How to Prune Spring Flowering Shrubs
Pruning your spring flowering shrubs will reward you with lush new growth and a lot more flowers next year. Many shrubs will only flower on new wood. It’s important to encourage this new growth.
Spring flowering shrubs kick off the gardening season with spectacular displays of flowers. Lilacs (Syringa sp.), mock orange (Philadelphus sp.), and forsythia are often considered beloved old-fashioned shrubs that can live for generations. Over time, however, the quality and quantity of bloom can decrease as plants grow too woody and mature. This guide will explain how to prune and maintain these beloved spring shrubs to ensure loads of flowers to enjoy for years.
Why Prune Your Flowering Shrubs?
Flowering shrubs, such as lilacs, mock orange, and forsythia, are all woody plants that bloom best on what horticulturists call ‘new wood.’ Generally, quick-growing shrubs will produce too much old wood or become over-run with suckers, and eventually will not bloom. Hard pruning is in order, but few gardeners will want to cut the shrub entirely to the ground.
Professional horticulturists practice a method called the ‘One-Third Rule,’ which involves removing one-third of the oldest branches every spring at ground level. This method ensures a continuous renewal of branches that will produce flowers in abundance.
When to Prune Flowering Shrubs.
Timing is everything, especially with spring-blooming shrubs. The window is small, between just after flowering and 2 or 3 weeks after. One should remember that the plant’s entire annual growth will happen in a month or two after flowering.
Never prune spring-flowering shrubs in mid to late summer, fall, or winter. Pruning must be done in early spring or immediately after they bloom to ensure that the shrub can produce flower buds for the following year. This applies to deciduous shrubs such as lilacs, mock orange, spirea, most woody roses (Rosa sp.), forsythia, deutzia, honeysuckle (Lonicera sp.), butterfly bush (Buddleja sp.), and weigela.
Pruning is very beneficial to some hydrangeas, though not all (see below). Furthermore, pruning should be avoided on evergreen shrubs such as rhododendron, camellia, and even deciduous azaleas as they will not benefit from such treatment.
How to Prune Flowering Shrubs.
There are multiple goals when it comes to pruning deciduous blooming shrubs. First, remove any dead or damaged wood. This could include branches that cross over each other or simply branches that have died or look too old. Second, pruning should allow air and light to reach through the entire shrub while being aesthetically pleasing. A common mistake is to try and shape such shrubs into a topiary, a standard, or a ball. Most blooming shrubs don’t comply, given their natural upright or vase-shaped habit, which one should encourage.
Remove one-third of the oldest branches to near ground level and any suckers. Avoid reducing the rest of the shrubs’ established stems that may impact future flower buds. Younger shrubs may benefit from just reducing the current years’ growth (such as with lilacs). In this case, cut just above a pair of leaves, as this is where flower buds will form and eventually stems next year.
What About Hydrangeas?
The trick to pruning hydrangeas is demystify them first. Common names and groups abound, if you don’t know the difference between a smooth hydrangea and a bigleaf hydrangea, a few tips will help.
First, research the species if you know what it is. Hydrangea macrophylla (commonly referred to as a ‘bigleaf hydrangea’) blooms on old wood and should only be carefully pruned to remove dead growth every spring. If you live in a mild climate and your hydrangea has colorful flowers, blue mop heads, or bright pink flowers, you probably have H. macrophylla. These hydrangeas often suffer in severe winter conditions.
On the other hand, if your hydrangea has large white flowers with big leaves, it’s probably H. arborescens. The names are familiar: ‘Incrediball’, ‘Annabelle’, ‘Little Lime’. These flowers don’t blush to pink but usually turn lime green through late summer and eventually buff-colored in fall. All H. arborescens must be pruned hard every spring to 6 inches above the ground. This will stimulate the strongest and most robust branches and the largest flowers. If you skip pruning them for a year, the flower will be smaller but still abundant.
The third group of common hydrangeas is often called tree hydrangeas: H. paniculata. The leaves are pointed at the tips, and the cone-shaped flower heads generally emerge completely white but age with a blush of pink or mauve tint. These can be trained to a tree form (a standard) or as a shrub. The cultivar names too are familiar (‘Pinky Winky,’ ‘Pee Gee,’ or ‘Limelight’). All H. paniculata bloom on new wood, so pruning is encouraged. Hard pruning will result in longer stems with flowers great for cutting. Light pruning (or none at all) will result in shorter stems and smaller flower heads.
Other hydrangeas, such as oakleaf and lacecap, require slightly different care. Oakleaf hydrangea (H. quercifolia) blooms only on old wood, so prune carefully just after bloom and only if needed. Lacecap types can be either H. macrophylla or H. arborescens, so you may need to do some research to identify what species you have. Hydrangea macrophylla ssp. serrata tends to be slightly more hardy in the north, blooming from previous buds in pink and blue. Flowering may be sporadic in cold climates, so prune only when winter-kill is present.
However, some cultivars of H. arborescens produce lacecap flowers, which should be pruned hard every spring before their buds break. This includes the summer blooming cultivar ‘Haas Halo,’ which can produce lace-cap flowers up to 14″ in diameter after a hard pruning nearly to the ground every late winter.
Best Tools for Pruning Flowering Shrubs
Hand Pruners- Hand-held pruning sheers or secateurs are best for the cleanest cuts. Look for brands that cut with a blade (known as bi-pass pruners) as they will produce the cleanest cut. Clean cuts are easier to make and reduce stress and damage to the shrub.
Loppers – Long-handled loppers (the kind that requires two hands to use) are essential for larger branches.
Saw – A shard pruning saw is often necessary for old or dead branches with a diameter of 2″ or more. Dead wood is often more challenging to cut with lopper or pruners.