Maintenance of Substrate Water Content to Control Growth of Gardenia jasminoides

Soil moisture sensor controlled, automated irrigation has been used to successfully maintain substrate volumetric water content (θ) of container plants.  By maintaining different θ thresholds and monitoring water applications, it is possible to relate plant growth to irrigation volume.  This would provide nursery growers with the ability to control plant growth via irrigation control.  To most successfully utilize this method for controlling plant growth, growth patterns of different species and different cultivars of the same species in response to θ need to be understood.  Using a soil moisture sensor controlled, automated irrigation system, we irrigated two Gardenia jasminoides cultivars, ‘Radicans’ and ‘August Beauty’, at θ thresholds of 20, 30, 40, and 50% (v/v) to assess their growth and flowering at two locations,  Studies were conducted at Watkinsville (USDA Hardiness zone 8a) and Tifton, GA (USDA Hardiness zone 8b) to look at environmental impacts.  Growth of both cultivars was related to θ threshold, and patterns of growth were similar at the two locations.  High mortality was observed at the lowest threshold (20%) with poor root establishment due to the low irrigation volume.  In Watkinsville at the 20% threshold ‘August Beauty’ received 1.9 L/plant and ‘Radicans’ received 1.4 L/plant over the course of the experiment.    Growth of plants at the 30% threshold was reduced compared to plants grown at the 40 and 50% thresholds.  Flowering was delayed at the 30% threshold, with blooming not occurring until after rainfall late in the experiment.  Irrigation volume for the 30% threshold in Watkinsville was 15.0 L/plant for ‘August Beauty’ and 14.1 L/plant for ‘Radicans’.  Plants grew similarly at the 40 and 50% thresholds, but with significant differences in irrigation volume.  In Watkinsville ‘August Beauty’ received 48 L/plant at the 40% threshold and 88 L/plant at the 50% threshold.  ‘Radicans’ received 33.5 L/plant at the 40% threshold and 136 L/plant at the 50% threshold.    The irrigation volume at the 50% threshold was likely in excess of what was needed for growth, resulting in leaching.  The growth reductions at lower θ show that θ adjustment is a possible means of controlling plant growth and crop timing.