<em>Diervilla</em>: planting, pruning and care

Description and botany

Genus Diervilla comprises only three species, lonicera, rivularis and sessilifolia, with an intergeneric hybrid D. x splendens between lonicera and sessilifolia. It belongs to family Caprifoliaceae like honeysuckle (Lonicera) and weigela (Weigela), which it closely resembles though with less spectacular flowers. The genus was placed in family Diervillaceae together with Weigela under APG II (2003) then reintegrated into Caprifoliaceae in APG III (2009). Genus is native only to North America, eastern United States.

These bushes have a divergent bushy habit that often reaches little more than 1.30–1.50 m in height and can be used to cover ground with varying speed depending on species. Diervilla lonicera, most vigorous of the species, is also used to counter invasive species or to stabilise slopes. Red colour of young wood makes its suckers easy to spot. Diervilla rivularis grows larger, reaching about 2 m in all directions. Other species remain around 1.30–1.50 m in height and spread.

Diervillas generally have deciduous, opposite, lanceolate leaves, light to medium green with slightly dentate margins. Species sessilifolia is evergreen and shows reddish venation on leaves borne on arching shoots. Cultivar Cool Splash has dark green leaves marginate with cream. Bronze-coloured young shoots in Diervilla sessilifolia and x splendens precede flowering in terminal or axillary clusters. Autumn also offers a display with lovely leaf colours in yellow, orange and purple shades. Diervilla x splendens delivers splendid hues from summer, when abundant clusters of golden-yellow flowers stand out against dark green foliage tinged with purplish red even in shade. D. splendens ‘Diva’ turns purple year-round before shifting to purplish-red in autumn.

Flower of Diervilla

Diervilla flowers have a calyx of 5 fused sepals elongated, a corolla in a tube 1–2 cm long, more or less bilabiate, opening to 5 narrow lobes, with 5 stamens and 1 prominent pistil. As flowers fade, colour becomes creamy white before small dark red berries form. Diervilla-honeysuckle (D. lonicera) is earliest-flowering, blooming in May–June. Others flower in early summer.

Diervilla owes its name to French surgeon and writer Sieur de Dierville (1699–1711), who brought back Diervilla lonicera from his voyage to Acadian region of Canada in 1699. Tournefort dedicated genus name to him.

Main varieties of Diervilla

Diervilla splendens

This horticultural hybrid with a spreading habit and compact growth, reaching 2 m across, makes a perfect groundcover for dry, shaded to partly sunny spots. Its handsome dark green leaves with purple tones in summer turn purple-orange in autumn. Clusters of pale yellow flowers appear in summer on new shoots with red wood. This Diervilla will provide abundant flowering, even in shaded positions, where it particularly thrives.

• Flowering time August, September
• Height at maturity 1,20 m

See the product sheet

Diervilla sessilifolia Cool Splash

Compact bush with green leaves variegated with white. Very hardy, fills shady areas of your garden.

• Flowering time July, August
• Height at maturity 1,20 m

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Diervilla splendens Diva

This new Diervilla develops purple foliage that takes on scarlet tones in autumn. Its summer flowering, with a sulphur-yellow corolla and red calyx, contrasts beautifully with the dark foliage. Ideal for carpeting awkward spots in your garden, in dry shade where nothing will grow!

• Flowering time August, September
• Height at maturity 80 cm

See the product sheet

Planting

Where to plant Diervilla?

Diervillas are understorey bushes that tolerate dry soil and all aspects but prefer part-shade to shade, under a tree or near a building. They can easily establish between tree roots thanks to their suckering habit and low requirements.

They tolerate all well-drained soils from fresh to dry, though not excessively calcareous. Their hardiness is excellent, around -25°C.

When to plant?

Diervilla is best planted in autumn or spring to ensure good establishment.

How to plant?

This plant is easy to grow and should suit all areas of garden disadvantaged by excess dry shade. Space plants 1 to 1.50 m apart to cover soil or form a semi-open barrier of medium height.

• Plunge the bucket into a tub of water to moisten it thoroughly.
• Dig a hole three times wider than the rootball and loosen soil around it with the tines of a digging fork.
• In heavy soil, add a few shovelfuls of sand or grit to ensure good drainage around roots.
• Add well-rotted manure or compost if soil is sandy.
• Place the plant in the planting hole.
• Replace soil and firm lightly.
• Water.

Planting in pots is not recommended given the bush’s suckering nature.

Diervilla sessilifolia ‘Cool Splash’ with variegated, bright foliage

Care, pruning

Diervillea is not very susceptible to diseases and pests.

Prune after flowering. Simply cut off inflorescences. Then, every 3-4 years, renew the whole clump by cutting back hard in winter or in early spring to give it a more compact shape.

Multiplication

Single most straightforward method of propagation is to separate suckers in autumn.

Separating suckers

Diervilla forms suckers that are easy to separate, preferably in autumn, with a sharp tool once they have formed roots. Replant them straight away, cutting back the stem if it is too long.

Uses and companion plants

Diervilla are groundcovers easy to establish in well-drained soils not excessively calcareous, for almost any aspect. They help to stabilise steep slopes and limit spread of invasive plants such as brambles or ivy.

They can also be used to green a woodland understorey, brightening it with their sulphur-yellow flowering, colourful shoots, autumnal display or the effect of cream-variegated foliage of certain cultivars such as Cool Splash. Their modest height is ideal for small gardens or enclosed spaces. They can establish in sandy soil and stabilise dunes or embankments thanks to their suckers (underground stems bearing roots).

An example of a planting combination: Diervilla splendens ‘Diva’, Nepeta faassenii ‘Six Hills Giant’, Rose ‘Iceberg’, Hardy geranium ‘Azure Rush’

They can also be planted within a plot of standard roses, accompanied by undemanding perennials such as Nepeta, hardy geranium or artemisia. Exposure should not be too hot. In southern France, prefer partial shade or woodland shade, for example alongside Mahonia, acanthus or laurustinus, or on the north-facing side of your house. Le Diervilla sessilifolia Scool splash spreads by root suckers, so plant it in shade. It will brighten dark areas of your garden, planted alone or in a bed to dress base of a tree. In a bed, pair with shade-loving perennials without mixing colours too much.