76 Gilman et al.: Nursery Planting Depth, Mulch Application, and Root Pruning and Day (2010) showed little impact on post- planting growth. Two studies found a significant but small increase in anchorage when root ball sides were shaved of peripheral roots at planting (Gilman and Wiese 2012; Gilman 2013). Experi- ence with one species (Acer platanoides L.) shows that remediation a decade or more aſter planting is much more difficult and may be ineffective (Tate 1980; Watson and Clark 1993). Earlier remediation may require fewer inputs and be more effective. Although mulch applied to the soil beyond the root ball appears to moderate soil temperature and moisture measured outside the root ball (Scharen- broch 2009), there are a limited number of instances where mulch has been shown to enhance establish- ment of landscape-sized trees. No studies showed increased survival rates resulting from applications of mulch around the root ball; however, at least four showed a slight increase in growth for some of the species tested (Litzow and Pellett 1983; Smith et al. 2000; Montague et al. 2007; Arnold and McDonald 2009). In most studies, trees planted in mulched landscape soil in different climates responded iden- tically to those surrounded by bare soil (Kraus 1998; Downer and Hodel 2001; Gilman and Grabosky 2004; Bryan et al. 2008; Ferrini et al. 2008; Singer and Martin 2009). Mulch placed on the root ball surface or on the surrounding soil has been associated with reduced survival (Arnold et al. 2005; Singer and Martin 2009), increased stress (Gilman and Grabosky 2004), or reduced post-planting tree growth (Hens- ley et al. 1988; Hild and Morgan 1993); these symptoms typically appeared soon aſter planting. With questionable benefits of mulch applica- tion over the root-ball surface, and limited data on the influence of planting depth in nursery containers and root remediation at planting, this study was designed to test the interactions of these three factors for two commonly planted taxa in temperate urban landscapes. Influence on physi- ological stress symptoms, crown and root growth, and anchorage was the focus of this investigation. MATERIALS AND METHODS Trees and Treatments Rooted cuttings with stem tips originally stuck ap- proximately 4 cm deep into propagation containers (5.1 cm diameter and 13 cm tall ribbed containers, ©2015 International Society of Arboriculture 38 Groovetube, Growing Systems, Inc., Milwau- kee, Wisconsin, U.S.) were planted into 11.4 L (#3) Air-Pots (29 cm tall × 26 cm top diameter, Superoots® Air-Pot™ black plastic (Caledonian Tree Company, Ltd., Scotland) either with the point where the topmost root emerged from the stem (i.e., the root collar) 13 mm below substrate surface or 64 mm below the surface. Excess sub- strate was removed above the topmost root just prior to planting all trees into 11.4 L containers. Ten months later, trees planted 13 mm deep were shiſted to 57 L (#15) Air-Pots (33 cm tall × 46 cm top diameter) even with the surface; trees planted 64 mm deep were planted another 64 mm deep. The result was two planting depths, shallow (13 mm) and deep (128 mm) into 57 L containers. Af- ter 14 months, all trees were shiſted into smooth- sided 170 L (#45) containers (47 cm tall and 75 cm top diameter, Nursery Supplies Inc., Fairless Hills, Pennsylvania, U.S.) even with the substrate surface. Eighteen months later (February 2008), 40 elms (Ulmus parvifolia Jacq.) and 40 maples (Acer rubrum L. ‘Florida Flame’) selected for uniformity in 170 L smooth-sided containers were planted in the landscape a few hundred meters away. No root pruning was performed in the nursery. Substrate in all containers was composed of 60 pine bark: 30 New Florida peat: 10 sand by vol- ume (Florida Potting Soil, Inc., Orlando, Florida, U.S.). New Florida peat is a compost of Florida peat and hardwood fine particles (Florida Potting Soil, Inc.). Irrigation and fertilizer commonly used in the region to foster rapid growth were applied while trees were in containers. Trees were irri- gated two or three times daily during the growing season and once daily at other times, except in wet and cool weather. Tree crowns were pruned twice annually to meet Florida Grades and Stan- dards for Nursery Plants (Anonymous 1998) with one dominant central leader and the first per- manent branches beginning 1.4 m from ground. Trees were arranged in the nursery in plots described in the literature (Gilman et al. 2010b). Soil at the landscape planting site was a Mill- hopper fine sand (loamy, siliceous, hyperthermic Grossarenic Paleudults) with less than 2% organic matter at University of Florida Great Southern Tree Conference demonstration site in Alachua County, Florida, U.S. (USDA hardiness zone 8b).
March 2015
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