Gardeners often neglect pruning until a shrub is severely overgrown. Other gardeners prune the shrub back to the same level every year until it becomes dense and tangled with many branches coming from large knots at the pruning site. Use rejuvenation pruning to restore overgrown or too dense shrubs or to encourage new growth on plants that flower on new wood.
What Shrubs to Rejuvenate
Not all shrubs can tolerate severe pruning. It works best on multistemmed, very twiggy shrubs. Abelia (Abelia spp.), deutzia (Deutzia spp.), dogwood (Cornus spp.), forsythia (Forsythia spp.), honeysuckle (Lonicera spp.), hydrangea (Hydrangea spp.), lilac (Syringa spp.), mockorange (Philadelphus spp.), spirea (Spirea spp.), rose of Sharon (Hibiscus syriacus), St. John's wort (Hypericum spp.) and weigela (Weigela spp.) are good candidates for rejuvenation pruning. Other shrubs can be rejuvenated more gradually. Most broadleaf evergreens will not survive heavy pruning but pyracantha (Pyracantha spp.) can be completely cut back and Oregon grape holly (Mahonia aquifolia) tolerates gradual rejuvenation.
To rejuvenate a shrub cut branches all the way back to the ground in early spring. Use heavy lopping shears and a pruning saw to completely remove the branches 6 to 10 inches above the ground. Such severe pruning stimulates heavy growth. In midsummer remove half the canes that have grown back and head back, or cut off, some of the remaining canes just above outward facing buds. Over the next few years thin out new canes, keeping the strongest and removing thin or weak canes at the ground.
Shrubs can be renewed more gradually by removing 1/3 to 1/5 of the canes each year. Remove the oldest, thickest canes. New growth will quickly replace the growth that was removed. Over time a shrub can be completely rejuvenated without sacrificing a season of bloom. This technique is especially useful for shrubs that are used as screens or barriers or for broadleaf evergreens, which may take two to three years to recover from complete rejuvenation.
Rejuvenation pruning should not be done on grafted shrubs as the new growth will be the rootstock rather than the grafted shrub. Shrubs with a great deal of dead wood, extremely overgrown shrubs with large, woody bases or shrubs in weed barrier and rock mulch may not recover well from rejuvenation pruning. Some shrubs are more like small trees, with a central, woody trunk. Do not cut these shrubs back to the ground. Thin out branches, cutting back to a side branch.