<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), to which it bears a strong resemblance despite less spectacular flowers. The genus was included in family Diervillaceae with Weigela under APG II (2003) then reincorporated into Caprifoliaceae in APG III (2009). The genus is native only to eastern North America.

These bushes have an outward-spreading bushy habit that often scarcely exceeds 1.30–1.50 m in height and can cover the ground more or less quickly depending on species. Diervilla lonicera, the most vigorous of them, is used to counter invasive species or to stabilise banks. The red colour of the young wood makes its suckers easy to spot. Diervilla rivularis reaches greater dimensions, up to 2 m in all directions. Other species remain between 1.3 and 1.5 m in height and spread.

Diervillas have leaves that are generally deciduous, opposite and lanceolate in shape, light to medium green in tone with slightly dentate margins. Species sessilifolia is evergreen and displays reddish venation on leaves borne on arching shoots. Cultivar Cool Splash has dark green leaves margined with cream. The bronze-coloured young shoots of Diervilla sessilifolia and x splendens precede flowering in terminal or axillary clusters. Autumn also provides spectacle with a pleasing range of foliage colours in yellow, orange and purple tones. Diervilla x splendens produces splendid colour from summer onwards when its abundant clusters of golden-yellow flowers stand out against dark green foliage tinged with purplish red even in shade. D. splendens Diva displays purple all year before turning violet-red in autumn.

Diervilla flower

Diervilla flowers have a calyx with 5 fused elongated sepals, a tubular corolla 1 to 2 cm long, more or less bilabiate, open to 5 narrow lobes, 5 stamens and a prominent pistil. On fading the colour becomes creamy white before small dark red berries form. The diervilla-honeysuckle (D. lonicera) is the earliest of them, flowering in May–June. Others bloom in early summer.

Diervilla owe their name to French surgeon and writer Sieur de Dierville (1699–1711), who brought Diervilla lonicera back from his 1699 journey to the Canadian region of Acadia. Tournefort dedicated the genus name to him.

Main varieties of Diervilla

Diervilla splendens

Hybrid of horticultural origin with a low-growing spreading habit reaching 2 m across, it makes a perfect groundcover for dry, shaded to partly sunny areas. Its handsome dark green leaves take on a purplish tint in summer and turn purplish-orange in autumn. Pale yellow clusters of flowers appear on new, red-wooded shoots in summer. This Dierville will provide abundant flowering, even in shaded positions, which it particularly favours.

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

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Diervilla sessilifolia Cool Splash

Compact bush with green leaves variegated with white; very hardy, it fills shaded 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 hues in autumn. Its summer flowering, with sulphur-yellow corolla and red calyx, contrasts beautifully with the dark foliage. Ideal for carpeting difficult spots in your garden, in dry shade where nothing grows!

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

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Planting

Where to plant Diervilla?

Diervillas are understorey bushes that tolerate dry soil and all exposures but prefer semi-shaded to shaded positions, under a tree or close to a building. They can easily establish between tree roots thanks to their suckering habit and low requirements.

They tolerate all well-drained soils from moist to dry, though not excessively calcareous. 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 excessive dry shade. Space young plants 1 to 1.5 m apart to cover soil or form a partially screening barrier of medium height.

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

Pot planting is not recommended given suckering habit of bush.

Diervilla sessilifolia ‘Cool Splash’ with bright variegated foliage

Care, pruning

Diervillea is not very susceptible to diseases and pests.

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

Propagation

Easiest propagation is by separating suckers in autumn.

Separation of suckers

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

Uses and associations

Diervilla are groundcovers easy to establish in all types of well-drained soil not too calcareous, for almost any aspect. They help to stabilise sloping ground and limit spread of invasive plants such as brambles or ivy.

They can also be used to vegetate a woodland, which they will brighten with their sulphur-yellow flowering, their coloured shoots, their autumnal display or with cream-variegated foliage of some cultivars such as Cool Splash. Their small stature is ideal for small gardens or confined spaces. They can establish in sandy soil and stabilise dunes or banks thanks to their suckers (underground stems bearing roots).

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

They can also be planted within a rose bed trained as standards, accompanied by undemanding perennials such as Nepeta, hardy geranium or artemisia. Position should not be too hot and exposed. In south of France, opt for partial shade or woodland shade, for example with Mahonia, acanthus or laurustinus, or on north-facing side of house. The Diervilla sessilifolia ‘Cool Splash’ spreads by root suckers, plant it in shade. It will brighten dark parts of garden, as a specimen or in a mixed border to dress the base of a tree. In borders, pair it with shade-loving perennials without mixing colours too much.