Adventitious rooting of stem cuttings of yellow-flowered magnolia cultivars is influenced by time after budbreak and indole-3-butyric acid

Certain cultivars of magnolia are desirable in landscapes for their uncommon yellow flowers. While cultivars derived from Magnolia acuminata L. (cucumbertree magnolia) are difficult to propagate by stem cuttings, some with mixed parentage appear easier to propagate in this manner. We propagated six yellow-flowered cultivars vegetatively by applying 0, 8, 16, or 30 g·kg^-1 (0, 8,000, 16,000, or 30,000 ppm) indole-3-butyric acid (IBA) in talc to bases of terminal stem cuttings collected 5, 7, 9, or 11 weeks after budbreak. Mean rooting percentage over all cultivars increased from 12% (in the absence of IBA) to 34% (after application of 30 g·kg^-1 IBA). Rooting percentage and basal stem diameter of a cutting did not seem related. For each collection date, more cuttings of 'Ivory Chalice' and 'Yellow Lantern' developed roots than the other cultivars. More roots (mean = 5) developed on cuttings of 'Yellow Lantern' collected 5 weeks after budbreak or when treated with 30 g·kg^-1 IBA than the other cultivars. 'Butterflies' largely remained unresponsive, whereas rooting of 'Golden Sun,' 'Hot Flash,' and 'Maxine Merrill' collected 5 weeks after budbreak was 31%, 22%, and 28%, respectively. When data were analyzed separately for selected cultivars, 63% rooting was observed among cuttings of 'Ivory Chalice' collected 7 weeks after budbreak. Rooting percentage was higher (22%) among cuttings of 'Hot Flash' collected 5 or 7 weeks after budbreak in comparison to later collection dates, but harvest date did not influence rooting of 'Yellow Lantern,' which ranged from 44% to 59%. Collection of stem cuttings early in the growing season (5 weeks after budbreak) was beneficial (31% rooting) for inducing rooting among cuttings of 'Golden Sun.' We conclude that 'Ivory Chalice' and 'Yellow Lantern' are promising choices for growers interested in clonal propagation of yellow-flowered cultivars of magnolia. To maximize rooting among these cultivars, terminal cuttings should be collected within 5 to 11 weeks after budbreak and treated with 16 or 30 g·kg^-1 IBA in talc. Early collection dates (5 to 7 weeks after budbreak) improved rooting among cuttings of other cultivars but these, particularly 'Butterflies,' remain variably recalcitrant and merit further study.