184 Gilman et al.: Tree Size, Root Pruning, and Production Method on Q. virginiana prepare them for sub-optimal soil moisture conditions common on many landscape sites. Later work appeared to confirm this when Gilman (2001) found that live oak root pruned regularly during production and held in the ground in a nursery for several months after digging (i.e., hardened-off), had a much higher survival rate in drought following planting than trees installed from contain- ers. However, there are few studies comparing production meth- ods that extend beyond approximately one year after planting. Reported effects of mechanical root pruning in small contain- ers (11.3 L size or smaller) on root growth and morphology vary. Blanusa et al. (2007) found that light cutting of circling roots of shrubs increased the amount of roots growing into container sub- strate outside the original root ball in agreement with Krasowski and Owens (2000). In contrast, Gilman et al. (1996) showed slic- ing (11.3 L, 25 cm tall x 25 cm top diameter) root balls top-to- bottom on Burford holly (Ilex cornuta ‘Burfordii’) at planting resulted in redistribution of roots, not an increase in roots com- pared to nonpruned controls. Harris et al. (2001) reported root pruning treatments (5, 10, or 15 cm below soil) on pin oak (Quer- cus palustris Münchh.) liners in containers increased number of lateral roots but not total root length following planting. Other than Weicherding et al. (2007) on 38 L (36 cm tall x 40 cm top diameter) containers and smaller, there are few published me- chanical root pruning studies on large landscape-sized containers measuring water stress and growth following planting. There may be an effect of root pruning on tree development, and perhaps this influences the magnitude of water stress following planting. Communities would use resources most efficiently by un- derstanding how trees from various planting treatments, pro- duction methods, and tree sizes become established. Objec- tives were to evaluate impact of 1) radial slicing of the outer edge of container root balls, 2) initial tree size at planting, and 3) nursery production method on post landscape planting wa- ter stress and growth. Live oak was tested because it is a com- monly planted shade tree in the southern one-quarter of the U.S. MATERIALS AND METHODS Tree Production Techniques In August 2001, 80 cutting-propagated liners in 3.8 L (20 cm tall x 18 cm top diameter) smooth-sided black plastic containers of Quercus virginiana Mill. ‘SNDL’, PP#12015, Cathedral Oak® were planted into a field with Millhopper fine sand (loamy, sili- caceous, hyperthermic Grossarenic Paleudults) with less than 2% organic matter. These are referred to as field-grown trees. Eighty of the same 3.8 L liners were planted into 57 L smooth-sided black plastic containers (41 cm tall x 43 cm top diameter, Nursery Supplies Inc., Fairless Hills, PA), 50 m from the field plot. Con- tainers were maintained on woven, black nursery ground cloth to prevent rooting into the ground. In December 2002, the 57 L containers were shifted into 170 L (47 cm tall x 75 cm top diameter) smooth-sided black plastic containers. Plants pro- duced in this manner are referred to as 170 L container trees. All 160 trees were spaced on 2.4 m centers in eight rows 3.6 m apart. An additional thirty 11.3 L smooth-sided black plas- tic container-grown (24 cm tall x 28 cm top diameter) cutting propagated Cathedral Oak trees were planted May 2004 into 57 L smooth-sided black plastic containers on 1.2 m centers on ©2010 International Society of Arboriculture woven, black nursery ground cloth. These trees are referred to as 57 L container trees. All trees were located at the University of Florida Great Southern Tree Conference demonstration site in Alachua County, Florida, U.S. (USDA hardiness zone 8b). Substrate in all containers was composed of 60 pine bark: 30 peat: 10 sand by volume (Florida Potting Soil, Inc., Orlan- do, FL). Irrigation and fertilizer commonly used for these pro- duction methods in the region were applied during production. Trees were irrigated two or three times daily during the grow- ing season and once daily at other times, except in wet and cool weather. Tree crowns were pruned twice annually to meet Florida Grades and Standards for Nursery Plants (Anonymous 1998) with one central leader and crown beginning 1.4 m from ground. Landscape Planting Treatments Thirty field-grown trees closest to mean trunk caliper (trunk di- ameter measured with a diameter tape 15 cm from ground) for the 80 field trees (80 mm) were root pruned November 2004 with a tree spade. All blades on the 91 cm diameter three-bladed tree spade (Caretree Nursery Equipment, Hilliard, OH) were inserted into the ground so they cut roots in three one-sixth circumfer- ence sections on an 81 cm diameter, centered on the trunk. This allowed us to use the same spade to dig the trees later with a larger (91 cm) root ball as described later in this paper. Root pruning prior to digging is standard practice for oaks in the Flor- ida field nursery industry. Root balls were irrigated two to three times daily for three weeks following root pruning depending on rainfall and temperature, then daily. Ten weeks later, in January 2005, trees were dug with the same spade using the full 91 cm diameter. Trees were placed into wire baskets lined with copper treated natural burlap to fit the root ball, and wire was secured tightly around root ball by twisting wire and tying top of basket tightly against top of soil ball as standard practice. All trees were lowered back into the same hole and irrigated four to six times daily for four weeks; then irrigation was reduced to three to four times weekly depending on weather. This root pruning, digging, and holding in the ground prior to transplanting to the landscape is considered hardening-off and is standard practice in many field nurseries in the region. Eight weeks after digging (March 24 to April 3, 2005), trees were lifted by a tractor with a sling and straps through the second rung of the wire basket, and moved to an adjacent field of the same type soil previously described. Sixty trees closest to mean trunk caliper (66 mm) for the 170 L container-grown trees were planted into the same field soil as field-grown trees March 24 to April 3, 2005. April 4 was con- sidered the first day after planting (DAP) since all planting was completed April 3. Half the 170 L container trees (30 trees) were root pruned at planting by cutting 3 to 5 cm deep radially into the root ball periphery with a hand pruner in six equidistant places from the top of the root ball to the bottom (referred to as 170 L container/ sliced). The remaining thirty 170 L container trees were planted without root ball cutting (referred to as 170 L container/ not sliced). All 30 trees (29 mm caliper) in 57 L containers were planted into the same field during the same time period. The top of the root ball surface on all 120 trees was po- sitioned even with landscape soil surface; no mulch was applied. Trees were arranged in the field in a randomized complete block design with one tree of each of the four landscape planting treat- ments (57 L containers, 170 L containers/ sliced, 170 L containers/
July 2010
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