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My parents have had a lilac tree in their backyard for about 25 years. It came from saplings given by a neighbour. Over the years, some secondary shoots ("suckers", I think?) were removed as they broke under the weight of snow. Up until a year or two, it bloomed consistently every spring.

This year, it looks like this:

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Large areas of the top have no flowers, only dried bits from last year, although all branches have leaves. There seems to be three main shoots that are devoid of any flowers.

enter image description here

On the older shoots, there seems to be moss or some white stuff on the bark. I don't know whether this is normal.

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I don't think my parents have done any pruning of this tree over the years, except for removing obviously broken branches. They are now thinking about cutting the base of all the trunks that have no flowers. I asked them to wait until I could verify whether they could be salvaged. Is there anything we can do to make these trunks flower again?

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    Those withered flowers will not be from last year, but what's important is the condition of the leaf growth on those stems particularly, so how are the leaves looking (droopy, soggy, dried out?) A shot of this white stuff you mention would be useful too, and knowing whether it's just on the woody parts or also on foliage. And how's your weather been? Unusually dry or cold or wet? – Bamboo May 23 '16 at 14:37
  • @Bamboo Leaves look fine. There's no visible difference between leaves on good or bad branches. Added picture of bark. No weird stuff on foliage. Weather's been average, I think. Why do you say the withered flowers aren't from last year? – isanae May 23 '16 at 14:43
  • The extra photo shows what appears to be lichen, harmless, but I thought you said it was white? Looks purplish brown in the photo. The image of a withered flower is still showing a hint of purplish colouration, and is still pretty much intact apart from some withering, ergo, it's one of this year's flowers that's shriveled, not sure why, maybe withered ones were the earliest flowers. I was wondering whether there was trouble at the roots, but if all foliage is fine, it's not that. – Bamboo May 23 '16 at 14:50
  • @Bamboo I've uploaded higher resolution pictures. The purple haze is a combination of lighting and a crappy phone camera. Stuff on the bark is white, withered stuff is very brown and dry. As for "maybe withered ones were the earliest flowers", the dry stuff has been like this for weeks. They never bloomed at all. – isanae May 23 '16 at 15:00
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Flowers are an apical meristem, which means flowers occur only at branch tips. Hence if one never prunes their lilac bush, it winds up being exactly like what you have - a bunch of long stems/trunks with possibly a flower cluster way up high at each stem/trunk end.

The basic principle of pruning is that decapitating (or pruning off the end of) a stem will release latent buds that will produce new shoots (more branches). With a lilac, one can remove a significant length of a stem to lower the canopy and this will as well produce more shoots which means it multiplies the number of potential flower sites. You can even go so far as to chop the entire lilac bush down within a foot or so of the ground in late winter, and it will regrow. You can do this if you are frustrated and just to start over creating the bush. However, it takes about 3 years before it will again bear flowers like you want.

So, instead, you might want hard prune about one-third of the stems this season (anytime until the flowers have completely faded/browned is okay). Prune down to a level below where you want to have the canopy in three years (about half that height). Next year do another third and the year after next, the remainder. Obviously, choosing stems scattered throughout the tree each time will give you the smoothest transition, but it is strictly up to your preference.

One should also do some maintenance pruning every year after flowering (that is if you want the flowers; else you can do it before) just to shape the plant - keep it looking open and pretty. You can easily make a bush covered with blossoms, a shorter broom with flowers only at the top, or some other style with just a simple annual pruning/shearing. If it starts turning into something you don't like simply change where you cut to make it be more like what you want.

  • I've reworded my question and changed the first image. Many areas on top of the tree have no flowers at all, which is the crux of my question. Is this still normal? – isanae May 23 '16 at 14:20

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