Post-transplant Irrigation Scheduling of Native Deciduous Shrub Taxa

Abstract

The effect of irrigation scheduling treatments on root and shoot growth and photosynthesis of Hydrangea quercifolia Bart. ‘Alice’ (‘Alice’ oakleaf hydrangea), Rhododendron austrinum Rehd. (florida flame azalea), and Itea virginica L. ‘Henry’s Garnet’ (‘Henry’s Garnet’ sweetspire) was evaluated using Horhizotrons in a greenhouse in Auburn, Ala. Irrigation frequency treatments included: backfill and root ball maintained at or above 20% moisture (well watered, WW); backfill and root ball re-watered when root ball moisture reached 10% (10RB); backfill and root ball re-watered when quadrant soil moisture reached 15% (15S). For all three species horizontal root length (HRL) increased linearly over time for all treatments. Overall, 10RB plants exhibited the longest HRL followed by plants in WW and 15S treatments. Shoot growth index (GI) and photosynthetic rates (Ps) were similar among treatments for H. quecifolia ‘Alice’ and R. austrinum. GI for I. virginica ‘Henry’s Garnet’ in general was highest in WW and 10RB followed by 15S. Dry weight of roots in soil was similar among all three treatments for H. quecifolia ‘Alice’, and was higher in WW and 10RB than 15S for I. virginica ‘Henry’s Garnet’. SDW of R. austrinum and I. virginica ‘Henry’s Garnet’ in general was highest in WW and 10RB followed by 15S. To further evaluate the effect of irrigation scheduling treatments on shoot growth and Ps, plants of I. virginica ‘Henry’s Garnet’ and R. austrinum were planted under shade structures in field plots in Auburn, Ala. Irrigation scheduling treatments included: root ball and surrounding soil maintained at or above 25 cb (centibar) (well-watered, WW); root ball and surrounding soil re-watered when root ball moisture reached 50 cb (50RB) or 75 cb (75RB); or root ball and surrounding soil re-watered when surrounding soil moisture reached 25 cb (25S) or 50 cb (50S). For I. virginica ‘Henry’s Garnet’ final GI was highest in WW and 25S treatments, followed by 50S, 50RB, and 75RB while there were no differences among treatments for R. austrinum. Both taxa had the largest increase in GI during the first growing season and the lowest in winter months. For R. austrinum Ps was higher in WW, 25S, and 50RB than in 75RB and 50S. Stem water potential (SWP) was similar among treatments. For I. virginica ‘Henry’s Garnet’ Ps was higher in 50S, 50RB, and 25S than in WW, followed by 75RB. SWP was higher in 50S and 75RB than in 50RB, WW, and 25S. For both experiments results indicate that until and even after roots grow into the backfill soil, monitoring both backfill soil and root ball moisture is important for scheduling and reducing post-transplant irrigation.