If you were about to start pruning lilacs, stop right now: Gardener Scott says that many people prune their lilacs all wrong and too early in the season, which results in lilac bushes that bear no flowers. Since most of us grow lilacs because of the fragrant spring blooms, you'll want to make sure you follow his tips on pruning shrubs to ensure you get to enjoy the lilac flowers every spring.
1. Pruning lilacs too early will stop them flowering
This is the most important thing to bear in mind before you get out those best secateurs. As Gardener Scott points out, 'Many gardeners don't understand how and when to prune lilacs,' he says. He explains that a very common question he gets is 'why is my lilac not flowering?', and that the most likely answer is: 'the gardener probably waited too long' after flowering.
Unlike most shrubs that are pruned in winter when they're dormant, if you prune your lilacs in winter, you're 'probably cutting off those flower buds'. Lilacs start producing fresh flower buds for next year's flowers right after this year's flowers fade, so you must seize the moment right after they finish flowering in spring and never prune them in winter before they've flowered.
2. Lightly prune lilacs that are in good shape
A lilac that is generally nicely shaped and not overgrown – 'six to eight feet or two meters is a good height' – will just need a light annual prune, 'just a maintenance function to clean up the plant'. You can even skip a year, if you like, and just trim off the spent flowers as soon as they've finished blooming. Gardener Scott recommends pruning off the tips 'where the seed heads are beginning to develop, we're adding some energy back to the plant' and 'stimulating more growth.'
For a light prune, cut back a branch by about a quarter, just above a fresh shoot; this will make the plant 'much more manageable'. He also points out that lilacs spread, so if you don't want the plant to spread too much around your garden, cut back any young plants next to your existing shrub.
3. Use the three-year rule for overgrown lilacs
- See more: how to prune hydrangeas
If your lilac is overgrown and lanky, it will need a more drastic prune. You'll want to take out some of the bigger stems all the way down to the ground, 'because they're no longer producing any flowers along the stem, only at the top'. Be careful not to kill your lilac by overzealous pruning, however: you 'only want to remove one-third of the plant in any given year', so you can do a staggered major prune over the next three years. By then you 'should have a relatively nicely-sized two-meter plant that is filled with flowers'.
Gardener Scott's top tip is to 'take out older stems first' – they're easy to identify because they are wrinkly, damaged, or discolored. You can cut these old branches all the way down with the best hand saw; for medium-sized branches you're not cutting all the way back, use the best loppers instead.
In rarer cases where you plant 'really is wild and you don't know what to do with it', cut it right down to 15-20 centimeters. Aim for leaving 10-12 stems.
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