Post-transplant Irrigation Scheduling for Two Native Deciduous Shrub Taxa

Abstract

The effect of five irrigation scheduling treatments on shoot growth [growth index (GI)] and stem water potential (SWP) of Itea virginica L. ‘Henry's Garnet’ (‘Henry's Garnet’ sweetspire) and Rhododendron austrinum Rehd. (Florida flame azalea) were studied. Plants were transplanted on 13 Mar. 2008 at soil grade level under shade structures in field plots of sandy loam soil on the Auburn University campus in Auburn, AL. Matric potential was continuously measured 7.6 cm from the stem in the root ball and 20.3 cm from the stem in the soil backfill for three plants per treatment per taxa. Irrigation scheduling treatments included (in order of decreasing irrigation frequency): root ball and surrounding soil matric potential maintained at or above –25 kPa [well-watered (WW)]; root ball and surrounding soil rewatered when root ball matric potential dropped to either –50 kPa (50RB) or –75 kPa (75RB); and root ball and surrounding soil rewatered when surrounding soil matric potential dropped to either –25 kPa (25S) or –50 kPa (50S). In both taxa, GI increased linearly over time in all five irrigation treatments. For I. virginica ‘Henry's Garnet’, GI increased most in WW and 25S treatments followed by 50S, 50RB, and 75RB. Shoot growth of R. austrinum was similar among treatments. Both I. virginica ‘Henry's Garnet’ and R. austrinum had a larger increase in GI during the first growing season (2008). For I. virginica ‘Henry's Garnet’, SWP was higher in 50S and 75RB treatments than in 50RB, WW, and 25S. For R. austrinum, SWP was not different among treatments. Results indicate that although plant growth might be diminished slightly, irrigation frequency can be reduced without compromising plant visual quality or survival if root ball and soil matric potential is monitored. Additionally, until roots grow into the backfill soil, monitoring both backfill soil and root ball matric potential is important for scheduling and reducing post-transplant irrigation applications.