Above-grade improves post-transplant root and shoot growth and physiology of native shrubs

Abstract

Morella cerifera (L.) Small (syn. Myrica cerifera L.) (wax myrtle)[11.3 L (3 gal)], Illicium floridanum Ellis (Florida anise tree)[11.3 L (3 gal)], and Kalmia latifolia L. (mountain laurel) [19 L (5 gal)] plants were planted in Horhizotrons in a greenhouse in Auburn, Ala. on 1 Mar. 2006, 6 June 2006, and 3 Jan. 2007, respectively. The experiment was repeated with the same species being planted 18 June 2007. Horhizotrons contained four glass quadrants extending away from the root ball, providing a nondestructive method for measuring growth over time of roots of the same plant into different rhizosphere conditions. Each quadrant was filled with 100% soil (Marvyn sandy loam) in the lower 10 cm (3.9 in). The upper 10 cm (3.9 cm) of the quadrants were filled randomly with either: 1) pine bark (PB), 2) peat (P), 3) cotton gin compost (CGC), or 4) more soil with no organic matter (NOM). Treatments 1-3 were intended to simulate an above-grade planting practice with the lower half of the root ball in soil and upper half in organic matter, and Treatment 4 was intended to simulate traditional at grade planting with no organic matter. Horizontal root lengths (length measured parallel to the ground, HRL) of the five longest roots visible along each side of a quadrant were measured weekly for M. cerifera and I. floridanum, and biweekly for K. latifolia, and when roots of one species reached the end of a quadrant the experiment was terminated for that species. M. cerifera had the fastest rate of root growth, followed by I. floridanum, and K. latifolia had the slowest rate of root growth. In most cases roots grew initially into the organic matter rather than the soil in treatments 1-3. In general, HRL and root dry weight (RDW) of I. floridanum and K. latifolia were highest in PB and P, while for M. cerifera they were highest in P. Differences in root growth among treatments were not as pronounced for M. cerifera as for the other species, perhaps due to its faster rate of root growth. Increased root growth in PB and P may be attributed to the ideal physical and chemical properties of these substrates. M. cerifera [11.3 L (3 gal)] and K. latifolia [19 L (5 gal)] were planted on 30 Oct. 2006 (Fall planting) and 12 Apr. 2007 (Spring planting) in a shade house in Auburn, Ala. At planting in fall and spring, four plants of each species in a row were randomly assigned one of four treatments. Three of the four treatments utilized a modified above-grade planting technique in which plants were planted such that the top 7.6 cm (3 in) of the root ball remained above the surface of the ground and pine bark (PB), peat (P), or cotton gin compost (CGC) was applied on and around the above grade portion of the root ball, tapering down from the top of the root ball to the ground at a distance of 30.5 cm (12 in) from the stem. In the fourth treatment, plants were planted at grade with no organic matter (NOM) using only the native field soil (Marvyn sandy loam). Net photosynthesis (net Ps) and stem water potential (?stem) were measured 15-23 Aug. 2007 for shrubs of each species planted in the fall and spring for all treatments before and after irrigation. Plants were harvested 18 Sept. 2007. Generally, for both species at harvest, shrubs planted in the fall had higher shoot dry weight (SDW) and root ball diameter (RBD) than when planted in spring. Plants also typically had higher RBD when planted in PB or P. M. cerifera had higher net Ps than K. latifolia, and both species had higher net Ps after irrigation than before. Differences in net Ps before and after irrigation were more pronounced for shrubs planted in the spring than in the fall. Highest net Ps and ?stem were generally observed for shrubs in PB and P. For easy-to-transplant species (such as M. cerifera) and especially for difficult-to-transplant species like K. latifolia, fall planting utilizing this modified above-grade planting t