I’ll admit I used to think ninebarks (Physocarpus spp.) were boring. When I went to horticultural college in Alberta during the 1980s, pretty much just the common ninebark was available. Occasionally, the yellow-leafed form ‘Luteus’ was offered, but perhaps we didn’t have a full appreciation of yellow foliage at that time—it was really considered more of an oddity. So until recently, I considered these medium- to large-sized, coarse-textured shrubs to have no particular redeeming features other than to fill space in large municipal plantings.
Almost 20 years later, the appearance of the purple-leafed selection ‘Diabolo’, developed in Germany and introduced here in the late 1990s, made me change my mind. In a flash, this plant was everywhere. Every nursery was selling it, and every magazine was raving about it—perfect timing, since gardeners suddenly had a new appreciation for unusual foliage colour.
‘Diabolo’ was a breeding breakthrough. As so often happens, this one development paved the way for further ninebark introductions, with a half-dozen new selections now available in a range of sizes and colours, including new dark purple to burgundy-red varieties, yellow- to gold-leafed selections and even a green-leafed Canadian cultivar, ‘Snowfall’.
Ninebark has other virtues as well. Its peeling bark (hence its common name) on older stems reveals contrasting inner bark in shades of red to tan in winter. As a wildlife plant, the flowers provide nectar for butterflies, the seeds are loved by birds and the stems provide nesting sites. Canadian gardeners don’t have a great many native shrubs to choose from, but common ninebark fits the bill, growing wild from Nova Scotia west to southern Manitoba, and south into Colorado and Florida.
Ninebark tolerates a wide range of soil conditions, preferring average to moist locations but tolerating dry sites once established. It has no particular pest or disease problems, which explains its long and successful use in civic plantings in the Prairies, where they receive little care. The shrub is hardy to Zone 3, though testing in Alberta has found that certain selections, such as ‘Diabolo’ and ‘Nugget’, do well in Zone 2. Full sun to part shade is fine, but the coloured-leaf varieties develop their brightest tones in sunny locations.
The little flowers, which appear in early to midsummer, are white to soft pink, held together in clusters 2.5 to five centimetres wide, similar in effect to their close cousin Spiraea. I’ve never thought of the green species as being particularly showy in bloom, but the flowers do stand out in the newer darker-leafed introductions. Ninebark produces flowers on old wood from the previous season, followed by dangling clusters of reddish brown seed capsules that are attractive in late summer and into autumn.
The shrub’s foliage is rounded, with enough of a lobe or indent to make it almost maple-shaped. Fall colour for green-leafed forms is basically a ho-hum yellow, but the red- to purple-leafed selections, and some of the yellow ones, produce reliable red autumn tones. While these shrubs are interesting enough to be used as specimens, they also make a nice informal hedge.
What I love best about ninebark is the way it tolerates different pruning methods. When left alone for years to grow into a medium to large shrub, the only care it needs is to remove dead wood, or to thin out a quarter of the stems each year in the winter or spring, right down to the base, to maintain a bushy habit. This rejuvenates the plant, helping to prevent a leggy appearance, and encourages the production of flowers and fruit.
Ninebark also responds well to being clipped back by half every few years in order to keep it in check, particularly if used in a foundation planting. This should be done right after flowering, so the shrub has time to form new buds for the following season.
The easiest pruning method of all, however, is to cut ninebark back to a height of 15 to 20 centimetres in the spring before it leafs out. This technique is known as coppicing, and results in a flush of new growth with particularly large and lush foliage—absolutely stunning in the coloured-leaf forms—though it comes at the expense of both blooms and seedheads. The lush growth and more compact size resulting from hard pruning makes ninebark especially wonderful in a perennial border, placed in the middle to back. Plants should be allowed to establish for two to three years before using this method, however, and it should be done only every other year since it temporarily weakens them.