERIALS AND METHODS Tree Production and Harvesting In February 2007, 120 genetically identical Cathe- dral Oak® live oaks (Quercus virginiana) from stem cuttings, averaging 13 mm trunk diameter mea- 235 sured 15 cm from ground (caliper) were obtained from a local central Florida nursery in 11 L black plastic containers (Accelerator™ Nursery Supplies Inc., Chambersburg, Pennsylvania, U.S.). Quercus virginiana was selected for study due to its popular- ity in warm temperate and semi-tropical climates. Trees had a dominant leader with short temporary branches along the trunk nearly to the ground. Treatments imposed on these 120 trees were: 1) 40 trees planted directly into field nursery soil (Millhopper fine sand—loamy, siliceous, hyper- thermic Grossarenic Paleudults), 2) 40 trees shiſted into 38 L plastic containers (Accelerators with containers touching one another—pot to pot), or 3) 40 trees shiſted into 57 L Accelerator contain- ers pot to pot. Container production facility was located on woven ground cloth several hundred meters from the field nursery in Gainesville, Flor- ida, U.S. (USDA hardiness zone 8b). Container substrate was 20: 60: 20 (New Florida peat: pine bark: sand, by volume; Florida Potting Soil, Inc., Orlando, Florida, U.S.). Fertilizer (18 N-5 P2 K2 Florida, U.S.) was incorporated into substrate prior to shiſting at (0.011 g/cm3 O, controlled release; Harrells, Inc., Lakeland, ), and no other fertilizer O5 was applied in containers. Each was irrigated two to three times, totaling 3.8 L daily, through one Spot-Spitter (Roberts Irrigation Products, Inc., San Marcos, California, U.S.) per container until autumn 2007 when irrigation frequency and vol- ume was reduced for the dormant season. Trees in containers were secured to a 10 mm diameter metal stake secured to a trellis system to develop a straight trunk and to hold them erect. Trees planted to the field nursery were secured to the same type of stake driven into the ground with- out a trellis system. Branches were pruned twice during the growing season to develop a dominant trunk and leader by reducing competing branch length with heading and/or reduction cuts on trees in containers and field nursery through 2009. The top of the 11 L root ball on all 120 trees in the study was washed with a stream of water and hand manipulated for a total of 10 seconds to expose circling and potentially girdling roots growing tangent to the trunk in the top 5 cm. These roots were then pulled away from the trunk and cut so the retained root segment was radially oriented straight from the trunk. Half of ©2016 International Society of Arboriculture -10
July 2016
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