Plant of the Month: May

Ceanothus 'Dark Star' Common name: California lilac Family: Rhamnaceae Vital statistics Height and spread: 2m (6ft6in) high by 3m (10ft) wide Form: Evergreen shrub Soil: Fertile, well-drained soil Aspect: Full sun but protected from strong, cold winds Hardiness: Hardy in some regions; can withstand temperatures down to -5ºC (23ºF).

Ceanothus 'Dark Star' can be found growing against the house wall in the Farmhouse Garden. The warm, south-facing situation is ideal site and the bright light emphasises the metallic blue buds of this wonderful cultivar as it comes into flower

Ceanothus

Ceanothus is a genus of around 55 species of deciduous and evergreen shrubs, mostly from western north America, California in particular, but also from eastern USA and Mexico.

Occurring in scrub and woodland on exposed, dry slopes from mountains to coastal regions they form a significant component of dense, scrubby chaparral, in association with plants including Yucca whipplei and Arctostaphylos species. Ceanothus can grow on poor soils as they have nitrogen-fixing bacteria which form nodules on the roots, much like Rhizobium does in legumes.

Ceanothus are cultivated for their small but profuse flowers which are usually borne in spring. Flower colours include white and pinks but it is the range of fine blues, from soft powder blues to indigo, which the genus is particularly known for. Individual flowers are only 3mm (1/8in) long but are borne in dense racemes up to 10cm (4in) long.

The name Ceanothus comes from the Greek name keanothos, spiny plant, though it was originally applied to a different plant before Linnaeus named this genus. Commonly called the California lilac, after its lilac-like flowers and origins in California.

Ceanothus 'Dark Star'

Ceanothus 'Dark Star' is a particularly fine evergreen cultivar with arching stems, believed to be a cross between C. impressus and C. papillosus. In late spring honey-scented, dark purplish blue flowers are borne in cymes up to 3cm (11/4in) across.

The upright pyramidal habit of Ceanothus 'Dark Star' lends itself to being used as an informal screen and the dense and dark foliage makes a great foil for other plants.

AGM

The RHS Floral Committee B has awarded Ceanothus 'Dark Star' an Award of Garden Merit.

Cultivation

Ceanothus grow well in fertile, well-drained soil in full sun but they should be sheltered from strong, cold winds as they are not fully hardy and winter cold can cause damage to the plant. Ceanothus are lime-tolerant but can become chlorotic on shallow chalk soils.

Suitable for growing in a shrub border Ceanothus can also be trained against a sunny wall where they will reach twice the height they would in an open border.

Pruning varies depending on the form and habit of the plant, vigorous wall-trained plants will require more pruning than the prostrate species, which require minimal pruning. Timing of pruning is dependent on the age of the wood on which the flowers are borne.

General advice on pruning summer-flowering shrubs can be found here.

Ceanothus are relatively free from pests and diseases but are susceptible to honey fungus.

Propagation

Seed can be sown in a seedbed, or in containers in an open frame, in autumn. To promote germination seed should be stratified for 30 to 90 days in moist compost, at 1-5ºC (34-41ºF) and should then be soaked in warm water for 24 hours before sowing. It should be remembered that most Ceanothus hybridise readily in mixed collections and may not come true.

Cuttings are the only way to raise new plants true to the parent plant from which material is taken. For deciduous Ceanothus root greenwood cuttings in mid-summer, evergreen Ceanothus should be raised using semi-ripe material in mid to late summer.