Using moisture sensors installed in the root ball and backfill soil to schedule post-transplant landscape irrigation

Adequate post-transplant irrigation remains critical for landscape plant establishment and survival despite water restrictions and increasing incidence of drought.  The frequency of irrigation applications required for establishment and survival appears to be less than what is applied in most situations.  Specific recommendations for precise irrigation scheduling to ensure plant establishment and survival are needed.  One confounding factor is that most landscape plants are produced in soilless substrates that tend to dry more rapidly than the backfill soil but in which roots exist exclusively until post-transplant root growth commences.  Installing moisture sensors in both the root ball and the backfill soil makes it possible to monitor the difference in drying rates between the soil and a typical pine bark-based container substrate.  Sensor size determines the amount of root ball disruption during installation, while sensor type affects measurement accuracy, presenting important considerations in sensor selection.  
Scheduling post-transplant irrigation based on moisture contents of the transplanted root ball and the backfill soil is an effective way to decrease irrigation volume without decreasing plant quality.  If only one method is employed, thresholds for scheduling irrigation must be higher (wetter) if using backfill soil moisture contents than if using moisture contents in the root ball.  With species that tend to exhibit relatively slow rates of root growth, monitoring root ball moisture for a longer period of time following transplanting is particularly important.  For these species, root growth appeared to persist initially only within the original root ball, thus irrigation frequencies based on root ball moisture may initially increase by one day, however in all cases frequencies were less than in the well-watered “control” treatments.  For plants with relatively faster rates of root growth, irrigation scheduling decisions change shortly after transplanting from being based on root ball moisture to being based on backfill soil moisture as roots grow out from the original root ball into the backfill.  In studies with several native shrub taxa, root and shoot growth were sustained if irrigation was only applied once root ball volumetric moisture reached 10% or if soil volumetric moisture reached 15%.  In all cases, even if post-transplant growth in these treatments was slightly less than in well watered “control” treatments, survival and plant visual quality were not compromised, and photosynthetic rates remained adequate to support continued growth.  In fact, limiting post-transplant irrigation applications actually resulted in increased post-transplant root growth compared to well watered plants in some species.