<em>Ceratostigma</em>, Chinese plumbago: planting, pruning and care

Description and botany

Ceratostigmas are herbaceous perennials or small spreading bushes with slender, densely ramified stems belonging to family Plumbagos, a name they formerly bore. Genus comprises eight species native to Himalaya and Far East, with one species native to Somalia. They are commonly called Chinese plumbagos to distinguish them from Cape plumbago, a much less hardy bush from South Africa in pale blue or white tones often seen on Mediterranean coasts. They belong to family Plumbaginaceae.

Ceratostigma plumbaginoides (syn. Plumbago larpentae) is sometimes called creeping plumbago, due to its aptitude to colonise soil via its running reddish rootstocks. Fast-growing, this herbaceous plant is extremely ground-covering even though it dies back in winter and starts late around May. Brought to Germany from western China in 1834 by Alexander von Bunge, it made a second notable appearance in England in 1846 from Shanghai via the noble Larpent family who helped spread it.

This perennial grows in rather dry, poor, sunny rocky sites. Plant does not exceed 30 cm in height but can spread over large distances in cool ground. It is also more frost-hardy (-20 °C) than the bushy species (-12 to -15 °C), which can nevertheless reshoot from the stump.

Ceratostigma plumbaginoides – botanical illustration

From its woody stump emerge slender stems with quadrangular section, often purplish, bearing alternate, overall ovate leaves with pointed tip, 3–5 cm long and 2.5–3 cm wide. Lamina is bright green, edged with dark cilia, brownish to purplish. Foliage often takes bronze hues then bright raspberry-red with first frosts from September. It falls fairly late in autumn. Colour intensity is all the stronger when plant is exposed to sun.

Bushy species are deciduous or semi-evergreen and display a bushy habit slightly wider than tall, reaching 80–100 cm depending on species. Wilmott’s plumbago (C. willmottianum), native to Sichuan, shows an open habit, more vigorous than Ceratostigma griffithi native to the Himalaya. Its dark green leaves, golden in ‘Sapphire Ring’, diamond-shaped, measure 5 cm and bear fairly long hairs on margins and both faces. Leaves of griffithi are evergreen in mild climates, pubescent and thicker with a spatulate shape, hardly exceeding 2–3 cm long, dull green margined with violet and borne on reddish stems. In all cases, leaves are sessile (without petiole) and alternate. Slender, quadrangular stems reshoot directly from the collar if aerial parts are frozen, and well-drained soil increases frost tolerance of the leadworts. Leaves take on red tones in autumn.

Flowering can begin as early as July depending on climate, notably for plumbaginoides, a little later for others, and continues until frosts in October–November. Corolla of the leadworts, supported by a reddish tubular calyx with fine teeth, forms a long slender tube ending in five triangular lobes marked by a reddish vein that enhances the gentian-blue or cobalt-blue brilliance. Flower diameter is between 1 and 2 cm. Flowers appear and follow one another in axillary corymbs, grouped in loose clusters appearing towards stem tips. They are rich in nectar, attracting butterflies and bees. Stamens and pistils are not very conspicuous. Flower clusters give way to bristly fruits, brownish to rust-coloured.

Genus name Ceratostigma means “horned stigma”, the stigma being the terminal part of the pistil that receives pollen from other flowers. It is divided into five parts, a character common to Chinese plumbagos.

Intense deep-blue flowers of Ceratostigma willmottianum (photo Daderot)

Main varieties of Ceratostigma

Perennial variety

Bush varieties

Ceratostigma plumbaginoides

Perennial with running rootstock that quickly forms an attractive groundcover, even in dry, stony soils. Its small, star-shaped flowers of an extremely vivid gentian-blue stand out from July against dark green foliage that turns bright red to bronze at season's end. Entirely hardy, it is also an excellent rockery or sunny bank plant.

• Flowering time September to November
• Height at maturity 30 cm

See the product sheet

Ceratostigma griffithii

Prostrate, spreading shrub bearing small cobalt-blue flowers on red branches clothed with thick green leaves edged with purple. Evergreen on coastal areas; otherwise it regrows from the stump. Plant in gravelly, rich, very well-drained soils, in a warm exposure on a dry bank or in a rockery.

• Flowering time September to November
• Height at maturity 80 cm

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Ceratostigma willmottianum

This bush, spreading to 1.5 m, produces a myriad of star-shaped flowers in a magnificent intense blue in late summer and autumn. Five-petalled cobalt-blue flowers with a purplish-red tube form clusters along flexible branches clothed with small, dark green, delicate leaves. It can be pruned as desired and tolerates frosts down to -10°C, which sometimes necessitates growing it in a pot.

• Flowering time September to December
• Height at maturity 1 m

See the product sheet

Ceratostigma willmottianum Sapphire Ring

Cultivar with very bright golden foliage.

• Flowering time August to December
• Height at maturity 80 cm

See the product sheet

Planting

Where to plant leadwort?

Plant Ceratostigma in sun or partial shade, in light soil, even poor, calcareous and stony. They appreciate free-draining, gravelly soils, rather neutral to calcareous and adapt thus very well to summer drought. Choose a sheltered spot to make the most of late-season flowering, especially for shrubby varieties. They can therefore be planted against a wall, at the foot of a clematis to shade its base even if soil is poor.

Limit excessive growth of Larpent leadwort in fresh, fertile soil by pulling out its rootstocks, which are easy to extract.

When to plant?

Plant Ceratostigma preferably in spring in cold climate or in autumn to ensure deep rooting before facing summer drought.

How to plant?

This plant is easy to grow.

• Soak rootball in a bucket of water to thoroughly moisten it.
• Dig a hole three times wider than rootball and loosen soil around it with tines of a garden fork.
• Add a few handfuls of sand and gravel to ensure good drainage around roots. In heavy soil, opt for planting on a raised mound or within a rockery.
• Add well-rotted manure or compost if soil is sandy.
• Place plant in planting hole.
• Replace soil and firm down lightly.
• Water.

In a large terracotta pot, place a layer of clay balls at bottom and add good potting compost for flowering plants mixed with draining materials (sand, vermiculite, perlite…). Add slow-release organic fertiliser or in capsule form in spring.

→ Read also : grow a Ceratostigma in a pot.

Ceratostigma (photo by Manuel m.v. – Flickr)

Care

Shorten shoots of shrubby plumbagos by about one third when growth restarts, to thicken the clump. Otherwise remove dried stems by pulling them.

Water during prolonged dry spells, especially the species C. willmottianum, which prefers cool soil, particularly when it resumes growth. Growth is generally fairly rapid unless water is lacking. Place the pot against a sunny wall in winter or bring it into a garage.

This plant is not subject to diseases and parasitic pests in our gardens. Don’t worry if you don’t see it start in spring; it can be rather slow to show its best until mid and late season.

→ Learn more about pruning plumbago in our tutorial: How to prune plumbago?

Propagation: propagation by cuttings, sowing

The simplest method of propagation is to divide the clump, but shrubby dentelaire varieties can also be obtained by propagation by cuttings of terminal parts in May–June.

Propagation by cuttings

Prepare a deep pot by filling it with potting compost mixed with sand.

1. Take tips of herbaceous shoots 10 cm long.
2. Remove leaves located near base of cutting.
3. Insert these to two-thirds of their length, taking care they do not touch each other.
4. Firm soil gently all around to remove air pockets and ensure good contact between potting compost and cutting.
5. Place them under cover in shade, for example by placing a cut transparent plastic bottle over them.
6. In autumn, separate rooted cuttings and plant them in buckets which you keep under a cold frame until spring.
7. Plant out in ground in spring.

→ Learn more in our tutorial : How to propagate Plumbago?

Sowing

Lift natural sowings you can find, particularly around shrubby varieties.

Uses and companion plants

These undemanding, robust groundcover plants accept competition relating to roots so well that they can be planted at the foot of a deciduous tree or bush or a climbing plant. Clematis montana ‘Freda’ offers, for example, pretty dissected leaves tinged with purple in spring and again in autumn, providing a delicate backdrop to the lush electric-blue and bronze clump of a Ceratostigma.

An example of a pairing for a colourful border: Ceratostigma plumbaginoïdes and red Salvia microphylla such as variety ‘Royal Bumble’ in a beautiful deep red

The sunny rockeries, slightly poor banks, low walls, path edges, the base of bushes planted in open ground or in a large pot provide Ceratostigmas with the opportunity to express themselves and spill over generously.
Bush species can form small low hedges with an open habit paired with drought-hardy bushes such as caryopteris, perovskia, potentilla, rosemary, cistus or summer spireas…
Create modern pot displays, flashy and graphic in autumn, by pairing Ceratostigma griffithii or willmottianum with grasses, asters, phormiums, Nandina, Sedum spectabile, silvery foliage of artemisia or santolina, and blue sages (Salvia nemorosa ‘Caradonna’, ‘guaranatica’, ‘uliginosa’)…

→ More pairing ideas with Plumbago in our Advice sheet !