2018 ASHS Annual Conference

Catberry (Ilex mucronata), rhodora (Rhododendron canadense), and sweetgale (Myrica gale) are three horticulturally promising shrubs of wetlands in North America. Although all three can be rooted from stem cuttings, root system quality may be inconsistent. To explore two factors that might improve quality of adventitious root systems, a minimum of 270 semi-hardwood cuttings of each species were collected in June or July 2016, treated with 0 to 15,000 ppm of the auxin K-IBA, and inserted into propagation media with 0, 25, or 50% peat moss by volume, with coarse perlite for the remainder. Cuttings were evaluated after eight weeks under intermittent mist. Rooting percentage of catberry cuttings was high, and averaged more than 80% across hormone treatments when either 25 or 50% peat moss was included in the medium; rooting averaged only 33% in perlite alone. Greatest root ratings, root lengths, root numbers, and root dry weights among cuttings of catberry occurred when they were treated with 5,000 to 15,000 ppm K-IBA, and inserted into media with 25 or 50% peat. The use of perlite alone was unsatisfactory, as it caused catberry cuttings to proliferate callus excessively instead of rooting. Rooting percentage of rhodora cuttings averaged more than 90% when either 25 or 50% peat moss was included in the medium, but only 64% in perlite alone. Curves fit to these rooting responses showed that maximum root ratings, root lengths, and root dry weights can be obtained with concentrations of K-IBA between 5,000 and 10,000 ppm for cuttings rooted in media containing peat. In contrast, cuttings inserted into perlite alone showed only modest gains in these measures when K-IBA was applied. Rooting percentage of sweetgale exceeded 80%, whether peat moss was included in the medium or not. The greatest average root ratings, root lengths, and root dry weights of sweetgale cuttings were produced when they were treated with 0 to 5,000 ppm K-IBA. Although the number of roots on sweetgale cuttings did not vary markedly with K-IBA application, auxin concentration had a consistently negative effect on root elongation, root rating, and root dry weight. Collectively, these results show that media with 25 to 50% peat are appropriate for all three wetland species, and only sweetgale tolerates pure perlite during propagation. Catberry and rhodora respond well to concentrations of K-IBA exceeding 5,000 ppm, whereas rooting in sweetgale is not enhanced, but may be diminished, by K-IBA application at concentrations used in this study.