2017 ASHS Annual Conference

Restricting irrigation can be a means for non-chemical growth control and resource conservation. There are a wide variety of spring bedding plant taxa and few species-specific reports of the effect of restricted irrigation. Our objectives were to quantify the effect of restricted irrigation on growth, quality, and water use of diascia (Diascia barberae) and lobelia (Lobelia erinus). Rooted cuttings of lobelia and diascia were individually planted into 11.4-cm-diameter petroleum-based plastic containers filled with a commercial peat-based soilless substrate amended with controlled release fertilizer, and grown in a glass-glazed greenhouse. Plants were provided with drip stakes connected to a capacitance soil moisture sensor-controlled irrigation system that maintained substrate volumetric water contents (VWC) of 0.20, 0.30, or 0.40 m³∙m^–3 throughout the experiment. Five and six weeks after treatments were initiated, data were collected for diascia and lobelia, respectively. Data recorded and calculated included the time to flower, height (diascia only), diameter, shoot dry mass (SDM), and total irrigation volume, and water use efficiency (WUE). Analyses of variance and mean separation using Tukey’s honestly-significant difference (hsd) test were performed on all data. Time to flower for diascia was not effected by VWC. Compared to plants grown at 0.40 (14.5 cm), diascia were 2.4 and 4.4 cm shorter when grown at 0.30 and 0.20 m³∙m^–3, respectively. Width was also suppressed by 36% for diacsia grown at 0.20 m³∙m^–3 (20.0 cm wide) compared to plants grown at 0.40 m³∙m^–3 (34.6 cm wide). Diascia SDM decreased from 5.4 g to 3.1 g as VWC decreased from 0.40 to 0.20 m³∙m^–3, respectively. Total irrigation volume provided to diascia decreased by 1.1 L as VWC decreased from 0.40 to 0.20 m³∙m^–3. The WUE for diascia was unaffected by VWC. Neither time to flower nor diameter of lobelia was affected by VWC. Lobelia SDM decreased from 4.6 g to 2.7 g as VWC decreased from 0.40 to 0.20 m³∙m^–3. Compared to plants grown at a VWC of 0.40 (3.3 L), lobelia grown at 0.30 and 0.20 m³∙m^–3 used 0.8 and 1.5 L less water, respectively. Lobelia WUE was unaffected by VWC. Based on the results of this study, restricted irrigation reduces water use for both species, but the effect on growth control varied between lobelia (insensitive) and diascia (sensitive).