Physocarpus opulifolius (L.) Maxim ‘Donna May’: A New Compact, Purple-leafed Landscape Shrub

‘Donna May’ is a new vegetatively propagated cultivar of Physocarpus opulifolius (L.) Maxim (common names include ninebark, common ninebark, and eastern ninebark). ‘Donna May’ has a compact, well-branched growth habit ( 1 m in height and width), purple foliage, powdery mildew resistance, and light pink flowers in midto late spring. It is adapted to USDA cold hardiness zones 3 to 7 and does well across a wide range of soil types. The compact, dense growth habit of ‘Donna May’ offers new versatility for purple purple-leafed ninebark. ‘Donna May’ can readily be integrated into containers and used in confined landscape beds as a manageable specimen, grouping, and/or hedge. Physocarpus opulifolius (L.) Maxim is a deciduous woody shrub in the family Rosaceace and subfamily Spiraeoideae. The species is diploid (2n = 2x = 18) and exhibits selfincompatibility (Dickson et al., 1992; Sutherland et al., 2008). Plants typically grow up to 3 m in height and are native throughout the majority of the eastern half of the United States and Canada with the notable exception of Texas, Louisiana, and Mississippi. Ninebark grows in diverse habitats, including woodland edges, near streams, upland slopes, and rock outcrops (U.S. Department of Agriculture, Natural Resources Conservation Service, 2012). The species is tolerant of variable soil conditions (including a range of soil moisture levels, pH, and textures) and grows well in partial to full sun (Dirr, 2009). Older stems on maturing plants develop peeling bark that separates in multiple thin layers (inspiring the name ninebark). Leaves are alternately arranged, simple, petiolate, and typically have three to five pronounced, pointed lobes (Dirr, 2009). Five-petaled, typically white flowers last for 3 d each and are produced in corymbs in midto late spring. Individual flowers open concentrically from the outer edge to the center of corymbs over approximately a 3-week period. Eastern ninebark is becoming increasingly popular as a landscape shrub. Greater variation in plant size and leaf color in newer cultivars has especially contributed to its popularity. ‘Monlo’ (Diabolo , frequently incorrectly spelled Diablo ) was the first commercialized purple-leafed cultivar. It was discovered as a unique seedling at the German nursery Kordes Jungpflanzen in 1968. ‘Monlo’ was issued a U.S. Plant Patent in 2000 (U.S. Plant Patent 11,211) and introduced in North America by Monrovia Nursery (Azusa, CA). The rich purple foliage and large plant size of ‘Monlo’ brought eastern ninebark to the forefront as a more densely growing alternative to purple-leafed sand cherry [Prunus 3 cistena (N. E. Hansen) Koehne], a predominantly used large purple-leafed shrub. In addition, golden-leafed forms (i.e., ‘Luteus’ and ‘Dart’s Gold’) and the compact green-leafed ‘Nanus’ have been landscape staples for decades. Several recent cultivars combine different foliage colors (gold and purple foliage on an individual plant) or foliage colors other than green on more compact plants. Helping to spur on the increase in new cultivars is the fact that purple and gold foliage color in ninebark are simply inherited and easily recovered in progeny. Purple and gold foliage are controlled by different single genes (Pur and Aur located on linkage groups I and III, respectively) and by dominant alleles (Sutherland et al., 2008). Additional reasons for the increased popularity of ninebark is that it is a durable, easy-to-grow U.S. native shrub and can be used as an alternative to purple and yellow-leafed barberry in regions where barberry is deemed invasive (Lubell et al., 2011) or in situations in which the spines of barberry are a liability. Ninebark is relatively pest-resistant but can be significantly impacted by powdery mildew (Phyllactinia guttata, Podosphaera aphanis var. physocarpi, and Podosphaera macularis) and feeding damage from the specialist ninebark beetle (Calligrapha spiraeae). Lubell et al. (2011) surveyed several popular purple, gold, and green-leafed cultivars of ninebark for powdery mildew resistance. Great variability was found among the cultivars; ‘Nanus’ was the only cultivar that did not exhibit infection, ‘Nugget’ had the greatest infection, and ‘Monlo’ was intermediate. Podosphaera aphanis var. physocarpi only infects ninebark (the others are generalists), and plants infected with this fungal species often display witches’ brooms covered in mycelium (i.e., thickened, stunted stem and leaf growth at the terminals). Infected tissue typically dies and becomes unslightly. The generalist fungi typically produce infections more uniformly over plants and do not lead to witches’ brooms. The specialist ninebark beetle feeds on the leaves of ninebark. Tenczar and Krischik (2007) compared feeding and reproductive tendencies of the beetle on ‘Monlo’, ‘Dart’s Gold’, and a wild-type ninebark. They found that the beetles preferred ‘Monlo’ the least and those forced to feed on only ‘Monlo’ had the lowest fecundity. They suggest this may be the result of the high anthocyanin content in ‘Monlo’. Anthocyanins are known to deter feeding of some insects and bind with nitrogen resulting in a less nutritious food source (Tenczar and Krischik, 2007).