Today’s photo features a purple-leaved cultivar of ninebark, Physocarpus opulifolius ‘Monlo’.
The wild, eastern North American Physocarpus opulifolius is an attractive species in its own right. It provides interest in the garden nearly any time of year. The tall, arching stems form a vase-shaped bush that can be used in borders, hedges, or as a specimen plant. Abundant, fluffy corymbs of white flowers in late summer are followed by dark glossy red fruits that persist into late autumn (fading to a rosy tan). In winter, the stems take the spotlight. The bark exfoliates in narrow strips, revealing layers of different hues that inspire this species’ common name.
The cultivar ‘Monlo’ takes the ornamental merits of Physocarpus opulifolius to the next level. The selection was discovered growing in a field along with 120000 other Physocarpus opulifolius seedlings at Kordes Nursery in Germany. One seedling had unusual red foliage; its potential was recognized by the nursery owners
, who designated it as the cultivar ‘Monlo’. Physocarpus opulifolius ‘Monlo’ offers all of the same gardening qualities as its native counterpart, but in addition boasts deep-burgundy foliage. As you can see from today’s photo, the crinkled leaves look particularly stunning when contrasted with this cultivar’s pink-tinged flowers or red fruits. The combination of these qualities has earned Physocarpus opulifolius ‘Monlo’ the Royal Horticultural Society’s (RHS) Award of Garden Merit. If you are seeking this cultivar for your garden, it is typically marketed under the registered trademark Diabolo® (though many sources incorrectly treat the trademark name as the cultivar name, or sometimes drop the first “o” from the name).
As a native plant enthusiast, I feel an unjustified sense of pride when a species from my own continent gains international acclaim. It’s nice to know that plants that are (at least) similar to species supporting wild North American ecosystems are finding their way into gardens worldwide. Still, a number of studies have indicated that native plant species result in higher insect diversity than exotic plantings (see Wilde, Gandhi and Colson 2014). I wondered, however, whether a Physocarpus opulifolius ‘Monlo’ planted within the wild ninebark’s range could contribute to local ecosystems as much as the native species would. Two entomologists from the University of Minnesota, Emily Tenczar and Vera Krischik, attempted to answer this question. They tested which ninebark plants that specialist ninebark beetles (Calligrapha spiraeae) preferred to eat and lay their eggs on. These beetles were presented with the native Physocarpus opulifolius as well as its cultivars ‘Monlo’ and ‘Dart’s Gold’. The ninebark beetles seemed indifferent to whether they were making use of the native species or ‘Dart’s Gold’, but avoided laying their eggs or eating the cultivar ‘Monlo’. At least for this species of insect, it didn’t matter whether a plant was wild-grown or cultivated. Instead, the species or cultivar’s specific characteristics were what made the difference. Tenczar and Krischik posit that the high levels of foliar anthocyanins found in the cultivar ‘Monlo’ were the likely reason that the beetles favoured the other ninebarks.
How does a gardener considering pollinators and wildlife decide whether to plant native species or cultivars (and if cultivars, which ones?)? The article, Native Cultivars – Good, Bad, and Ugly by Vincent Vizachero provides a straightforward answer. The article suggests staying away from any cultivars that promise “improved insect resistance” (as Physocarpus opulifolius ‘Monlo’ does). Vizachero also recommends sticking with cultivars that have similar flower shape, berry size and leaf colour as the original species. Sounds like good advice, but I’m not sure I am willing to give up the deep purple leaves of ‘Monlo’ ninebark in exchange for my region’s native ninebark, Physocarpus capitatus.