Ceanothus are arguably the best blue-flowered hardy shrubs in the UK. The evergreen types provide fantastically dark-green glossy leaves throughout the year and blue flowers are produced in profusion in spring or summer.
There are actually quite a few species — around 55. They can be either shrubs or small trees and evergreen or deciduous. The majority have blue flowers, but some are white or pink, held in tight panicles. Ceanothus originate throughout the USA, parts of southern Canada and Mexico, but by far the largest concentration is found in California, which leads to their common name, "Californian lilac".
Some are prostrate plants but the majority grown in UK gardens are the taller shrubs, either free-standing specimens or grown as wall shrubs. They are generally highly vigorous, making a decent sized plant within years of planting.
This can fill a space but in a small garden they can takeover and, being the type of plant that has adapted to the Mediterranean climate, they "live fast, die young". So sometimes that beautiful large garden specimen you really treasure may start to fail and there is not much you can do.
They like a position in full sun and well-drained fertile soil. Being Mediterranean they appreciate shelter and planting them against a wall is ideal. Avoid exposed positions, frost pockets and windy corridors, though the deciduous types may tolerate these sites.
They are somewhat lime-tolerant but will become chlorotic on really chalky soil. They seem to do well in seaside gardens. Mulch in late winter or spring but keep the base of the shrub free from mulch. Apply a balanced fertiliser after pruning in the spring.
While the evergreens can be quite vigorous, the textbook method is not to prune them overmuch. However, I have found that not pruning the evergreen types means they get oversized quickly and some careful management is necessary. The RHS handily splits the evergreen types into early- and later-flowering.
The late-summer-flowering group, including C. ‘Autumnal Blue’ Award of Garden Merit (AGM), bear spring flowers on shoots that grew the previous summer along with summer flowers on the current year’s growth. Therefore their shoots are trimmed after spring flowering by one-third to a half.
The late-spring- and early-summer-flowering group, including C. arboreus ‘Trewithen Blue’ AGM, should be pruned after flowering, again by cutting long, flowered shoots by one-third to a half. Should more bushy growth be desired, trim lightly again in late summer. Do not to cut into old wood though because it is unlikely to regenerate.
The deciduous Ceanothus (such as C. × delileanus ‘Gloire de Versailles’ AGM) should be pruned in early to mid spring to produce strong new shoots to bear summer flowers. Prune the main flowered stems of established plants by about a half and cut weak side shoots back hard.
What the specialists say
Kevin Scott, director, Amati Plants, Dorset
"Ceanothus is one of my favourite shrubs for their incredible display of blue flowers, ranging in colour from the very palest blue to dark. The flowers last a long time and from a distance produce this amazing show.
"We find the evergreen shrubs are the most popular. Our top sellers include C. ‘Concha’ with deep-blue flowers and the much lighter-blue-flowered ‘Puget Blue’. Also popular is the large growing C. arboreus ‘Trewithen Blue’, which has a nice rounded dark-green leaf and can become a small tree reaching up to 6m high. Another favourite, especially in the amenity market, is C. thyrsiflorus var. repens, which is
a good low-growing species that spreads well.
"I do not think people realise that they can be quite vigorous, which is in one way a good thing, covering a space or wall quickly, but then they may need quite a bit of regular pruning to keep them within bounds".
Ben Wighton, assistant head gardener, Lincoln’s Inn, London
"Ceanothus is a fantastic plant, providing a stunning backdrop and foil for many other plants at the back of a border. I mainly know the evergreen varieties. We sadly lost two large specimens of C. ‘Trewithen Blue’ and ‘Autumnal Blue’ recently. We don’t have an exact record for when they were planted but it may have been 1997, so we estimate they were around 20 years old, so they do really ‘live fast, die young’.
"I’ve seen newly planted specimens in town gardens romp away and occasionally make a nuisance of themselves with the size they become, but on the way it is a glorious sight, being smothered in gorgeous clusters of blue flowers.
"They don’t like it when you cut into the old wood so if it really is outgrowing its space it may be a case of replacing the plants, rather than cutting them back hard, as they are unlikely to survive.
"I’ve never grown the deciduous types but I came across C. ‘Gloire de Versailles’ last year when
visiting a garden and I was hugely impressed by its form and lovely powder-blue flowers. I think we should give it a try as a mid- to front-of-border plant."
Species and varieties
C. ‘Autumnal Blue’ AGM (H4) is a handsome evergreen shrub with fluffy clusters of sky-blue flowers from late summer into autumn and small, glossy, bright-green leaves. It has a strong upright habit and is ideal for training on a sunny wall. Height and spread: 3m.
C. arboreus ‘Trewithen Blue’ AGM (H4) is a large, vigorous evergreen spreading shrub or
small tree with broadly oval or rounded leaves and slightly fragrant deep-blue flowers in long panicles. Height and spread: 6m.
C. ‘Blue Mound’ AGM (H4) is a bushy medium-sized evergreen shrub of broad, dense habit with glossy leaves and compact heads of bright-blue flowers in late spring. Height: 1.5m. Spread: 2m.
C. ‘Burkwoodii’ AGM (H4) is a compact medium-sized evergreen shrub with fine-toothed dark-green oval leaves. It produces large clusters of rich blue flowers in late summer and autumn. Good for a small garden. Height: 1.4m. Spread: 2m.
C. ‘Concha’ AGM (H4) is an evergreen shrub of dense habit bearing narrow dark-green leaves and deep-blue flowers in late spring. Height and spread: 3m.
C. ‘Italian Skies’ is a vigorous medium-sized evergreen shrub with small, narrow, dark-green leaves and masses of small bright-blue flowers in compact conical clusters in late spring. Height: 1.5m. Spread: 2.5m.
C. ‘Pershore Zanzibar’ (PBR) is an evergreen shrub with yellowy-green leaves blotched with dark green in the centre. Light-blue flowers open in late spring and early summer. Height: 2m. Spread: 1.2m.
C. ‘Puget Blue’ AGM (H4) is a medium-sized evergreen shrub of upright habit with small dark-green narrow leaves. In mid spring it is smothered in dense clusters of deep-blue flowers. It needs a protected site because the leaves are easily scorched by cold, drying winds. Height and spread: 3m.
C. ‘Skylark’ AGM (H4) is a compact evergreen shrub with finely toothed, glossy dark-green leaves and clusters of deep-blue flowers in late spring and early summer. Ideal for smaller gardens. Needs protection from cold, drying winds. Height 1.5m. Spread: 2m.
C. thyrsiflorus ‘Cool Blue’, a new variety, is an evergreen with small dark-green and creamy white variegated foliage as well as clusters of soft-blue flowers in late spring. Height: 1.5m. Spread: 2m.
C. thyrsiflorus var. repens AGM (H4) is a low-growing vigorous evergreen shrub that becomes wider than it is tall. It has glossy dark-green leaves and is covered in powder-blue flowers in spring and early summer. Good for the front of a mixed border. Height: 1m. Spread: 2.5m.
C. × delileanus ‘Gloire de Versailles’ AGM (H4) is a deciduous medium-sized vigorous shrub with a rounded habit. It has light-green oval leaves and large panicles of scented powder-blue flowers in summer into autumn. Drought-tolerant and attractive to bees and butterflies. Height and spread: 1.5m.
Thank you to Floramedia, which supplied the images for this article from its photo library www.floramedia-picture-library.com