It is generally recommended to put it on a three year plan: Hack out a third of the trunks each year for a total renewal by the end of the third year. As you know, the older trunks look much older than the newer shoots.

You probably know that it blooms on old wood, so whatever you cut this year won't bloom next year.

I definitely wouldn't hack it all the way down like with say, Forsythia. When you cut out a third, it will send up plenty of new suckers. Those will be fresh-looking, and you'll notice an improvement right away the first year.

It really was a good year for Lilacs, wasn't it? Everybody's look great, regardless of the level of care they are getting.

I prune mine while they are blooming and give the blooms away. Then I know I won't be cutting off any of next year's blossom buds.

From your description, your lilac is just a moderately sized bush. Still, it must fit into your landscaping plan.

Working as Gardenscout suggests will preserve bloom yet allow you to make it smaller. If you want to go the extreme route, you can just cut it all back to just above ground level in the winter/early spring. It will put out tons of new shoots, which in a couple years will give you a beautiful small bush loaded many times over with blooms. This small bush, however, will turn into a much bigger bush over time if you don't keep it pruned just after blooming.

And, you don't HAVE to do anything to it except clean out dead stuff. If it fits into your landscape as it is, leave it alone.

thanks for all the responses! i'm out of town but will post a picture when i return. you all obviously know what you're talking about, and i am a total gardening newbie so i really appreciate your insights. i really don't think i need to change it's look, but i got the idea from somewhere that it might make it stronger to give it some attention and radical pruning. i don't know why i listen to such ideas; it has obviously thrived without any attention for a very long time. the blooms were gorgeous this year and smelled divine, but as soon as i cut them and brought them in, they wilted within 24 hours. someone told me blooms from old, woody bushes will do that. finally, it sure has been a good year for lilacs (and all the blooming trees in my yard). i bought a small lilac bush for my husband for father's day 3 years ago and almost dug it up last year because it never has done anything. i'm so glad i didn't. this year it went crazy. we planted it under a window, and the smell of the blooms wafting in on the breeze was grand.

If you want to start it over and get rid of the heavy trunks cut everything off at about 18 inches near the end of winter. You won't see bloom that year and maybe not for a few years but in future years it will probably look quite like you want it to. The only thing would be if the abundant dead wood indicated a problem and this prevented it from growing back as strongly and evenly as it would otherwise do.

I went after my older lilac, took out all the shoots except the largest ones and the oldest wood trunks. Just cleaned out the whole area. Mine was about 8ft across! Not blooming well, too many suckers.

It came back well the next year, with plenty of flowers, started more shoots. My older shoots bloomed because they were large enough, had bloom spray stems from the previous year to show they could bloom.

I would never cut a bush totally to the ground, loose bloom time for a couple years! Too much of a loss for me!! I did reduce the old trunks as the younger shoots increased in size. It is now pretty much renewed, had all the blooms in the years between.

I now keep my new shoots limited on the lilacs. I let a couple grow each year to rejuvenate, but trim off other new growth at the ground. I watch the bloom quantity on old wood, sometimes remove them if not getting much production. I am braiding some small shoots, going for textured trunks. I have seen some old bushes that were braided and like the looks. Maybe I will have more one-trunk looking lilacs in the future!

With new shoot removal, a bush is able to focus more energy into flowers, not just stem growth.

I hammer the woody stems on cut flowers, if they are very old or thick. Seems they can absorb better that way in vases. I still don't get a long vase time before wilt, but good for several days. On newer stems, softer wood, a daily trim can help when you change water, like with roses.

Being further north, my lilacs are just getting started, smell DIVINE on the breeze!! The small white one has no buds, but all the old bushes are loaded. Not sure if the sustained winter cold had something to do with blooming.

Lilacs are great plants(Syringa) but they do tend to sucker like mad,and surely heavy pruning would just encourage this?

Thanks for all the posts. It sounds like I should either leave it alone or cut it back some, but not all the way to the ground, and not all of it... I think I have figured out how to post a photo of it, in case that changes anyone's mind. Thanks!

Here is a link that might be useful:

Best not to prune. The less they are pruned, the more they will flower. New branches formed after any pruning won't mature enough to flower at all for around 7 years.

Resin

I just wanted to add my voice to Gardenscout's and support the "three-year plan." Lilacs respond extremely well to pruning and it has not been my experience, as resin suggests, that it takes new growth seven years to flower.

Danielle, I also agree with Gardenscout and Mlevie. Your shrub is little crowded, so I would thin it out some. After it has finished blooming, you can cut out up to 1/3 of the oldest, leggiest wood each year for 3 years and rejuvenate an old leggy shrub. Cut them at the base, don't leave half-cut big woody stems, that would look weird.

You could also thin it out a bit and prune it more as a multi-stemmed tree. But common Lilacs have a naturally suckering habit, so to leave the large older stems and cut off the suckers would be a never-ending battle. Personally I think it looks weird to have big trunks and little suckers and no stems that are in between.

Or you could do what my neighbor does, which is to prune his row of Lilacs with an electric hedge clipper into a rectangle with a flat top as if it were a Boxwood hedge or something. Poor things never really flower, just a few pathetic blooms along the edge. (J/K, please don't do what my neighbor does! :)

well, after much thought and consideration, and the hugely beneficial (and much appreciated) input from everyone here, this afternoon my mother and i went after the lilac. apparently her parents had lots of lilacs when she was growing up, and she talked with me about her fond memories of pruning the lilacs with her mother every year. that made the effort worthwhile, regardless of the outcome. we went with the 1/3 every 3 years plan - she selected the spots to cut and i did the cutting. tree already looks better, and now i have a new fond memory of a special time spent with my mom. cool all the way around!

Will these survive?
We neglected our lilacs for 5 years - they were now almost 12 feet tall & looked awful. I had enough, not knowing what exactly to do, I just trimmed (whacked) away - only to find they were grossly attacked by VINES, VINES & more VINES! When I finished there is almost nothing left. They look better??-yet "weak & thin" compared to what was there.

Will they bounce back next spring (wrong time of year for me to do this) or should we take them out completely?
What problems might we have next spring or this winter?