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 fl owers. While cultivars derived from Magnolia acuminata L. (cucumbertree mag- nolia) are diffi cult to propagate by stem cuttings, some with mixed parentage appear easier to propagate in this manner. We propagated six yellow-fl owered 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) de- veloped on cuttings of 'Yellow Lantern' collected 5 weeks after budbreak or when treated with 30 g·kg -1 IBA than the other cultivars. 'Butterfl ies' 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' col- lected 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 infl uence rooting of 'Yellow Lantern,' which ranged from 44% to 59%. Collection of stem cuttings early in the growing season (5 weeks after budbreak) was benefi cial (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-fl owered 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 'Butterfl ies,' remain variably recalcitrant and merit further study. Species, varieties, and cultivars of mag- nolia are popular fl owering trees that can be propagated by seed (species and varieties only), softwood cuttings, semi-hardwood cuttings, micropropagation, layering, budding, and grafting (Callaway, 1994; Dirr and Heuser, 1987). Plants are valued for their showy fl owers, foliage, or fruits, but all taxa are not equally easy to propagate, nor do they respond similarly to a single method of propagation (Hartmann et al., 2002). Demand for yellow-fl owered selections of magnolia has increased, and a large number of hybrids are derived from Magnolia acumi- nata and M. denudata Desr. (yulan magnolia) (Dirr, 1998). Other taxa used to obtain yel- low-fl owered hybrids and cultivars are M. kobus DC. (kobus magnolia), M. acuminata L. var. subcordata (Spach) Dandy (yellow cucumbertree), and Magnolia ×soulangiana Soul. -Bod. (saucer magnolia) (Knox, 2001). Hybrids derived from M. acuminata tolerate variable soil conditions and exhibit delayed fl owering, which prevents frost damage to flowers (Callaway, 1994). Nevertheless, because stem cuttings of M. acuminata and M. denudata do not root easily, many of their yellow-fl owered hybrids also carry this trait, and rooting generally is less than 40% (Dirr and Heuser, 1987; Knox, 2001; Owen et al., 2001). Consequently, these primarily are propagated by budding, grafting, or tissue culture. For most growers, however, inducing root formation on stem cuttings is the most feasible method of vegetative propagation. Despite clonal propagation, some variation in fl ower color is observed among yellow-fl ow- ered cultivars of magnolia in northern Florida, and color can vary from year to year (Knox, 2001), due perhaps to variation in weather and/or soil conditions (Tessmer, 1998). The yellow color (listed in the order of decreasing color intensity) is consistently more intense among fl owers of 'Butterfl ies,' 'Golden Sun,' 'Maxine Merrill,' and 'Hot Flash' compared to the fl ower color of 'Ivory Chalice' and 'Yellow Lantern,' which are cream or pale yellow (G.W. Knox, personal observation). These cultivars are of interest to growers and consumers, but experimental methods and statistical treatment of results have not been reported for their vegetative regeneration. Physiological developmental stage of the cuttings or time of year when cuttings are harvested infl uences rooting among cuttings of many woody species (Hartmann et al., 2002). While stem cuttings of certain plants produce roots easily at any time of year, species and cultivars of magnolia generally have a narrow 'window of rootability,' which also may vary among cultivars (Dirr and Heuser, 1987). Deciduous taxa of magnolia produce roots when softwood cuttings are used (Ellis, 1988; Hartmann et al., 2002), and although some cultivars have been rooted by using softwood cuttings (Ellis, 1988), experimental methods and results are not available, to our knowledge, to indicate the sensitivity of abovementioned yellow-fl owered cultivars to time of cutting collection. Auxins induce adventitious rooting on stem cuttings of many woody plants, including spe- cies and cultivars of magnolia (Hartmann, et al., 2002). A range from 5 to 20 g·kg -1 IBA in talc often is used to propagate cuttings of magnolia, but variation among taxa is known (Dirr and Heuser, 1987; Ellis, 1988; Hartmann et al., 2002). Although the effect of indole-3-butyric acid (IBA) on cuttings of the abovementioned yellow-fl owered cultivars is unknown, some general, non-statistical reports describe vegeta- tive propagation of a few cultivars and parent taxa. For example, rooting of some yellow- fl owered cultivars, including 'Hot Flash' and 'Ivory Chalice,' ranged from 50% to 100% (Knox, 2001), but the infl uence of IBA, number of cuttings, physiological stage, date of collec- tion, and environmental conditions during the experiment were not reported. Among parent taxa, there are no reports, to our knowledge, to describe root formation on cuttings of M. acuminata, which is one of the parents of all yellow-fl owered cultivars included in this study. However, 85% to 100% of the wounded cuttings of M. denudata collected in early June in the northeastern U.S. developed roots after application of 3 to 8 g·kg -1