Go to the front pages of every Royal Horticultural Society A to Z Encyclopaedia of Garden Plants and you will discover a visual guide to the 12 pruning groups, the different techniques recommended for pruning the plants covered in the encyclopaedia.
Number seven is the most dramatic as it requires the stems and branches of the shrub to be lopped to just above ground level leaving just stumps. This ‘Coppicing’ is the technique that we use at Harlow Carr every year between March and early April to cut our coloured Dogwoods and Willows down to between one and three buds from the base. It’s also the pruning technique that gets the most reaction from visitors, “Won’t you kill it if you do that”? comes the question. The answer is in the strong, straight growth that comes from the base of the Cornus or Salix during summer creating fresh coloured stems to be admired through the winter.
Of course, cutting for colour is really only the ornamental gardening cousin to the ancient woodland craft of coppicing trees to provide a constant supply of young straight stems for firewood, tool handles, beanpoles, and for weaving into hurdles. The woodland is worked with a rotation of several years between cutting so that the trees are kept in a state of perpetual youth, and just like in our gardens the technique relies on the natural biological response of the plant to grow new wood after severe pruning.
“Well that’s all very well” I hear you say “But what do I do if my shrubs are old, overgrown and congested”? It is all too common to see shrubs and trees that were planted decades ago become hulking monsters looming over the garden with all the flowers high up out of reach. Well, luckily, you won’t have to dig them all out to start again because pruning groups six and seven are also used to renovate overgrown shrubs. Many common garden plants like Lilac, Philadelphus, Cherry Laurel, evergreen Mahonia x intermedia, Viburnum tinus and most deciduous Viburnums will take hard pruning to gain a new lease of life as young shrubs once more.
There is still just time to carry out this renovation work, as late winter through to spring is the traditional time to hard prune evergreen and deciduous shrubs. You will need, apart from courage, a sharp pruning saw, remembering that these cut backwards on the pull stroke. A set of good quality bypass loppers will be worthwhile and a good pruning book with illustrations is an investment that will last a lifetime.
So how low do you go? John Evelyn writing in 1786 advises “cut not above half a foot from the floor” more modern pruning books suggest between one to two feet (300-600mm). For deciduous shrubs cutting towards the lower measurement gives a neater appearance to the rejuvenated shrub, it matters less on evergreens as the stumps will eventually be fully hidden by foliage. Completely cut out all dead, damaged, weak and crossing stems to leave a set of well-placed stumps at the chosen height, apply a general fertiliser around the base then mulch well. Drastic pruning stimulates dormant buds to grow and during the summer large clusters of shoots will appear on the stumps. For deciduous shrubs these ‘water’ shoots will be very upright and vigorous, all that is left to do will be to revisit the shrub in October to thin out this cluster growth to leave two to three of the strongest and best placed shoots on each stump, these will form the new framework of the renovated shrub. Within two to four years the shrub should be flowering well on its newly formed framework of branches.