Among the most persistent myths in horticulture is the one that insists you must remove the faded blooms of the common lilac, also called the French lilac (Syringa vulgaris), so it will bloom more abundantly the following year. But it simply isn’t true. Removing the spent flowers (this is called deadheading: I just love that term!) will not give you a extra single flower the following year.
I know that some readers will accuse me of lying (just try to explode a horticultural myth and you’ll see: no one likes to be told they’ve been doing something for no reason!). Often these people claim to have proof! They offer up in evidence the fact that the one year they didn’t deadhead their lilac, it barely bloomed the following spring. But I’ll have to ask these people to be honest. Does their lilac really bloom abundantly year after year? Most common lilac cultivars bloom more or less on a biennial basis: a year of abundant bloom is usually followed by a year of poor flowering and there is nothing you can do about it. It is simply the nature of the beast. Deadheading changes nothing.
Now, if you say that you deadhead your lilac because the presence of its seed capsules bothers you, I’ll grant you that. Green at first, brown at the end of the season, they are not the prettiest sight. But personally, the presence of a few seed capsules has never bothered me. All the better, because as a laidback gardener, I try to work as little as possible. Let Mother Nature do her job, that’s my motto. And Mother Nature leaves the capsules on.
The Origin of the Myth
The idea that we must remove faded lilac blossoms comes from the concept that flowering and producing seeds saps a plant’s energy. And that is true… to some degree, for some plants.
It is especially apparent with short-lived plants, such as annuals. These plants absolutely must produce seeds during their one year of life to ensure the survival of their lineage. If you remove their flowers, they’ll usually bloom again rapidly in an effort to compensate and produce at least some seed.
This is much less true for most permanent plants (trees, shrubs, conifers, long-lived perennials, etc.). These plants usually store up ample reserves to ensure the success of their blooming process. If their effort to flower and set seed is thwarted (if, for example, frost, an animal or a human eliminates their blooms before their seed can mature), the plant will usually simply resume blooming the following year, with no more flowers than usual. These are plants that don’t put all their eggs in one basket: if one attempt to produce seed fails, they simply try again next season.
Still don’t believe me? Try removing all the flower buds of a perennial in your garden this summer (a daylily or phlox, for example), never letting it produce even a single bloom, and you’ll see: it will bloom again next year, but no more heavily than usual.
But There are Other Reasons for Pruning a Lilac
For example, removing the abundant suckers that tend to form at the base of your shrub will reduce competition for resources and will stimulate better bloom. Or you may want to cut back a lilac that is too tall to bring the flowers down to nose level. Or to remove an old branch or two that is less productive. These are all legitimate reasons for pruning lilacs and even a laidback gardener would probably carry them out… eventually.
Prune at the Right Season
Do note that if you need to prune a lilac for any reason and you want it to flower the following year, you should do so within the two or three weeks after its flowers fade. That’s because lilacs “bloom on old wood”, that is, flower buds for the following spring are produced over the summer and remain dormant on the plant over the winter. So the plant blooms on growth produced the previous summer. As a result, if you prune too late, you’ll be cutting off next spring’s blooms.
This is not just the case with lilacs, but with all spring-flowering shrubs. If you need to prune them for any reason, it is always better to do so within two or three weeks after they bloom. That way the plant has time to produce plenty of buds for the following spring.
What About Other Lilacs?
The text above concerned the common or French lilac (Syringa vulgaris). But what about other species?
The rule still holds: there is no need to deadhead them, except… for the reblooming varieties, like the Boomerang series. Removing their faded flowers can help stimulate follow-up blooms.