Arboriculture & Urban Forestry 40(1): January 2014 for reducing root growth on the periphery of container r oot b alls (B urdett 1978; S truve 1993; Marshall and Gilman 1998). Or lander (1982) and Ortega et al. (2006) found that exposing the open container bottom to air (air pruning) resulted in fewer deflected roots in the propagation container. ἀ e number a nd t otal len gth of Acer r ubrum L . roots from stem cuttings deflected up, around, and down b y co ntainer wa lls w ere a pproximately a n order of magnitude greater in f our types of plas- tic containers compared to those made from thin paper (Gilman et al. 2012). ἀi s was presumably due to a combination of root tip dieback on roots growing t hrough the paper and into the air out- side the container (i .e., air pruning), and growth of s ome o f t hese r oots in to ad jacent co ntainers. ἀ e objective of t his s tudy was t o find a n urs- ery p roduction sys tem t hat p roduced a r oot b all with a ttributes simi lar t o t hose o f w ell-anchored landscape trees; i.e., with straight roots, some close to t he s urface. M ahogany [S wietenia ma hagoni (L.) J acq.] wa s c hosen d ue t o: 1) i ts p opular- ity a s a n urb an l andscape t ree in t ropical a nd subtropical r egions o f Flo rida, U .S., a nd in t he Caribbean, and 2) a general lack of nursery produc- tion research on t ropical shade t ree root sys tems. MATERIALS AND METHODS On F ebruary 11, 2009, in L oxahatchee, Flo rida (USDA h ardiness zo ne 10a), m ahogany s eeds were p laced in to p ropagation (lin er) co ntain- ers in substrate consisting of 45% super fine pine bark, 20% Florida peat, 10% horticultural p er- lite, 15% A llgro compost, and 10% co arse s and. ἀr ee p ropagation co ntainer t ypes t ested w ere: 1) B ottomless Ellepot (EP) co nstructed of paper 50 mm diameter × 90 mm t all, with a volume of 137 cm3 per made by Ahlstrom Stalldalen AB, S talldalen Sweden from spruce, pine, and polyester long fi- bers, 27g/m 2 17 (SM) black plastic containers 40 mm top diameter × 90 mm tall (volume 105 cm3 pered cone and a single drainage hole at the bot- tom. Trays (e ach with 40 t o 55 co ntainers) were arranged in a ra ndomized fashion on wire mesh benches 80 cm f rom the ground in f ull sun in a non-climate co ntrolled, o pen-sided g reenhouse. Retained in Propagation Container (5 months) On July 27, 2009 (5 months retention time in propa- gation co ntainer), t rees w ere ei ther 1) wa shed o f substrate for root e valuation, 2) s hiſted into 3.8 L containers, or 3) r etained in t he propagation con- tainers. On 10 ra ndomly c hosen, wa shed t rees, roots >1 mm di ameter were evaluated for number of roots in the top half of root ball that branched, estimated % of total root ball root length that was in the top half o f the root ball, t ap root deflected by liner bottom or not, tap root length aſter deflec- tion, number of primary lateral roots deflected by the co ntainer b ottom, number o f p rimary l ateral roots deflected downward by t he co ntainer sides, and a v isual es timate of where active root growth was occurring: either mostly in the top half of root ball, mostly in the bottom half of root ball, or evenly distributed in the root ball. Tree height and trunk diameter a t s ubstrate le vel w ere a lso r ecorded. One-hundred lin ers o f e ach p ropagation co n- tainer type were shiſted into either 3.8 L, 1) b lack plastic sm ooth-sided s lightly-tapered co ntainers (SC1; 15.5 cm top diameter × 15.5 cm tall; Nursery Supplies, Inc., Chambersburg, P ennsylvania, U.S.) or 2) into containers with exceptionally porous walls and bottom (Pioneer pot® ; PC1; 19 cm top diameter (Ellegaard, Esbjerg, Denmark, Ellepot pa- , 190 micr ons t hick, 1320 N/m dr y tensile strength in m achine direction, 2.0 N t ear strength), arranged 10 mm apart in a p lastic tray (27 cm × 53 cm), w hich exposed 100% of the pa- per sides to air and rested on a plastic ring (8 mm wide) as part of the holder tray; 2) EP w ith same dimensions placed in a tray of smooth (EPS) black plastic cells (60 mm tall × 50 mm wide), spaced about 5 mm apart; and 3) a t ray of smooth-sided × 17 cm t all, a ll co ntainer s urfaces composed o f about 15% plastic and 85%, air including a bottom elevated 8 cm f rom ground, Pioneer Farms, Visa- lia, California, U.S.) and placed several cm apart on woven ground cloth, on the ground, pot-to-pot in a randomized fashion. Side of PC1s w ere lined with paper (as described in EP) to ensure substrate would not leach through the large (10 mm square) openings in the side. ἀ e resulting experimental design was a complete factorial with three propagation container types × two 3.8 L container types, totaling 600 trees. Substrate volume was equivalent in both 3.8 L con- tainers; it reached the top in the PC1 containers and was 1 cm below the top in the SM1 containers. ἀ e ©2014 International Society of Arboriculture ) with a slightly ta-
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