Organic Matter Type Affects Growth and Physiology of Native Plants Planted Above-Grade

Abstract

Experiments quantified the response of three native taxa to different substrates utilized in above-grade planting. In a greenhouse study, Hydrangea quercifolia Bartr. ‘Alice’ (oakleaf hydrangea), Chionanthus virginicus L. (white fringetree), and Rhododendron austrinum Rehd. (Florida flame azalea) were planted in Horhizotrons™ on 28 Feb. 2008 (run 1), 22 Aug. 2008 (run 2), and 25 Feb. 2009 (run 3), and root growth into each of four quadrants was monitored. Bottom halves (10 cm depth) of each quadrant were filled with soil, while top halves (10 cm depth) of quadrants were filled with one of four substrates [coconut coir (CC), chipped pine trees (PT), peat moss (PM), or pine bark (PB)] to simulate above-grade planting. Horizontal root length (HRL) was measured throughout the experiment and root dry weight (RDW) was recorded at experiment termination. HRL of all taxa increased linearly over time in all substrates and runs. Based on RDW, more roots grew into soil than in substrate. HRL and RDW for H. quercifolia ‘Alice’ were generally highest in CC and PT substrates, while HRL for C. virginicus and R. austrinum was generally highest in PM. C. virginicus RDW only differed among treatments in run 1, in which RDW was highest in CC and PB. A field study was conducted to evaluate effects of the same substrates on growth of the same taxon planted above-grade compared to planting at-grade with no organic matter (NOM). For C. virginicus and R. austrinum, net photosynthesis (Ps) was measured once in summer 2008 and twice in summer 2009 (before and after irrigation), and stem water potential (SWP) was measured twice (before and after irrigation) in summer 2009. Growth index (GI) was recorded at planting (17 Mar. 2008), 23 Oct. 2008, and 6 Jul. 2009. Visual ratings (VR) and root ball diameter (RBD) were determined at experiment termination (Jul. 2009). GI increased linearly in response to treatments in C. virginicus and R. austrinum, and quadratically or linearly in H. quercifolia ‘Alice’ depending on treatment. VR and RBD of C. virginicus and H. quercifolia ‘Alice’ were not affected by treatments. GI was not different among treatments in C. virginicus, while GI was highest in PT and lowest in NOM for H. quercifolia ‘Alice’. RBD for R. austrinum was highest in PM; VR was highest in PM and CC; and final GI was not different among treatments. In 2008, Ps was not different among treatments in all taxa; however, in 2009, Ps was lower in PB than other substrates for C. virginicus and highest in PM for R. austrinum. SWP in C. virginicus was highest in CC and PB and lowest in NOM, but there were no differences in SWP among treatments in R. austrinum. Physical and chemical properties of soil and substrates as well as taxon affects results of this technique, and CC or PT are acceptable replacements for PM and PB in above-grade planting.