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The leaves are falling off the 6'x6'x10' beast of a lilac I inherited from the previous homeowners, and I'm wondering whether now is an appropriate time to start reducing the height.

The plan was to take the front 1/3 or 1/4 of the canopy away this year, leaving the rest to flower, look pretty, and shade my pond through the next season; then repeat each year until it is a manageable size.

So I have two questions:

  • Is pruning at this time of year likely to cause problems, and if so, when would be a best time to prune?
  • Am I correct in assuming that the pruned section would likely produce leaf and/or flower (pond cover) within a year?

Thanks, James

Addenda from the responses so far:

  • Image from mid July (pointing roughly north). The main bush hasn't changed much, but I've thinned out the front (below that sandstone slab), excavated a waterfall and created an alpine rockery: thePatient
  • Why: It's a bit unweildy so needs reining in, blocking late afternoon sun down half of the garden.
  • Health: Seems quite happy. We did not have the house when it was in bloom, but there is evidence it put out a fair number of flowers.
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    It would be helpful to indicate what part of the world you live in since weather is different in every region. Also including some pictures of the lilac will give everyone a visual of what your plant looks like in terms of maturity
    – JStorage
    Commented Oct 26, 2016 at 18:06

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This is the perfect time to be pruning. Lilacs can be absolutely beautiful small trees not so much as a bush. By your description of size I am assuming you have a Common Lilac or Syringa vulgaris. Please send a picture so that we know for sure what you are dealing with and we can guide your pruning without assuming. Why is it that you want to reduce the height? It is doubtful that Lilac will grow any taller than 10'. Chopping it in half will look awful all winter long and will cause lots of new growth looking like a tree chopped down and trying to come back.

We can help you THIN this shrub, removing dead, dying branches, any branches that turn and grow towards the center, branches that rub together, branches that are thin in comparison to the branch it grew out of, choose the healthiest of the multiple trunks Lilac as a tree. If you would answer a few more questions such as; is your lilac blooming well? Does it look healthy? Please send a picture. There are lilacs that are supposed to be a shrub not a tree so we'd have to see it to ID and help you prune. You never want to prune more than a third of a plant at one time, so if you need to renovate this plant pruning might have to be done over a period of years.

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  • As you can now see, there's a fair bit of wood underneath it, so you may have a point about thinning it too: not to provide area for more plants (not possible, as it's thick with roots), but to give the thing a bit of space!
    – James Finch
    Commented Oct 26, 2016 at 20:10
  • This is what I imagined. Oh, James, this is a beauty! Definitely could use pruning. Got great bypass pruners? Felco makes the best. Get a pair of hand pruners and a larger one with handles. Clean with alcohol before and after. I'd start by pulling up the ivy/ground cover. Then go in an get all those thin, small caliper stems from the ground. Do this then send me another picture. What a beautiful specimen!! I am so glad you asked this question on this site! To imagine chopping this tree down....arghhhh!
    – stormy
    Commented Oct 26, 2016 at 20:37
  • I also adore the dove grey wood, stone...please maintain that patina and grey color as your architectural additions. This highlights the plants so beautifully. Get that ivy off your fence or plan on building another fence by next year! Just pull it out. Will not be a permanent solution but a good start. You'll see thin branches/trunks coming out of the ground and those need to be cut off at the surface of the soil. Oh, I would so love to actually be there to help you prune. What color are the lilac flowers? Are they dark dark purple or pure white???
    – stormy
    Commented Oct 26, 2016 at 20:47
  • So with regard to reducing height, is the following the suggested approach: To thin around the ground level, leaving only a few main "trunks", then prune off some of the branches from the trunk at the desired height to encourage bushing at head height?
    – James Finch
    Commented Oct 27, 2016 at 8:09
  • You've got plenty of a canopy on that guy. What I would do is just thin it and forget heading. It won't get any taller, pretty sure. It will try going side ways. I took another look and was wondering why you want to reduce the height? I love the rock waterfall (you could also make a bubbler for the sound of water and just use tiny pea gravel to be the 'water'. I like the ivy on the fence and you could just thin that as well and the ivy on the ground. At least sequester and control the ivy. The fence beneath might not look so pretty...thinning your lilac will make it feel more in...
    – stormy
    Commented Oct 27, 2016 at 18:43

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