Physocarpus is a name familiar to many, yet these shrubs are rarely grown in gardens or specified in designs, which is a shame because they offer a dramatic foliage backdrop of purple or yellow leaves, contrasting with clusters of white flowers in early summer and red fruits in autumn.
There are ten species of these deciduous shrubs, whose foliage has three or five lobes, not unlike currants or maples. They range from the 1m tall P. monogynus to the full height of P. opulifolius, which can mature to 3m, though often stays around 2m in cultivation.
Most commonly grown are the cultivars of P. opulifolius. Arguably the most popular are P. opulifolius 'Diabolo' (PBR) Award of Garden Merit (AGM), which has young bronze-green leaves that turn dark purple, and P. opulifolius 'Dart's Gold' AGM, which is a yellow-leaved variety. Dart's Gold and P. opulifolius 'Nugget' are seen as improvements on the older yellow-leaved variety P. 'Luteus'. The leaves of Dart's Gold are a brighter yellow than those of P. 'Luteus' and the colour is longer lasting.
Physocarpus has the common name ninebark because all the species have exfoliating bark. The genus is within the family Rosaceae and the flowers are cup-shaped, white to pale pink and held in clusters. Because they are generally medium to tall shrubs, they suit being planted in the middle or toward the back of a border. They can also look effective when used in mass plantings in an urban setting.
Whites and pastels, as well as hot colours, stand out well against the dark-purple backdrop of a plant like P. opulifolius 'Diabolo' (PBR) AGM. Nurserywoman Jennifer Matthews from Moorland Cottage Gardens likes planting Crocosmia 'Severn Sunrise' AGM, with its orange and pink flowers in front of its dark leaves. Designer Dan Boyer from Fisher Tomlin would use silvery-leaved Astelia or white flowered specimens such as Viburnum plicatum f. tormentosum 'Mariesii' AGM to contrast with it.
Physocarpus are all hardy plants, some species coping down to -25 degsC. They like to be planted in acid soils, though P. opulifolius will thrive almost anywhere. Physocarpus like full sun but moist conditions. P. malvaceus is more tolerant of dry soils.
Pruning should take place after flowering in June, if necessary. The shrubs can be thinned out by cutting back old and overcrowded shoots. Propagation can be carried out by removal of suckers, by seed or from softwood cuttings in a closed case with bottom heat. There are few pests that affect Physocarpus but growers should watch out for leaf spot, powdery mildew and fireblight.
What the Specialists Say
- Jennifer Matthews, owner, Moorland Cottage Plants, Pembrokeshire: "People like them for their foliage colour and shape. Physocarpus opulifolius 'Dart's Gold' AGM has bright yellow foliage and doesn't scorch like P. opulifolius 'Luteus' does, at least not here in West Wales.
"Then there's P. opulifolius 'Diabolo' (PBR) AGM, with dark bronze-purple foliage and pale pink flowers. These are followed by tiny red fruits that resemble rosehips. It's a good seller, and grows to about 2.5m tall. P. opulifolius Lady in Red (PBR) is more compact, with similar flowers and fruits. Its foliage is purply-red, like a Cotinus or Berberis ottawensis f. purpurea 'Superba' AGM. P. opulifolius 'Diablo d'Or' also sells well. Its leaves are sage green, turning orange and yellow through to rustic red. It's slightly smaller, between P. opulifolius 'Diabolo' (PBR) AGM and P. opulifolius Lady in Red (PBR)."
- James Toone, owner, Goscote Nurseries, Leicestershire: "It's the foliage that makes them - all of sudden they come into their own. We have two here in our show garden, the two most established varieties - Physocarpus opulifolius 'Diabolo' (PBR) AGM and P. opulifolius 'Dart's Gold' AGM. A few variants have been introduced onto the market, including P. opulifolius 'Diablo d'Or', but the originals are still hard to beat.
"As for cultivation, they need a sunny aspect, otherwise they lose their colour. Because they can reach 2-2.5m high, they are best in the middle or back of the border. They are quite a thug in some ways. If they stay in a pot they grow quite big quite quickly and they dry out - I wouldn't recommend using them as patio plants. But they respond well to being hacked down so can be controlled."
- Kevin Marsh, grower, Beeches Nursery, Essex: "They are becoming popular and are being bred left, right and centre. One of the new, improved varieties is the yellow-leaved P. opulifolius 'Nugget'. It can be used instead of the more established yellow-leaved variety P. opulifolius 'Luteus', which becomes scorched in the sun.
"There is also P. opulifolius Lady in Red (PBR) and P. opulifolius 'Summer Wine' (PBR) or P. opulifolius 'Seward', both of which have red to wine-coloured foliage. Physocarpus are no trouble at all. It is a simple, deciduous shrub, with pinky-white flowers in late spring. It has no major pests but should not be allowed to get too dry."
- Dan Bowyer, director, Fisher Tomlin: "It's quite an unusual shrub. We've used P. opulifolius 'Diabolo' (PBR) AGM in schemes before. The genus has a fantastic shape and foliage colour. They have a nice dark tone to their leaves, similar to Corylus.
"The purple foliage types are great as a backdrop for lighter colours. Try planting silvery plants such as Astelia or white-flowered specimens like Viburnum plicatum f. tormentosum 'Mariesii' AGM against them. The silvery or white tones really stand out. Yellow-flowered plants also contrast well."
Species and cultivars
- P. malvaceus is an elegant, medium-sized shrub that resembles Spiraea. It has dark-green leaves and dense umbels of white flowers in June.
- P. monogynus is a small shrub with leaves with three to five lobes, resembling currant foliage. They are dull green above and paler below. It has white or pink-tinged flowers in the summer. Height: 1.2m. Spread: 2m.
- P. opulifolius is a vigorous, medium-sized shrub. It has mid-green three-lobed leaves that turn yellow to bronze in autumn. Whitish-pink flowers in May and June. Height: 2-3m.
- P. opulifolius 'Angel Gold' has lemon-yellow leaves, with a thin copper margin in the spring, which turns yellow later. White flowers are borne in the summer, followed by red fruit that turns purple.
- P. opulifolius 'Center Glow' has interesting leaves that have a lovely red colour and a bright yellow centre when young. The foliage matures to a deep wine colour. Light-pink buds open to clusters of white flowers in summer. Height: 3m. Spread: 2.4m.
- P. opulifolius 'Dart's Gold' AGM is a tall shrub with golden yellow foliage on arching branches. Small pink-tinged white flowers are borne in early summer, followed by clusters of bladder-like red fruit with green flushes. Height: 2m. Spread: 2.5m.
- P. opulifolius 'Diablo D'Or' (PBR) has rich dark foliage, burnished gold when young, and pink flowers that appear in early summer. Height and spread: 2m.
- P. opulifolius 'Diabolo' (PBR) AGM is a tall variety with white flowers and dark purple leaves, which start out bronze-green when young. Height: 2m. Spread: 2.5m.
- P. opulifolius Lady in Red 'Tuilad' (PBR) has brilliant chestnut red to wine foliage on arching branches. Small pink-tinged white flowers are borne in early summer. Height and spread: 2.5m.
- P. opulifolius 'Luteus' has yellow spring foliage that turns to green by early summer and develops bronze tones as the season progresses.
- P. opulifolius 'Nugget' has yellow to lime-green foliage that stands out against the exfoliating, cinnamon-coloured bark. White flowers are borne in clusters along the stems in early summer, followed by dark brown seed capsules that look good in winter.
- P. opulifolius Summer Wine 'Seward' (PBR) is the result of a cross between 'Nana' and 'Diabolo'. It has attractive burgundy foliage and a dense compact habit, remaining about half the size of 'Diabolo'. Recommended used against yellow and bright green plants, it produces white button-like blooms in spring.