Timing, Root Zone Temperatures, and Auxin Concentrations Affect Quantity and Quality of Adventitious Roots Regenerated on Shoot Cuttings of Native Texas Groundcovers

During previous selection efforts, four commercially promising native Texas groundcovers were identified; Borrichia frutescens (L.) DC., Erigeron procumbens (Houst. ex Mill.) G.L. Nesom., Oenothera drummondii Hook. and Phyla nodiflora (L.) Greene.  Studies, conducted independently for each species, were designed to test cutting propagation methods that would yield optimum adventitious root quantity and quality.  Factors tested included concentrations of root growth regulator applied (potassium salts of indole-3-butyric acid (K-IBA); 0, 1000, 2000, 4000, or 8000 mg∙L^-1), bottom heat treatments [none, low (26.7°C) and high (35.0°C)], and propagation time (warm season and cool season).  Measurements at the end of the experiments included a binomial indication of the presence of roots (0 = no roots; 1 = rooted), cutting height, height of root system, width of root system, root growth index (RGI), length of five longest roots, and root and shoot dry masses.  Visual quality ratings were assessed by two independent raters and a mean of the two scores were used for analysis.  All species rooted at 90% in at least one of the factorial combinations tested with greater rooting percentages generally observed in cuttings treated with 1000–2000 mg∙L^-1 K-IBA and low bottom heat when propagated in warm seasons.  Species-specific results included similar rooting percentages (~100%) for E. procumbens treated with 0, 1000, and 2000 mg∙L^-1 K-IBA, all warm season treatments of O. drummondii rooting at ~100%, and all treatment combinations including the low and high heat treatment rooting at near 100% for P. nodiflora.  Root quality was more variable among species than rooting percentage.  Mean root length and mean root dry masses were greater in the cool season for E. procumbens while all other species exhibited greater measurements in the warm season.  Mean root length measurements decreased for O. drummondii with increasing K-IBA concentration, a result that was not observed for any other species.  Increasing K-IBA levels for O. drummondii decreased mean root dry masses during warm season propagation while greater K-IBA concentrations increased the measurement in the cool season for the non-heat treatment.  Generally, the mean visual quality ratings confirmed what had been observed by the empirical measurements.  The current work demonstrated that cuttings of B. frutescens, E. procumbens, O. drummondii, and P. nodiflora could be rooted successfully utilizing conventional propagation techniques.