The Effects of Parboiled Rice Hull-amended Substrates on the Growth and Water Use of Petunia and Zinnia

Utilizing renewable and low-cost materials to create growing mixes is an important strategy for sustainable greenhouse and nursery production.  Parboiled rice hulls has been suggested as a viable substitute to peat moss, because it is inexpensive, light in weight, and excellent to increase drainage and aeration. Our previous results indicated that altering the composition of commercial potting mix (CPM) by substituting 40% peat moss with ground- or whole-rice hulls provided as comparable physical properties as CPM. This research was conducted to evaluate the effects of parboiled rice hull amendment on plant growth and to compare the amount of water consumed by plants grown in different formulations of potting mixes. Petunia 'Easy Wave Neon Rose' and Zinnia ‘Benary’s Giant Golden Yellow’ were grown in the greenhouse in 0.6L pots filled with commercial potting mix (CPM, 70% peat moss), or amended mix either with ground rice hulls (GRH, 30% peat moss and 40% ground rice hull) or whole rice hulls (WRH, 30% peat moss and 40% whole rice hull), and were fertigated to provide two different levels (high and low) of volumetric water content (WVC) throughout production periods. The growth of Petunia was similar regardless of the types of growing mix used, however, shoot dry weight was slightly smaller in rice hull grown plants due to the reduced shoot growth. Meanwhile, the growth of Zinnia was slightly reduced by WRH as evidenced by reduced plant height, and branch and leaf numbers especially during the first 4 weeks of vegetative phase, however, it was recovered by the end of production period. The number of flowers was slightly decreased in rice hull grown Petunia due to the overall reduction in plant size, while it was not different in Zinnia grown in different types of mix. The daily water use increased in both plant species as plants aged. Significantly higher amount of water was required to maintain higher VWC regardless of the types of potting mix. However, plants grown in CPM consumed significantly higher amount of water to maintain target ranges of VWCs compared to the ones grown in rice hulls. In both species, GRH produced quality crops with the least amount of water. It is concluded that peat-based medium amended with 40% ground rice hulls is a sustainable option for the production of both Petunia and Zinnia as it helps reduce water use of crops without major adverse impacts on the crop quality.