Hot Topics: How to Trim a Large Lilac Bush

woman trimming lilac bush blossoms

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Lilacs are purple spring flowers with an attractive scent. This DIYer wants to know how to trim their lilac bush, and if it's even the right season to do so. As always, the forum has some suggestions.

Original Post: Trimming a Lilac Bush?

bsa_bob2 Member

I have this huge lilac bush I want to trim back. Any tips on how and what and where? It's so huge and the limbs are maybe 6" or 8" in diameter. Please share what to trim and what not to.

chandler Forum Topic Moderator

chandler Forum Topic Moderator

All trimming of deciduous vegetation should take place in the dead of winter when the sap is down. Trimming in the summer may cause the bush to bleed to death. I would concentrate on the lower branches as they don't appear to produce any flowers. It will make the plant more well-rounded. I have heard they are quite hearty and can stand drastic cutbacks, but I won't advise that without first-hand knowledge.

marksr Forum Topic Moderator

I've trimmed mine as/when needed, but mine isn't anywhere that big. Minor pruning can be done anytime, but I wouldn't do any major cutting until fall/winter.

bsa_bob2 Member

Yes, I think you're right. I have cut this one down many times before in all types of weather. It's just gotten too big.

Vermont Member

I learned the hard way that lilacs set their buds for the next year in late summer/early fall, so the best time to prune them is immediately after they've completed flowering this season. Otherwise, you'll have no flowers next year and have to wait until 2019.

YaddaYadda Member

Consider cutting it down to the ground. It should recover OK and send up straight shoots, some of which you can cut for flower arrangements and the others you leave alone.

Norm201 Member

Does that mean cutting the blooms for the house will not affect the following year's flowering? Or do those cut blooms become the next year's buds? I often wondered if I should cut them now and fill the house with scent and color and not have any next year. (Actually, it's my wife's question.)

Vermont Member

I'm not a botanist, but I think you're safe cutting this year's blossoms. This year's blossoms don't relate to where buds for next year's blossoms will form.

New buds for next year's blossoms will form elsewhere later on this year. I'd wager that varies a bit from climate to climate. I also think that new buds for the following year will only form on shoots that are at least a year old, and not the shoots that immediately sprout up after a severe pruning.

Severe pruning usually results in no buds for the following year—at least that's been my experience. Buds will not form on "old wood" until the new shoots are at least a year old.

In the end, I reckon lilacs have a mind of their own; it's up to us to figure it out so that we can predict their behavior. It's a good thing that we live more than just one season so that we have the time necessary to recognize these patterns.

I've already said enough to make a fool out of myself regarding lilacs; I'm in the midst of trying to relate my understanding of biennial raspberries, where a bumper crop can only be had if the new canes from the previous year are well protected over winter to produce fruit the following year. The considerations compared to lilacs are quite similar.

Norm201 Member

No fool are you. What you say makes sense. My lilac bush (weed, tree, scrub, call it what you will) is only about five feet high. I will try and keep it under seven feet by continuous trimming or pruning, but the missus wants flowers every year.