Post-Transplant Irrigation Scheduling for Two Native Deciduous Shrub Taxa

The effect of five irrigation scheduling treatments on shoot growth and photosynthetic rates of Itea virginica L. ‘Henry’s Garnet’ (‘Henry’s Garnet’ sweet spire) and Rhododendron austrinum Rehd. (Florida flame azalea) were studied.  On 13 March 2008, twenty-five 11.4 L (3 gal) plants of each taxa were planted 1.2 m (4 ft) on center in field plots in Auburn, AL.  I. virginica ‘Henry’s Garnet’ was planted under 30% shade and R. austrinum was planted under 47% shade.  Watermark® soil moisture sensors (model 900M) were installed in the root ball and surrounding soil of plants in three replications per treatment.  Five irrigation scheduling treatments were used:  root ball and surrounding soil maintained at or above -25 kPa (well-watered, WW);  root ball and surrounding soil re-watered when root ball moisture reached -50 kPa (50 RB) or -75 kPa (75 RB); root ball and surrounding soil re-watered when surrounding soil moisture reached -25 kPa (25 S) or -50 kPa (50 S).  When irrigated, plants received 2.5 cm (1 in) of water applied by hand in a 31 cm (12 in) radius around the plant [7.4 L water (1.9 gal)].  Growth index (GI) of plants was measured at planting and at the end of the first growing season (7 October 2008), and relative growth index (RGI) was calculated.  Photosynthetic rates (Ps) of plants were measured on 29 August 2008 for R. austrinum and 6 October 2008 for I. virginica ‘Henry’s Garnet’.  Shoots of I. virginica ‘Henry’s Garnet’ grew almost twice as much as those of R. austrinum.  In I. virginica ‘Henry’s Garnet’, RGI was highest in plants that were WW and re-watered at 25 S moisture followed by those re-watered at 50 S, 75 RB, and 50 RB moisture.  R. austrinum RGI was highest in 75 RB, 50 RB, and WW treatments and lowest for plants in the 25 S and 50 S treatments.  In spite of growth differences, Ps was not significantly different between taxa or among treatments.  Data for soil and root ball drying rates suggest faster root growth rates in I. virginica and the importance of monitoring root ball moisture for species like R. austrinum that may have slower rates of root growth.  Both taxa were able to maintain shoot growth, Ps, and visual quality in all treatments, indicating that scheduling irrigation based on soil and root ball moisture can result in decreased irrigation frequency without decreasing plant survival.