Growth and Photosynthesis of Selected Native Shrubs Planted Above-Grade with Organic Matter

In modified above-grade landscape planting, substrate is mounded around the upper portion of a root ball that remains above soil grade at planting in order to mimic the natural organic litter layer that supports root growth in naturalized landscapes.  On 17 March 2008, 3-gal plants of Chionanthus virginicus L., Rhododendron austrinum Rehd., and Hydrangea quercifolia Bartr. ‘’ were planted in field plots 1.2 m (4 ft) on-center, and the planting hole was backfilled to soil grade with existing field soil.  Plants were planted at grade (AG) with no organic matter or above grade with one of four organic substrates:  peat moss (PM), pine bark (PB), pine tree substrate (PT), or coconut coir (CC).  In above grade plantings, plants were planted such that the top 10 cm (4 in) of the root ball remained above grade, and organic matter was mounded around the above-grade portion of the root ball, extending outward from the stem in a radius of 31 cm (12 in).  C. virginicus and H. quercifolia ‘’ were planted under 30% shade, while R. austrinum was planted under 47% shade.  On 19 September 2008, net photosynthesis (PS) was measured for each plant.  Growth indices [(widest width measurement + perpendicular (^) width measurement + height)/3] were recorded at planting (initial growth index, IGI) and on 23 October 2008 (final growth index, FGI).  IGI and FGI were used to calculate relative growth index [RGI=(FGI - IGI)/IGI].  Treatments were arranged in a randomized complete block design with five blocks.  C. virginicus plants were smallest at planting, yet had the highest RGI (1.7) among taxa, followed by H. quercifolia ‘’ (0.8) and R. austrinum (0.6).  RGI was not different among treatments for C. virginicus and R. austrinum, however, RGI was lowest in PB for H. quercifolia ‘’.  PS rates were not different among treatments, yet were highest in R. austrinum (19 umol/m²/s), followed by C. virginicus (16 umol/m²/s), with H. quercifolia ‘’ being lowest (6 umol/m²/s).  Differences in growth and PS among taxa are likely related to inherent genetic differences, and similar PS rates among treatments correspond with the lack of growth differences among treatments.  Correlation between PS and RGI for C. virginicus and H. quercifolia ‘Alice’ but not for R. austrinum may explain differences in ease of landscape establishment of these taxa.  All substrates appeared to be suitable for use in this planting technique with these taxa.