The easy thing to remember about pruning Viburnums - deciduous or evergreen - is that they should ideally be pruned soon after flowering. Within a month of finishing flowering is good, but a little later is also suitable. As some of them flower in winter, then the weather conditions outside may be the determining factor!
Viburnum tinus French White
Viburnum are one of the most popular groups of shrubs with a wide range of growth habits and flowering times. Most flower either in winter or late spring. They all flower on stems or growths that we made in the year previous - starting from early spring and carrying on through summer. That is to say that they produce either flower buds or flowering wood/twigs in the year before actually flowering.
Prune any of the Viburnum too late in the year and you will forfeit the flowers for a year or more.
Most Viburnums can be rejuvenated by cutting back hard. This method is specifically for overgrown shrubs or those that simply need a new lease of life - It should not be carried out as a 'norm'!
This type of (drastic) pruning should be carried out in the early part of the year in order that the plant may put on growth before the following winter. Deciduous types are best pruned hard in very early spring before growth starts.
Viburnum x bodnantese Dawn
Deciduous Viburnums include - Viburnum x bodnantense, V x carlecephalum, V. carlesii, V. judii, V. lantana, V. opulus, V. plicatum, V. sieboldii.
Whilst the evergreen Viburnums are often touted as being 'winter' flowering, their peak, full, flowering period is more realistically late Spring. The flowering times can be as varied as the seasons, with up to 4 weeks difference in successive years.
They tend to start growing new foliage whilst in the latter stages of blooming. This foliage will be the basis of flowering stems for the following year.
As most of the evergreen Viburnums are dense in habit of growth and foliage, and generally of a 'uniform' shape, the pruning required is generally light - just enough to tidy the shrub, and maybe re-form some of the framework.
They rarely need any further pruning than simply trimming the flowered growths back a few inches. The cut should be back to a pair of newly sprouting leaves - simply taking the untidy flowered stems off.
Because of the dense habit, evergreen viburnums can sometimes suffer from heavy snow damage. this ultimately spoils the symmetry of the shrub. Cut back any damaged branches as soon as possible after flowering. It may well be that other non-damage branches will need formative pruning in order to regain the formal look of the shrub. But, do bear in mind that growth can be quite rapid after cutting back damaged branches and the Viburnum will soon 'close up' and gaps caused by snow or other damage.
Deciduous Viburnums fall basically into two groups - being those that flower on bare wood in late winter - such as Viburnum x bodnantense types - and those that flower early to late spring with foliage emerging or established. These include Viburnum plicatum, V. lantana, and V. opulus.
The winter flowering types bloom on bare stems that are devoid of foliage, and tend to have woodier, brittle branches and stems with generally an upright habit of growth. If left un-pruned, they tend to grow into an untidy state with most foliage and flowers at the top of the shrub.
The best way of pruning this group, is to cut back hard, one in three or four stems back to a few inches from the ground or main support stem. Do this right after flowering, and in the ensuing years, again cut back the older stems - leaving the younger stems to form the new-look shrub. This will help to provide a much wider spread of flowers and foliage from top to bottom of the shrub.
Late Spring Flowering Viburnums such as V. opulus; V. lantana; V. plicatum types generally have a clothing of foliage and flowers over most parts of the shrub. It is normally only necessary to trim back the flowered stems UNLESS you are wanting the berries which are also an important feature of Viburnum lantana and Viburnum opulus. If you cut out the flowering growths of these, you will also be cutting out the berrying stems for the following autumn.
Viburnum opulus and Viburnum lantana - if they need pruning at all - can be kept under control, by cutting out 1 in 3 branches at near to ground level. Do this each year, always cutting out the older branches of course, and you will have a steady supply of new growths, and flowers/berries on the older growths. Better still not to prune these at all - if there is space to let them grow.