Eastern redbud, Cercis canadensis L. (Fabaceae, legume family) is a widely distributed small tree (or large shrub) species native to the United States from Connecticut south to Florida and west to Oklahoma, Texas, and northeastern Mexico (Fritsch et al. 2009; Hopkins 1942; Kidwell-Slak and Pooler 2018; Robertson 1976). Every spring, it can be counted on for pink to magenta clusters of flowers to appear throughout the leafless canopy, just before vegetative budbreak. Over the past century, a number of white-flowered cultivars of C. canadensis have been widely distributed in the nursery trade, including ‘Alba’, ‘Royal White’, and ‘Texas White’, each of which produces pure white flowers, with no trace of red pigmentation. ‘Alba’ originated in the nursery of John Teas and Son in Carthage, MO, around the turn of the last century (Anonymous 1922; Rehder 1907) and ‘Royal White’ was discovered as a seedling in Bluffs, IL, in the 1940s (Kidwell-Slak 2023); both are cultivars of the common eastern redbud, C. canadensis var. canadensis. ‘Texas White’, derived from C. canadensis var. texensis (S. Watson) M. Hopkins, was released in the late 1960s by the German Nursery in Fort Worth, TX (Burns and Raulston 1993; Werner and Snelling 2010). White-flowered varieties of the western US species Cercis occidentalis Torr ex A. Gray, European and western Asian species Cercis siliquastrum L., and the Asian species Cercis chinensis Bunge have also been discovered and propagated (Kidwell-Slak and Pooler 2018). White-flowered sports have also been observed in Cercis yunnanensis H. H. Hu & W. C. Cheng and Cercis chingii Chun (Burns and Raulston 1993). Thus, although sports that lack any red pigmentation in the flowers of redbuds are not rare, to the best of our knowledge, a redbud cultivar with flowers whose petals are mostly white, as is the case here, has only been reported once before (‘Sjo’ US PP29,290 P2, Westervelt 2018). The cultivar we describe, C. canadensis ‘Arnold Banner’, is a sport of the common eastern redbud (C. canadensis var. canadensis) with extraordinarily unusual and beautiful nearly white flowers that retain the pink striping of the nectar guides on the banner petal; it was first brought to the public’s attention in 2013 in Arnoldia (Friedman 2013).
Origin
‘Arnold Banner’ arose as a spontaneous somatic mutation on a plant (Accession 10-68*B) growing in the Arnold Arboretum of Harvard University. This plant was germinated in 1968 from seed collected from another C. canadensis plant in the Arboretum’s collection (Accession 22870*J, which was an existing plant in the landscape of unknown origin accessioned in 1950). On 4 May 2009, a lateral branching system with aberrant flowers was first observed by Abby Meyer (née Hird), then a Putnam Fellow at the Arboretum. After this initial notice, Arboretum staff began to monitor and characterize the sport, and to propagate it clonally. In 2012, flowers of this mutant branching system were carefully examined to more fully characterize the mutation. The shoot system bearing mutant flowers died in 2015, although the rest of the plant (with wild-type flowers) remains alive.
Redbuds are cauliflorous (flowering on woody axes; flowers are found only on shoots 2 or more years old, but not on a current year’s new shoots). Thus, the juxtaposition of wild-type flowers with mutant flowers along the main axis of the lateral branching system is an indicator of the time when the mutation occurred in the history of the branching system (Fig. 1). Analysis of leaf and bud scars on the mutant branching system indicated that the mutation in the shoot apical meristem likely occurred sometime in 2002.
Branch of the original redbud tree (Accession 10-68*B) showing typical cauliflory with clusters of flowers on old wood. On the left side of the shoot, wild-type flowers appear in typical clusters. On the right side of the axis, mutant flowers can be seen. Beyond the uppermost set of flowers in the image, all distal flowers were mutant.
Citation: HortScience 58, 12; 10.21273/HORTSCI17393-23
Branch of the original redbud tree (Accession 10-68*B) showing typical cauliflory with clusters of flowers on old wood. On the left side of the shoot, wild-type flowers appear in typical clusters. On the right side of the axis, mutant flowers can be seen. Beyond the uppermost set of flowers in the image, all distal flowers were mutant.
Citation: HortScience 58, 12; 10.21273/HORTSCI17393-23
Branch of the original redbud tree (Accession 10-68*B) showing typical cauliflory with clusters of flowers on old wood. On the left side of the shoot, wild-type flowers appear in typical clusters. On the right side of the axis, mutant flowers can be seen. Beyond the uppermost set of flowers in the image, all distal flowers were mutant.
Citation: HortScience 58, 12; 10.21273/HORTSCI17393-23
Description and Performance
In Boston, flowering has ranged from 21 Apr to 15 May, depending on the year, and is coincident with other C. canadensis in the Arboretum’s living collections. Peak bloom typically lasts 7 to 10 d. The flower pedicel is uniformly light pink and significantly less red than is the case in wild-type flowers. The calyx is strikingly pink with streaks of very light yellow-green (Figs. 2 and 3), whereas the calyx of the wild-type flower displays significantly more red pigmentation (although close observation shows that there are equivalent streaks in the parent plant wild-type flowers that are slightly lighter pink than the rest of the calyx). Flower petals are mostly, but not entirely, white. Careful examination shows that red anthocyanin (see Perkins-Veazie et al. 2017 for high-performance liquid chromatography analyses of anthocyanin types in C. canadensis flowers) pigmentation is present in patches, although in significantly lesser amounts (compared with wild-type). As a result, the two wing and two keel petals of the Arnold Arboretum mutant redbud flowers appear white from a distance, but on closer inspection, display subtle areas of light pink pigmentation (Fig. 3B and 3F). In the banner petal, there is strong expression of red pigmentation in radiating streaks that lie between the veins of this specialized member of the corolla. Small numbers of darker punctate areas can often be seen randomly distributed within the broader nectar guide streaks (Figs. 2D, 3B, and 4). The banner petal of wild-type flowers, while clearly pink throughout, also has more intense zones of deep magenta that radiate out and lie between the veins (Fig. 3B). This is also true of the wild-type flowers of other species of Cercis (personal observation). This set of magenta streaks is characteristic of what are commonly called “nectar guides,” displays of pigmentation that help insect pollinators (in this case, short- and long-tongued bees) orient properly as they approach the flower during the process of pollination (Dafni and Kevan 1996; Leonard and Papaj 2011; Robertson 1976). The cultivated standard for C. canadensis ‘Arnold Banner’ is deposited in the Herbarium of Cultivated Plants of the Arnold Arboretum (A).
Wild-type Cercis canadensis flowers in bud just before anthesis and in full flower (A = Accession 115-2017*A; B = Accession 10-68*B). C. canadensis ‘Arnold Banner’ flowers in bud just before anthesis and in full flower (C = Accession 46-2011*A; D = Accession 46-2011*B). C. canadensis ‘Alba’ flowers in bud just before anthesis and in full flower (E = Accession 372-2001*A; F = Accession 372-2001*A).
Citation: HortScience 58, 12; 10.21273/HORTSCI17393-23
Wild-type Cercis canadensis flowers in bud just before anthesis and in full flower (A = Accession 115-2017*A; B = Accession 10-68*B). C. canadensis ‘Arnold Banner’ flowers in bud just before anthesis and in full flower (C = Accession 46-2011*A; D = Accession 46-2011*B). C. canadensis ‘Alba’ flowers in bud just before anthesis and in full flower (E = Accession 372-2001*A; F = Accession 372-2001*A).
Citation: HortScience 58, 12; 10.21273/HORTSCI17393-23
Wild-type Cercis canadensis flowers in bud just before anthesis and in full flower (A = Accession 115-2017*A; B = Accession 10-68*B). C. canadensis ‘Arnold Banner’ flowers in bud just before anthesis and in full flower (C = Accession 46-2011*A; D = Accession 46-2011*B). C. canadensis ‘Alba’ flowers in bud just before anthesis and in full flower (E = Accession 372-2001*A; F = Accession 372-2001*A).
Citation: HortScience 58, 12; 10.21273/HORTSCI17393-23
Banner petal of Cercis canadensis ‘Arnold Banner’ showing prominent nectar guides.
Citation: HortScience 58, 12; 10.21273/HORTSCI17393-23
Banner petal of Cercis canadensis ‘Arnold Banner’ showing prominent nectar guides.
Citation: HortScience 58, 12; 10.21273/HORTSCI17393-23
Banner petal of Cercis canadensis ‘Arnold Banner’ showing prominent nectar guides.
Citation: HortScience 58, 12; 10.21273/HORTSCI17393-23
Comparison of individual flowers of Cercis canadensis ‘Arnold Banner’ (B = Accession 46-2011*B, F = Accession 46-2011*A) with C. canadensis ‘Sjo’ (C = 88-2020*A, G = 88-2020*A), wild type (A = 633-83*C, E = 115-2017*A), and C. canadensis ‘Alba’ (D = 372-2001*A, H = 372-2001*A).
Citation: HortScience 58, 12; 10.21273/HORTSCI17393-23
Comparison of individual flowers of Cercis canadensis ‘Arnold Banner’ (B = Accession 46-2011*B, F = Accession 46-2011*A) with C. canadensis ‘Sjo’ (C = 88-2020*A, G = 88-2020*A), wild type (A = 633-83*C, E = 115-2017*A), and C. canadensis ‘Alba’ (D = 372-2001*A, H = 372-2001*A).
Citation: HortScience 58, 12; 10.21273/HORTSCI17393-23
Comparison of individual flowers of Cercis canadensis ‘Arnold Banner’ (B = Accession 46-2011*B, F = Accession 46-2011*A) with C. canadensis ‘Sjo’ (C = 88-2020*A, G = 88-2020*A), wild type (A = 633-83*C, E = 115-2017*A), and C. canadensis ‘Alba’ (D = 372-2001*A, H = 372-2001*A).
Citation: HortScience 58, 12; 10.21273/HORTSCI17393-23
Based on the Royal Horticultural Society Color Chart (Royal Horticultural Society 2015), the pedicel falls within the Grayed-Purple Group (186 C to A). The apex of the calyx is also in the Grayed-Purple Group (186 C to A), whereas the base falls within the Grayed-Red Group (178 A). The banner petal is in the Red-Purple Group (N74 B, C), with a greater intensity of color on the abaxial side. The wing and keel petals are white with flecks of pigment in the Red-Purple Group (69 B to D).
In ‘Arnold Banner’ redbud flowers, the style differs in pigmentation from the wild type. In typical redbud flowers, the distal portion of the style is a reddish color. Under the microscope, it becomes evident that the mutant flowers have styles that lack any obvious red pigmentation (Fig. 5).
Style and stigma of ‘Arnold Banner’ flower (A) and wild-type C. canadensis (B).
Citation: HortScience 58, 12; 10.21273/HORTSCI17393-23
Style and stigma of ‘Arnold Banner’ flower (A) and wild-type C. canadensis (B).
Citation: HortScience 58, 12; 10.21273/HORTSCI17393-23
Style and stigma of ‘Arnold Banner’ flower (A) and wild-type C. canadensis (B).
Citation: HortScience 58, 12; 10.21273/HORTSCI17393-23
In wild-type C. canadensis, the lamina and prominent stipules of young leaves display a reddish hue during the early flushing of buds. In ‘Arnold Banner’, young leaves appear to lack any blush of red pigmentation, similar to the very young leaves of white-flowered cultivars of C. canadensis (Fig. 6).
Coloration of young leaves after budbreak in wild-type Cercis. canadensis showing light red tint (A = 9042*A), ‘Arnold Banner’ showing no discernable red tint (B = 46-2011*A), and ‘Alba’ also showing no red tint (C = 372-2001*A).
Citation: HortScience 58, 12; 10.21273/HORTSCI17393-23
Coloration of young leaves after budbreak in wild-type Cercis. canadensis showing light red tint (A = 9042*A), ‘Arnold Banner’ showing no discernable red tint (B = 46-2011*A), and ‘Alba’ also showing no red tint (C = 372-2001*A).
Citation: HortScience 58, 12; 10.21273/HORTSCI17393-23
Coloration of young leaves after budbreak in wild-type Cercis. canadensis showing light red tint (A = 9042*A), ‘Arnold Banner’ showing no discernable red tint (B = 46-2011*A), and ‘Alba’ also showing no red tint (C = 372-2001*A).
Citation: HortScience 58, 12; 10.21273/HORTSCI17393-23
‘Arnold Banner’ appears to be a stable periclinal chimera of the shoot apical meristem. An earlier investigation of the shoot apical meristem in C. siliquastrum (Slade 1957) found that there is only a single tunica layer along with a standard corpus of perpetually embryonic meristematic cells. Examination of median longitudinal sections of shoot apical meristems of C. canadensis indicates that there is likely a single tunica layer over a corpus of meristematic cells (Fig. 7). Thus, there appear to be two discrete populations of perpetually embryonic initial cells in the shoot apical meristems of ‘Arnold Banner’.
Median longitudinal section of the shoot apical meristem of Cercis canadensis. Cells have been outlined to indicate that they are part of the shoot apical meristem. A single tunica layer of cells (T) lies above the corpus (C). LP = leaf primordium.
Citation: HortScience 58, 12; 10.21273/HORTSCI17393-23
Median longitudinal section of the shoot apical meristem of Cercis canadensis. Cells have been outlined to indicate that they are part of the shoot apical meristem. A single tunica layer of cells (T) lies above the corpus (C). LP = leaf primordium.
Citation: HortScience 58, 12; 10.21273/HORTSCI17393-23
Median longitudinal section of the shoot apical meristem of Cercis canadensis. Cells have been outlined to indicate that they are part of the shoot apical meristem. A single tunica layer of cells (T) lies above the corpus (C). LP = leaf primordium.
Citation: HortScience 58, 12; 10.21273/HORTSCI17393-23
Rare occurrences of chimeric and revertant wild-type flowers (Fig. 8) indicate a slight instability in the maintenance of discrete populations of meristematic cells in the shoot apex. Chimeric flowers of C. canadensis ‘Arnold Banner’ (Fig. 8A) appear to be the result of meristems in which the mutant population of non–anthocyanin-producing cells has been invaded by wild-type meristematic cells, a classic example of a sectorial periclinal chimeric shoot apical meristem (see Marcotrigiano 1997; Tilney-Bassett 1986). A very small number of flowers also show evidence of complete reversion to wild type (Fig. 8B), and some flowers have converted to a fully white corolla, much like the purely white cultivars ‘Alba’, ‘Royal White’, and ‘Texas White’. All of these variant flowers on ‘Arnold Banner’ redbuds are congruent with a largely stable and discrete population of mutant cells in the shoot apical meristem. Although it is possible that the mutation represents a new pattern of tissue-specific anthocyanin expression (“non-cell lineage variegation” sensu Marcotrigiano 1997), the conversion of meristems giving rise to either entirely white flowers or wild-type flowers on ‘Arnold Banner’ still argues for a two-layered periclinal chimeric apical meristem. Clearly, without further study, the specific developmental mechanisms resulting in the anthocyanin patterning characteristic of this new cultivar, remain unknown.
Flower with sectorial (incomplete) reversion to wild type (A = 10-68*B). Flower clusters with ‘Arnold Banner’ type flowers and fully reverted wild-type flowers on an ‘Arnold Banner’ plant (B = 10-68*A).
Citation: HortScience 58, 12; 10.21273/HORTSCI17393-23
Flower with sectorial (incomplete) reversion to wild type (A = 10-68*B). Flower clusters with ‘Arnold Banner’ type flowers and fully reverted wild-type flowers on an ‘Arnold Banner’ plant (B = 10-68*A).
Citation: HortScience 58, 12; 10.21273/HORTSCI17393-23
Flower with sectorial (incomplete) reversion to wild type (A = 10-68*B). Flower clusters with ‘Arnold Banner’ type flowers and fully reverted wild-type flowers on an ‘Arnold Banner’ plant (B = 10-68*A).
Citation: HortScience 58, 12; 10.21273/HORTSCI17393-23
An examination of flowers on C. canadensis ‘Sjo’ indicates that the flowers of this eastern redbud sport are very similar to those of ‘Arnold Banner’ (Fig. 3) and that ‘Sjo’ also resulted from a mutation that led to a stable periclinal chimeric shoot apical meristem. ‘Sjo’ was discovered in 2001 in Riley County, KS, in a field production plot as a branch mutation and is sold under the moniker Cotton Candy™. Flowers on an accession of ‘Sjo’ growing at the Arnold Arboretum (88-2020*A) show clear pink nectar guides (although less intensely pink, compared with ‘Arnold Banner’) and a minimal amount of very light scattered pink blushing on the other petals (again similar to ‘Arnold Banner’).
The habit of ‘Arnold Banner’ is similar to that of the species—low-branching and multistemmed close to the ground with a spreading vase to rounded form. Twelve-year-old trees at the Arboretum have a height span of 4.3 to 5.8 m and a width of 5.5 to 5.8 m. ‘Arnold Banner’ has an intermediate growth rate and has performed well in full sun to light shade in a wide range of soil moisture levels. Proper structural pruning at a young age helps negate included bark and tight branch angles typical of C. canadensis.
Dissemination of Cultivars
Grafted stock plants and scions were provided to Arnold Arboretum nursery partners through the “Arnold Selects” PI program (https://arboretum.harvard.edu/plants/plant-introductions/arnold-selects/)—Broken Arrow Nursery, Hamden, CT; Brotzman’s Nursery, Madison, OH; Cricket Hill Nursery, Thomaston, CT; Heritage Seedlings & Liners, Salem, OR; Pleasant Run Nursery, Allentown, NJ; and Weston Nurseries, Hopkinton, MA—in 2021. Arboretum propagators have had the highest success pot grafting ‘Arnold Banner’ in winter using a whip and tongue graft. C. canadensis cultivars are traditionally budded in late summer onto seedling rootstocks.
C. canadensis reliably grows in US Department of Agriculture (USDA) hardiness zones 4 to 9 (Kidwell-Slak and Pooler 2018). Our nursery partners report robust growth of ‘Arnold Banner’ in their respective USDA hardiness zones ranging from 6a to 8b.
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