Knowing when to prune lilacs is vital because if you do it at the wrong time you could lose a whole year of deliciously fragrant flowers.
Timing isn’t the only factor—you also need to know how to prune lilacs properly to keep your shrub or tree reliably blooming and a star of your border ideas.
“The sweet scent of lilac blossoms signals that spring is finally here to stay. By pruning lilacs at the right time, you won’t miss out on next spring’s festive floral display,” says Bloomscape’s gardening expert Lindsay Pangborn.
The good news is that plants don’t necessarily need severe pruning. Left to their own devices, old-fashioned lilacs can get out of control, with bushes reaching about 15 feet and tree varieties reaching 25 feet tall. However, many newer varieties are much more compact.
“Many of the newer varieties of lilac are smaller, making them perfect for urban gardens and requiring less pruning,” says Gail Pabst of the National Garden Bureau. “There are also reblooming lilacs that bloom again in summer/fall.”
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When to Prune Lilacs – Expert Advice
When pruning lilacs, it is not just the season that matters, but also the development of the plant. Lilacs don’t usually need pruning for the first two or three years, so wait until the plant has matured and reached a height of 6 feet.
“Annually pruning isn’t necessary, but cutting off faded flower heads within a month of flowering will help the plant produce more flowers for the next year,” says Pabst.
However, once the plants are fully grown, lilacs benefit from an annual pruning. The exception is if you have a dwarf variety that needs a “haircut” rather than a plum.
“Prune lilacs after flowering and before seed production,” says Stuart Mackenzie, gardener, arborist and expert at Trees.com. “They seem to be responsive and can recover easily before becoming dormant.”
This happens in spring to summer. “If you miss the crop window, you can crop later, but you’ll lose blooms,” adds Mackenzie.
Be careful when pruning your lilacs in hot weather. “In general, pruning during hot weather and periods of drought can stress plants and should be avoided,” says Pangborn.
In what month to prune lilac bushes?
“The best month to prune lilacs is usually late May or the first half of June — just after they’ve finished flowering,” says Pangborn. “This timing ensures you can enjoy full flowering and prune plants before summer fully sets in.
“For most areas of the US, the best time is the first half of June, after the flowers have withered. Those in more southern areas may be able to prune a little earlier, in late April.’
However, lilac pruning is more about when the plant is ready than a strict schedule. Observe the plant and wait for it to bloom before making the cut.
Don’t let it develop fresh growth for next year or you will sacrifice precious flowers. This happens fairly quickly after the flowers have faded, so keep your eyes peeled.
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Can I prune lilacs in the fall?
Avoid pruning lilacs in the fall. “Lilacs set their flower buds in summer and bloom on old wood. So if you prune in the fall, you are cutting off the lilac buds for the next year,” says Pabst.
If you do this, you will drastically reduce the amount of flowers you receive.
When to hard prune lilacs
“If your lilacs aren’t blooming as much as they used to, or are too big to enjoy the blooms, a rejuvenation pruning can be done,” says Pangborn. The best time to hard prune lilacs is in late winter.
“Cut off all but one foot of the stem from the ground. New stems will emerge from the base of the plant and will bear flowers in 2-3 years.’
Alternatively, if you do not want to do without so many flowers, you can cut back a third of the plant every year for three years.
“Know your lilacs and have a plan for pruning,” adds Mackenzie. “Make sure you know whether or not the lilac is a graft, as you don’t want to remove your desirable stock through poor pruning. Do not obstruct at the branch collar.’
How far can you cut back lilacs?
Try to keep your lilacs at around 2 to 8 feet tall to keep them under control and ensure you can enjoy their blooms and scent. Remove no more than a third of the plant’s stems and try to remove a balance of old and new stems.