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Clethra - Growing Guide
Clethra are a genus of 60 species from East Asia and North America. We grow only a few of them although more are under trial. The deciduous species tend to be hardier than the evergreen ones. The evergreen C. arborea comes from Madeira and will only survive the mildest winters outside here. This is a pity as this ‘Lily of the Valley’ tree is one of the finest species.
Clethra like peaty soil and partial or dappled shade. They make large shrubs (C. alnifolia, C. barbinervis) or small trees (C. delavayi, C. pringleyi, C. monostachya). C. tomentosa forms a suckering thicket.
The key reason for growing clethras is that they flower at the very end of the woodland garden season in July and August. They all have panicles of upright or pendant flowers up to 12in in length. The individual flowers are bell to cup shaped and predominantly white or yellowish white. Quite a display!
At Burncoose the small tree species are 10-12ft tall with a nearly equivalent spread. The more shrubby forms are from 4-8ft. C. alnifolia is the only one to need any pruning and then only to reduce the number of older stems to make room for more suckering growth which will produce better flowers next season.
C. alnifolia and C. barbinervis are perfectly hardy with us. They are however quite shortlived shrubs and, in our climate, they live only for 20 to 25 years. The small tree species have a similar longevity here and need protection from wind and frost so do not grow them in a valley bottom. C. delavayi will stand several degrees of frost. C. pringleyi has yet to prove itself and becomes semi evergreen and stunted after a colder winter.
One of the reasons clethra are short lived may well be that they overseed copiously in early autumn. The seed can be sown in containers in spring or autumn. Cuttings off the tree forms seem easiest as softwood cuttings. The shrubby species seem to do better from riper new growth. Clethra arborea seeds will keep for a few years so if you have a disaster with this plant, as you well may, there is a backup.