156 Gilman et al.: Container Wall Porosity and Root Pruning Influence on Swietenia mahogani studying the more complicated issues of growing much larger trees in field and container nurser- ies. Evaluating and growing high-quality trees for landscape markets may be more complex because instead of germinating in a seed bed or propagation container from which they are field planted, trees are typically moved from one field to another (Hewitt and Watson 2009) or they are shiſted into larger con- tainers up to four or five times during the course of production. Root architecture imposed by nursery practices in a landscape nursery can impact anchor- age in the landscape (Gilman and Harchick 2014). Trees with some large diameter, straight roots close to the soil surface oſten become well anchored in shallow (Coutts et al. 1990) and deep soils (Gilman and Wiese 2012), compelling development of field and container nursery trees with some large straight roots close to the surface of the root ball. Roots on established trees oſten proliferate close to the sur- face in soil with low oxygen content typical in dis- turbed urban zones (Watson and Kupkowski 1991). These can elongate from existing short roots within the root ball, from cut roots at the top portion of the root ball, or adventitiously from the root collar; how- ever, certain nursery production systems induce a vertical root architecture (Hewitt and Watson 2009; Gilman and Paz 2014) that seems counterintuitive. Some attempts have been made to standardize nursery tree quality. Florida Grades and Standards for Nursery Plants (Anonymous 2015) includes a formula for evaluating roots that circle the inte- rior and/or periphery of the root ball. Specification Guidelines for Container-grown Trees in California (Anonymous 2009) states that there should be no circling roots on the periphery. The American Stan- dard for Nursery Stock (Anonymous 2014) excludes soil over the root ball as part of the root ball depth measurement. Despite these advances, more detail than those provided by these sources may be nec- essary to describe the ideal nursery root system. Anchorage aſter planting a common shade tree in tropical regions (Swietenia mahogani L., Gilman and Harchick 2014) from 9.5 L containers was attribut- able to root architecture within the original planted root ball (Gilman and Paz 2014). Objectives of the current study were to evaluate anchorage and root architecture on Swietenia mahogani planted from 57 L containers. Specifically tested was the influence of root architecture within the root ball imposed by ©2015 International Society of Arboriculture growing trees in solid- and porous-walled contain- ers on growth, root attributes, and anchorage, one growing season aſter landscape planting. Also tested was impact of removing—by pruning away—all roots growing on the root ball periphery at planting. MATERIALS AND METHODS Growing Trees in the Nursery On February 11, 2009, in Loxahatchee, Florida U.S. (USDA hardiness zone 10a; mean low temperature -1°C), Swietenia mahogani seeds were placed into propagation (liner) containers in substrate consist- ing of 45% super-fine pine bark, 20% Florida peat, 10% horticultural perlite, 15% Allgro compost, and 10% coarse sand. The two propagation con- tainer types tested were 1) Bottomless Ellepot (EP) with sides constructed of paper 50 mm diameter × 90 mm tall, with a volume of 137 cm3 (Ellegaard, Esbjerg, Denmark; Ellepot paper made by Ahlstrom Stalldalen AB, Stalldalen, Sweden, from spruce, pine, and polyester long fibers, 27g/m2 , 190 microns thick, 1320 N/m dry tensile strength in machine direction, 2.0 N tear strength) arranged 10 mm apart in a plastic tray (27 cm × 53 cm) exposing 100% of the paper sides to air and rested on an 8 mm wide plastic ring as part of the holder tray; and 2) a tray of smooth-sided (SM) black plastic containers 40 mm top diameter × 90 mm tall (volume 105 cm3 ) slightly tapered cone with a single drainage hole at the bottom. Trays (each with 40 to 55 contain- ers) were arranged in a randomized fashion on wire mesh benches 80 cm from the ground in full sun in a non-climate controlled, open-sided greenhouse. On July 27, 2009 (five months retention time in propagation container), trees were washed of sub- strate for root evaluation (described in Gilman and Paz 2014) or shiſted into 3.8 L containers. One hun- dred liners of each propagation container type were shiſted into either 3.8 L black plastic solid-walled smooth-sided slightly-tapered containers (SC1; 15.5 cm top diameter × 15.5 cm tall; Nursery Supplies, Inc., Chambersburg, Pennsylvania, U.S.) or into containers with exceptionally porous walls and bottom (Pioneer Pot; PC1; 19 cm top diameter × 17 cm tall, all con- tainer surfaces composed of approximately 15% plas- tic, 85% air including a bottom elevated 8 cm from ground, Pioneer Farms, Visalia, California, U.S.), and placed several cm apart on woven cloth on the ground
May 2015
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