Soon after spring-blooming shrubs have bloomed is the time to prune them (if they need pruning) and the only time you can do so without cutting off next year’s buds. Prune before June 15 to be safe, though some sources give July 4 as the last okay-to-prune date.
Which shrubs? Forsythia, evergreen azalea, rhododendrons, lilac, deutzia, kerria, mockoranage, weigela, viburnum, st. johnswort, redtwig and yellowtwig dogwoods, Virginia sweetspire, quince, spring-flowering roses, and spring-blooming spirea.
How to Prune
“SHRUB-PRUNING ADVICE” by horticulturist Melinda Myers is a good place to start.
“HOW TO MAKE PRUNING CUTS” from the University of Kentucky is essential.
Spring Pruning for Shrubs
“HOW TO PRUNE FORSYTHIA“ by the University of Maine applies equally to any mature multi-stemmed shrub that blooms in the spring. They recommend removing 1/4 of the stems each year, targeting the oldest.
“RHODODENDRONS ARE DIFFICULT TO PRUNE, BUT HERE’S HOW“ by Cass Turnbull covers the tricky subject of pruning rhododendrons.
Spring-flowering shrubs are generally pruned by the renewal method. Each spring after flowering, prune out the largest stems to the ground to stimulate new growth from the crown and remaining stems. Remaining stems can then be shortened to shape.
“REJUVENATION/RENEWAL PRUNING FOR MORE BLOOMS ON OVERGROWN SHRUBS” by Robert Pavlis of Garden Fundamentals is excellent!
“HOW TO PRUNE AZALEAS“ by Cass Turnbull for Plant Amnesty illustrates that well. She notes that “radical rejuvenation” option is “scary” and that you need to really know what you’re doing. (See also her video about pruning evergreen azaleas, commissioned by Washington State and the U.S. Forest Service. She calls them one of the easier shrubs to prune.)
Infrequent Radical Rejuvenation
Rejuvenation is used when multi-stemmed plants become too large, with too many stems. In other words, the shrub is a tangled mess! Radical rejuvenation means removing all stems down to 4-6 inches from the ground. U. Illinois Extension says that these shrubs respond well to rejuvenation pruning: ‘Anthony Waterer’ spirea, abelia, honeysuckle, beautybush, snowberry, slender deutzia and privet. The experts seem to agree that this can be done all at once on these deciduous shrubs: forsythia, weigela, privet, honeysuckle, spirea, and hydrangea. Alternatively, the same all-the-way pruning can be done over three years, for a less scary, less risky approach.
“HOW TO RADICALLY REJUVENATE AN OLD AZALEA” demonstrates fearless pruning by woody-plant expert Phil Normandy of Maryland’s Brookside Gardens. He radically rejuvenates overgrown azaleas all the way to ground and gets good results there in Zone 7 with this method. Such radical surgery is considered riskier in colder climates.
To learn the basics of rehabilitative pruning and the all-too-common ways we mal-prune, these four videos by Cass Turnbull are a great education.