Theer are a couple of lilac-pruning articles on the blog already, so have a look: http://awaytogarden.com/pruning-lilacs and http://awaytogarden.com/lilac-pruning-and-perfuming This is the last week I’d recommend pruning this year, because as Kenn implies in his answer, flower buds will be set shortly for next year and you’ll be cutting those off if you work July through next spring. Ideal pruning time: right after bloom. However (and there is always a however) if you are going to cut out entire limbs, it matters not when you do it because there aren’t going to be any buds because there isn’t going to be any limb for them to form on! ;-) From the photo your trunks look somewhat thin for the height of the plants, and the plants look somewhat stretched or lanky–as you indicate they are really not getting enough sun to be full and bushy. I have not had good results with trimming leggy lilacs like this back part way, hence the cutting at the base. To bring them back in scale and fullness, you basically cut out approximately one-third of the old stems to the ground over three years, yielding a shorter, fuller bush in a few years…unless there is still not enough sun, in which case the plants will just grow back in similar fashion. Is there any pruning that can be done on adjacent things (trees or other shrubs) to let in more light while you are at it? I think that has to be addressed as well, and sometimes a nearby tree limb of two can make all the difference, so your lilac pruning efforts really yield good results. Anyhow, do read the blog articles as they go into more detail.
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Welcome! I’m Margaret Roach, a leading garden writer for 25 years—at ‘Martha Stewart Living,’ ‘Newsday,’ and in three books. I host a public-radio podcast; I also lecture, plus hold tours at my 2.3-acre Hudson Valley (NY) Zone 5B garden, and always say no to chemicals and yes to great plants.