Picking a Ceanothus for your garden depends on the size you would like and hardiness. Very few of the evergreen varieties are fully hardy, (what is hardy?) listed as H4 is C. thyrisiflorus (Blueblossom) which is a low growing, spreading variety, evergreen, and with light blue flowers growing up to 1.5 metres but taller if grown against the shelter of a wall up to 2.5 meters. C. x delilianus 'Henri Desfossé' is a hardy variety although it is deciduous with dark blue flowers from July to September. Most other evergreen varieties are less reliably hardy and may require some winter protection.
Some Ceanothus do grow very large, up to 6 metres tall, with a spread of 8 meters, such as Trewithen Blue, which makes checking the plant label important to know the eventual size of the shrub Good strong blue varieties with the RHS garden merit award are 'Trewithen Blue' and 'Cascade.' C. ‘Autumnal Blue’ is attractive, upright growing to around 3 meters with sky blue flowers from mid summer to early Autumn, C. 'Blue Mound' is more compact growing up to around 1.5 meter in the right conditions. C. ‘Concha' is around 3 meters with darker blue flowers in late spring. There are lots to choose from around 55 varieties in all. Ceanothus can be quite vigorous and although Treweithen Blue is popular is grows large, more of a small tree than a shrub. All these named varieties have the RHS GMA .
Confusingly many are described as H4 which should be hardy to -10 but are also described as 'needing winter protection'. All I can offer is that in an exposed garden, which I have, Ceanothus fail to thrive and often the leaves are wind burnt and brown. Despite the hardy description, I would only grow the evergreen varieties in a sheltered warm spot preferably against a wall.
The early spring flowering varieties of Ceanothus look good with Clematis Montana illustrated in the image above right. Once planted in a sheltered spot away from chilly winds Ceanothus is trouble free and will reward with lovely blue flowers year after year. It's evergreen foliage is attractive small bright green leaves.
Ceanothus is also suitable and ideal for planting in coastal areas.
There are also a few deciduous varieties of Ceanothus, such as Ceanothus x Pallidus 'Perle Rose' illustrated below left, which have delicate, attractive pale pink flowers from mid summer to early Autumn. This variety is also ** hardy and it needs to be planted in a sheltered spot on light well drained soil.
If Ceanothus is not what you are looking for check out shrubs and bushes; spring flowering shrubs; summer flowering shrubs; shrubs with autumn and winter interest; and evergreen shrubs.