Evaluation of well-composted pine bark as an alternative to peat moss in a standard rooting medium for Leucospermum

The Fynbos nursery at the Agricultural Research Council (ARC) Infruitec-Nietvoorbij in Stellenbosch, South Africa (33°55'28.1”S; 18°52'26.1”E), maintains the national Proteaceae genebank and supports related research. The maintenance protocol requires the rooting of various genotypes. A sufficiently aerated rooting mixture is essential in rooting Proteaceae cuttings. The rooting medium used in the ARC's protocol comprises of peat moss, coarse sterilised sand and polystyrene granules. This mixture has the ability to absorb and retain irrigation water, but still allows aeration. The aim of these trials was to evaluate alternative rooting mediums on three genetically diverse Leucospermum cultivars: 'Jyoti' (L. cuneiforme × L. conocarpodendron), 'Raziya' (L. patersonii × L. cordifolium) and 'Rigoletto Gold' (L. glabrum × L. cordifolium). The trial was performed in March 2016 and repeated in May of the same year. The standard rooting medium was compared with two mediums composed of well-composted, fine pine bark and coarse sterilised sand. The root length and weight was determined after 13 and 29 weeks respectively, and rooting success was evaluated according to a standard scale ranging from callused to well-rooted. It was found that, overall, the cultivar differences had a more significant effect on rooting than variations in rooting media composition. 'Raziya' produced well-rooted cuttings with the longest roots independent of the rooting medium used. However, the root length of 'Rigoletto Gold' was significantly influenced by the medium used, with the best results on pine bark and sand in a 1:1 v/v ratio. During the May trial a cultivar-treatment interaction was revealed on the category “well-rooted”. Rooting success increased with longer exposure to the rooting environment. Pine bark can, therefore, be considered as an alternative to peat moss, since rooting is not significantly influenced by replacing the peat moss and polystyrene granules with pine bark in the growth medium.