Abstract

Hongyan Sun , Michael D. Dukes , Roger Kjelgren and Richard Beeson Jr.

We used drainage lysimeters to study water balance and plant growth of mixed (woody plant-turf) landscapes irrigated at three rates of root zone water depletion based on local reference evapotranspiration (ETO) in a subtropical-monsoonal climate: slow/dry (60%), intermediate (75%) and fast/wet(90%) of ETO. Planting design for each lysimeter was identical: a Magnolia virginiana tree imbedded in St. Augustine grass (Stenotaphrum secundatum) turfgrass bordered by two Viburnum odoratissimum shrubs grown as a hedge. Landscape monthly actual evapotranspiration (ETA) was calculated as the difference between inputs (irrigation and precipitation) and output (drainage). Magnolia (height, projected canopy area, and trunk diameter) and viburnum (height and canopy volume) growth was measured every three weeks. Total drainage over three years from the 0.6 depletion (driest) treatment was 20 and 45 cm for the dry and wet season, respectively, half that of the 0.9 depletion (wettest) treatment. The dry treatment had the lowest ETA both seasons, and average dry season Plant Factors (PF=ETA÷ETo) was about 15% lower than the wet season (0.76 versus 0.98). Plant growth was largely unaffected by irrigation-depletion treatments. For subtropical climates, dry season irrigation scheduling based on a mixed landscape PF of 0.75 is reasonable to maximize growth following transplanting. Once established, dry season irrigation schedules can be based on a 0.6 PF value to maintain performance while minimizing drainage. Irrigation of mixed landscapes during the monsoonal wet season is only needed during exceptional dry periods.