A genetic mutation discovered at the Arboretum gives rise to a new cultivar

Among the wonderful benefits of the Arnold Arboretum—where plants gathered from around the world grow side-by-side under the watchful care of staff experts—is that when something interesting or unusual happens, it typically gets noticed. Case in point is a spontaneous mutation of a single branch (or sport) of an Eastern redbud tree in the Arboretum’s collection first observed by a staff member in 2009. After more than a decade of research and testing, the Arboretum has introduced a beautiful new redbud cultivar, Cercis canadensis ‘Arnold Banner’, published last month in HortScience magazine.

What sets ‘Arnold Banner’ apart from other Eastern redbuds, which are distributed across a wide swath of the eastern U.S. from New England to Florida and west to Texas and northeastern Mexico, is its flower color—or rather, the almost total absence of color. This species in the pea family grows as a large shrub or small tree and is known for its pink to magenta clusters of flowers that appear in early spring before the plant leafs out. Flowers emerge directly from the trunk and branches on old wood—a botanical phenomenon known as cauliflory—producing a spectacular floral display which covers the entire plant. While this is also true of ‘Arnold Banner’, the petals are mostly white and retain pink striping on the banner petal. Termed “nectar guides,” these markings are presumed to play a role in alluring pollinators.

Composite showing comparsion of redbud flowers and variation of color between magenta for the species and white for 'Arnold Banner' and other cultivars.
From the research published in HortScience on the introduction of the ‘Arnold Banner’ redbud cultivar by the Arnold Arboretum. This composite image compares 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). HortScience 58:12.

The original plant bearing the ‘Arnold Banner’ mutation germinated in 1968 from seed collected from another Eastern redbud in the Arboretum’s collection (AA accession 22870*J), a plant of unknown origin originally accessioned in 1950. In May 2009, Arboretum Putnam Fellow Abby Meyer (nee Hird, now executive director of Botanic Gardens Conservation International-US) first observed an aberrant white-flowering branch on the tree. Arboretum staff monitored the sport to characterize the mutation and harvested cuttings for clonal propagation at the Dana Greenhouses. Although the original shoot bearing the mutant flowers died in 2015, the clones cultivated from it form the basis for this new introduction.

Grafted stock plants and scions of ‘Arnold Banner’ were planted at the Arboretum and shared with commercial nursery partners to field test as part of the evaluation protocols the Arboretum follows to officially introduce new plant selections. These tests—ongoing over the past decade since its discovery—show robust growth of ‘Arnold Banner’ across the USDA hardiness zones of testing sites ranging from 6a to 8b. While its flowers make it stand out among typical Eastern redbuds, ‘Arnold Banner’ appears to hold true to other characteristics recognized in the species. This includes a low branching and multi-stemmed habit, which tends toward a spreading vase to rounded form. It performs well in full sun to light shade in a wide range of soil moisture levels.

Over the past century, several white-flowered cultivars have appeared commercially, including ‘Alba’, ‘Royal White’, and ‘Texas White’, which produce pure white flowers without a trace of red pigmentation. ‘Arnold Banner’ appears to be only the second instance of a mostly white flowered redbud to be recorded. “We are thrilled to introduce this new redbud variant as an example of how nature contributes to horticulture,” remarked Arboretum Director William (Ned) Friedman), Arnold Professor of Organismic and Evolutionary Biology at Harvard University, who wrote about ‘Arnold Banner’ for the Arboretum’s magazine Arnoldia a decade ago, shortly after its discovery in the landscape. “Many of the plants that have become superstars in American nurseries are the product of the natural process of mutation, which underpins all evolution. Random mutations that lead to novel and desirable characteristics, like the flower color and nectar-guide coloration exhibited by ‘Arnold Banner’, are rare. Our introduction of this plant is a celebration of this ongoing evolution, and the role that institutional plant collections can play in their discovery, evaluation, and appreciation.”

As a botanical institution committed to advancing horticulture and plant research as part of its mission, the Arnold Arboretum has introduced many new cultivars of woody plants over the course of its 152-year history. Some of these plants selected by Arboretum propagators have become notable in the trade include the ‘Arnold Promise’ witch-hazel (Hamamelis × intermedia ‘Arnold Promise’), ‘Donald Wyman’ crabapple (Malus ‘Donald Wyman’), and ‘Lilac Sunday’ lilac (Syringa × intermedia ‘Lilac Sunday’). Like ‘Arnold Banner’, some were discovered by staff growing in the Arboretum and were identified as something unique. In recent years, the Arboretum has renewed its commitment to introducing new plant varieties and promoting historical ones through the Arnold Selects program.