What to Do With Lilacs After They Finish Blooming in May – 5 Essential Jobs (and the Mistake That Could Cost You Next Year’s Flowers)
What you do straight after flowering determines next year’s display – these simple steps help lilacs recover, stay healthy, and produce more blooms.
When lilac shrubs finally bloom in spring, it’s a joyful riot of seasonal color and intoxicatingly sweet scent. Gardeners wait all winter for this peak moment, when these classic shrubs burst into flower and fill the garden with fragrance.
That brief flowering window can feel all too fleeting, though, and it’s easy to slip into simply enjoying the display while it lasts. But what you do immediately after blooming has a direct impact on how well your lilacs perform next year. A little timely attention now helps set the stage for strong growth and abundant flowers in seasons to come.
While it’s tempting to just sit back and enjoy the show, some targeted lilac care in May will keep your shrubs healthy, vigorous, and blooming beautifully next season. To help you get the best out of your beloved lilacs, I’ve compiled a list of the most important tasks to tackle this month.
1. Deadhead the Spent Flowers
Depending on where you grow lilacs, the flowers will peak from April to May and almost certainly will be done blooming by mid-May or a little later in the month. Deadheading those spent blooms is an important garden chore for this month. One reason to remove spent flowers now is simply to keep your shrubs looking tidy.
Another, even more important reason to deadhead lilacs after flowering is to ensure the plants direct energy into strong roots and new shoots. If you leave the flowers in place, the bush will spend energy creating seeds you don’t need. Remove the flowers to redirect that energy to strong, healthy growth that will support abundant blooms next year.
To deadhead lilacs, use a sharp, clean pair of shears – I like Fiskars' Bypass Pruning Shears, available on Amazon. You can use your fingers, but this risks damaging the stems. A clean snip is better. Cut the spent flowers off just above a set of healthy leaves.
2. Prune After Flowering
While deadheading removes spent blooms, pruning goes further by reshaping the shrub and removing older wood. Different shrubs have different ideal pruning times. For lilacs, it’s right after blooming, which makes May the right time for this chore. It’s essential that you don’t wait too long, as lilacs set their next year’s flower buds very soon after flowering. If you prune lilacs in summer, you will remove buds and reduce the number of flowers next season.
Start by removing dead wood and some of the oldest canes, which you can cut back all the way to the ground. Cut back any canes that are too tall and remove all suckers. Ideally, a lilac bush should have about ten healthy canes, so prune as much as you need to achieve this. A very old, overgrown lilac can be rejuvenated by cutting the entire bush back to about six or eight inches (15 or 20 cm) from the ground.
3. Add Mulch
If the mulch around your lilac shrubs has deteriorated over the winter, now is a good time to refresh it. If you haven’t mulched them at all, consider adding a good layer this May. Mulch helps keep down weeds, regulates soil temperature, and helps keep moisture in the soil.
Use shredded bark, wood chips, or coconut husk, like Back to the Roots' Expanding Mulch, and spread it around the shrub to a depth of two to three inches (5 to 8 cm). Keep a small ring around the canes clear of mulch to reduce the risk of rotting due to excess moisture.
4. Check for Pests and Disease
Catching lilac problems early is the best way to keep them from getting out of control and becoming unmanageable. Common pests of lilacs are aphids, scale, and leaf borers. You can remove these mechanically by picking them off or using a strong stream of water to spray them off, as long as you catch the infestation early. You can also use an insecticidal soap or neem oil to manage these critters. Bonide Captain Jack's Neem Oil is ideal for this purpose.
Common diseases are leaf spot, powdery mildew, and bacterial blight. The first two are fungal infections, which you can treat with a fungicide. Leaf spot manifests as spots on the leaves, which curl and drop prematurely. Powdery mildew looks like a white or gray powder on leaves. It looks unsightly but is rarely serious or fatal to the plant.
Bacterial blight causes brown and black spots on leaves, stems, and flower buds. The best way to manage this disease is to remove affected parts of the plant and prune to improve airflow. Excess moisture triggers and spreads this disease.
5. Water and Fertilize as Needed
Lilacs are generally low-maintenance and don’t need a lot of feeding or supplemental watering. However, May is a good time to ensure your shrubs are getting all the nutrients and water they need for healthy growth that will support flowering next year.
Established lilac shrubs don’t generally need watering unless you’re going through a dry period with no rain. If this is the case, water deeply around the base of the shrub, trying to avoid getting water on the leaves and shoots. Do this once or twice a week only during very dry spells.
These shrubs are not heavy feeders, but you should fertilize lilac once a year if your soil is poor. You can add a slow-release fertilizer to the soil in late winter, or if you missed that application, apply it now, in May. Use a balanced product only, like TPS Nutrients' Lilac Fertilizer, available on Amazon. A fertilizer high in nitrogen will promote leaf growth at the expense of flowers.
May is the time to celebrate and enjoy the eruption of lilac flowers on your shrubs. But, if you want the same show next year, it’s also a time to provide essential care. Don’t skip these steps if you want healthy, heavily flowering lilacs every year.
