Influence of Soilless Substrates on Irrigation Frequency and Plant Growth of Leucospermum

Many growers create their own potting mixes using two or more substrates based on available local resources. Volcanic cinder is a highly porous and the most commonly used substrate in Hawaii because it is inert and inexpensive, and provides good drainage for plants. Our aim was to quantify water use of plants grown with different soilless media and to evaluate plant quality during production period. Rooted cuttings of Leucospermum reflexum ‘Brandi Dela Cruz’ were planted into 3.78 L pots filled with either commercial potting mix (CPM) [60% peat moss: 40% perlite, v/v] or cinder-based potting mix (VCM) [50% volcanic cinder: 50% peat moss, v/v]. Irrigation was automatically controlled with substrate moisture sensors set at five set-points to provide different levels of volumetric water content (VWC, m³/m³). Due to the different physical properties of the two media, the set-points ranged from 0.33 to 0.41 for CPM and from 0.58 to 0.66 and VCM, respectively. Daily irrigation volume gradually increased as plant size increased. Plants grown with VCM required higher irrigation frequency and higher daily irrigation volume, however, plants were significantly bigger and greener compared to the ones grown with CPM. Although plant growth was not much affected by VWC regardless of substrates, less leachate was generated at lower set-points. It is concluded that substrates are important in determining the irrigation frequency and volume, and the plant growth of Leucospermum. Optimizing potting substrates for crop production will help maximize crop quality and minimize water loss to the environment.