Lilacs set their flower buds the year before they bloom.
When the first really fine spring days arrive, I’m eager to get out into the yard and start cleaning up the twigs and leaves strewn about the lawn, the fallen stems of last year’s perennials, and frost-heaved annuals. My favorite job isn’t at the end of a rake, though. It involves a very sharp pair of red-handled pruning shears.
The snipping feels decisive and the pile of woody debris on the ground looks productive and satisfying. Everything from poorly placed tree limbs and winter-damaged roses to gnarly, overgrown shrubs is fair game when I’m wearing my “Runs With Scissors” T-shirt.
Well, not quite everything. Knowing what to cut and when to cut it are part of the art and science of pruning. There’s more to it than simply wielding a pair of pruners, just as painting a masterpiece is more than owning a brush and palette of watercolors. For most people, the “when and how” of pruning flowering shrubs seems to be their undoing.
Shrubs fall into two general categories and knowing when to prune them lies in knowing to which group they belong. They either bloom on new wood (from the current year) or on old wood (from the previous year). Here’s how it works:
- Shrubs that bloom on new wood usually flower in mid- to late summer. Prune these in late winter to increase the new growth and the number of flowers. Examples include butterfly bush (Buddleia), pee gee hydrangea (Hydrangea paniculata), summersweet (Clethra), cherry laurel, rose of sharon, photinia, gardenia, and roses.
- Shrubs that bloom on old wood produce their flower buds the year before they bloom. These shrubs bloom in spring to early summer. The best time to prune this group is immediately after they bloom. Pruning them in the winter or early spring will remove the flower buds for that year. Examples include lilac (Syringa), azalea and rhododendron, broom (Cytisus), daphne, forsythia, fothergilla, spring-blooming or bigleaf hydrangea (Hydrangea macrophylla) and weigela.
The next step is deciding what to prune and what to leave. The goals in most cases are to establish and preserve the plant’s natural habit and to increase flowering potential. To achieve both goals, I cut stems either back to the ground or to another branch or main stem. Here’s what I remove in order of priority:
- Dead wood. Cut it back to living wood, preferably to a healthy bud or branch that’s growing toward the outside of the shrub.
- Poorly placed branches and stems. These may be crossing or rubbing on one another, growing out of the ground far from the original plant, sticking out into the driveway, or brushing the house. Either remove the stem entirely or cut it back to a branch that’s growing in the right direction.
To read more on this subject, check out the following articles:
- A Guide to Successful Pruning from the Virginia Cooperative Extension
- Pruning Flowering Shrubs from Colorado State University Cooperative Extension