Ceanothus is the evergreen shrub whose dark green foliage and bright blueset off so well the dignified stone walls of some of Oxford’s colleges. A native of California, it can be grown in the open on only the warmest sites in this country, and usually needs the protection of a wall. C. dentatus is the species most commonly grown, and is usually the one you get when the label simply says ‘ceanothus’. It can grow up to 3 m (10 ft) on a good wall site, and it produces its bright blue spikes of flowers in May and June. C. thyrstflorus is about the hardiest of the evergreen species, and will grow anywhere in Britain, given the protection of a wall.
There are both deciduous and evergreen species and they require different treatment in the garden. Most of the evergreen kinds flower in spring and many are best grown against sunny walls as they are not fully hardy. If they are pruned at all it should be immediately after flowering when unwantedcan be cut out and side shoots shortened a little. By contrast the most useful deciduous varieties flower in late summer and early autumn and can, with advantage, be pruned hard each spring when they will make long stems terminated by sprays of the typical thimble-shaped clusters of small flowers. All like good, well-drained soil and a warm, sunny .
Ceanothus impressus, 4 ft. high by 6 ft. wide, with deep blue flowers and a densely branched habit, and C. thyrsiflorus, with light blue flowers, are two of the hardiest evergreen kinds. C. thyrsiflorus is variable in habit, tall with arching growths in the variety Cascade, but making low, wide-spreading mounds of growth in the variety repens. Typical deciduous varieties are Gloire de Versailles, light blue; Topaz, deep blue, and Perle, rose pink.
It produces masses of light blue flowers in May and June, is very floriferous, and grows to 6m (20 ft) on a good site. C. impressus, 2.7m (9 ft), bears masses of dark blue flowers in April and May. C. rigidus, with an upright habit of growth and a normal height of 1.5m (5 ft), is suitable for a restricted wall space. Its violet flowers are sometimes produced as early as March, and it can keep flowering through to June. ‘Autumnal Blue’ is a hybrid of uncertain parentage. It is a vigorous grower, up to about 2.7m (9 ft), is evergreen and hardy and, as its name suggests, produces its dark blue flowers from late summer through to the autumn.
General care: Ceanothus like a sunny site, preferably a west-or south-facing wall. They do best in a light, well-drained soil, and many are not happy if there is a lot of chalk in it. The spring-flowering species need only lightwhen flowering is over, but ‘Autumnal Blue’ should have the previous year’s growth cut back in spring.
Propagation: From heeledtaken in July and struck in of a 50-50 peat and sand mixture.
Pests and diseases: Usually trouble-free, but too much chalk in the soil will show up in a chlorotic yellowing of the– yellow marks between the veins. Treat with chelated iron.