2019 ASHS Annual Conference

This study was conducted to provide a holistic view on the effects of substrate properties on the growth and water use of Petunia ‘Easy Wave Neon Rose’ and Zinnia ‘Benary’s Giant Golden Yellow’. Our objectives were 1) to identify ideal blends of parboiled rice hulls (PRH) by determining the physical and chemical properties of PRH-containing substrates using either parboiled ground rice hulls (GRH) or whole rice hulls (WRH), and 2) to evaluate how such properties affect the growth and water use of plants grown in the substrate mixtures. Fourteen growing substrates were formulated by blending 0%, 20%, 30%, 40%, 50%, 60%, or 70% GRH or WRH (by volume) with the remainder being peat, 20% perlite and 10% vermiculite. Physical and chemical properties were determined and compared with commercial potting mix (CPM: 70% peat, 20% perlite and 10% vermiculite). Inclusion of either GRH or WRH in the peat-based substrates increased total porosity (TP), air space (AS), and particle density (PD), but decreased water holding capacity (WHC). Physical parameters of GRH were similar to CPM at the mixing rate of 20% to 50%. The pH and EC increased as the percentage of PRH increased, but were significantly lower than those of CPM at 60% or lower mixing rate. Based on the results, the peat-based substrates containing 40% GRH (GRH-40) and 40% WRH (WRH-40) were identified as the ideal mixes for the plants evaluated in this research, and selected for plant growth and water use study. Each plant was grown in a pot containing CPM, GRH-40 or WRH-40 maintained at two different volumetric water contents (VWC): high (25% to 30% and 20% to 25% in both CPM and GRH-40) and low (15% to 20% and 10% to 15% in WRH-40). Water use of plants rapidly increased from 3 weeks as their size increased. Petunia required significantly less volume of water compared to zinnia in a given time period. Initial growth reduction was observed in petunia and zinnia grown in the substrates of GRH-40 and WRH-40, and at a higher reduction rate in zinnia, particularly when grown in WRH-40. This led to a significant decrease in plant biomass of WRH-40 grown zinnia at 7 weeks, compared to the CPM or GRH-40 grown plants. Low VWC more negatively affected the growth of plants in CPM than in GRH-40. Plants grown in GRH-40 were compact and produced marketable quality products similar to those grown in CPM, while using significantly less volume of water to maintain the target ranges of VWC. Our results show that substrates are one of the major factors affecting plant water use, and the peat-based substrates containing 40% GRH can be utilized for the production of petunia and zinnia using less irrigation water, without compromising plant growth.