Cut all the stems of them both down to 18 in. tall at the end of next winter. They will then come back shorter and straighter, with lots of full sized flower heads after the start budding up a few years later.

So chop it down from like 15 feet "tree" to 18 inches? Would new grow have to come from the root then?

And what about doing that now? The picture is just taken today, so nothing is budding yet on the red one.

It will come from the remaining 18 in. stem sections. If it's not really spring there yet do it now. Again, there will be a delay in flowering for a few years but based on what you have said you are really going to like how they look later. If you start them over from 18 in. high.

These common lilacs were pruned down from over 8ft tall to level with the ground. They didn't bloom for ~3 years but when they did! About 5 ft. tall now in this picture.

I do like it being more in a "tree" form then a wide shrub. Plus it's still a rather tight space for getting sun if it's short. And my main deck is on upper level. Would it make sense to cut down to say half way shown in the picture so it'll regrow starting at taller height? Note all the red-lines are a single plant, so it'll be a 3-split trunk if doing that, and they won't be super straight but should be ok if re-grow with nice flower heads and buds.

Should be fine.

The only thing I can think of is the tremendous amount of sprouting that will occur up and down the remaining 3 original stalks and from the base. So you may have to manicure things a bit to keep them in a tree form. Also there will be a 'bloom' of sprouts coming out of the top end of the 3 remaining original stalks which may or may not be to your likings.

The positive result from cutting them down to a shorter height is that the flush of sprouts tend to support each other from 'flopping over' as they all grow at about the same rate.

Guess if I want to re-train it to tall tree form after cutting to 18", I'd still have to manicure over few years anyway. So no getting away from manicuring.

Looking up close, just starting to see some green buds now. Will that be too late to do hard prune?

Pic of the bottom. First trunk split is about 24" off ground.

Flower buds are formed on last years growth (old wood), so anything cut from the tops this time of year will pretty much end any chance of blooms until next year. Wait until it stops blooming and prune soon after that. Any new shoots that grow after pruning may contain flower buds for next year. No guarantees. These are full sun plants and flowering energy directly corresponds to how much light they get. Little sun = little blooms.

boy im really feeling like this is no lilac ive ever run across ...

before you do anything .... get some pix of leaves and flower buds.. so we can confirm the ID .. and then suggestions ...

i dont understand what you did with the first pic.. but all i want to see is what you blanked out.. lol ...

lilac flower buds were set last fall ... very tiny.. but you can see them if you knew to look for them ... if you prune them off before bloom.. thats it for this year .. ergo .. the usual suggestion.. is to prune spring flowering shrubs after they bloom ...

this is how easy it can be to understand what is being suggested by embo ...: https://duckduckgo.com/?t=ffnt&q=lilac+rejuvenation+pruning&iax=images&ia=images

and if you watch a few.. see if anyo of them look like your tree ...

welcome to the forums ... big city location is usually helpful ... zone alone isnt as much ... in CA you could be toronto or windsor.. and that might make a difference .,.. or anywhere down around lake erie and ontario ... big lakes change weather patterns...

ken

Picture from couple years ago of the leaves. Don't have any flower pic

Current pic

those are NOT lilac leaves .. as far as i know ... ken

https://duckduckgo.com/?t=ffnt&q=lilac+leaves&iax=images&ia=images

Ok interesting. Any idea what it is? It has pinkish flowers. There are 3 of these. The other 2 plants with darker purple flowers and darker green leaves. All same height just under 2 story tall. Flowers do have some fragrance.

I agree,

According to your picture, you have multiple varieties of trees/shrubs growing in that clump, none of which resemble lilac.

One in front is possibly Viburnum?

The second picture idk for sure, maybe lilac.

Well the mystery thickens...

All the pictures here so far are from the same plant... or at least from the same main truck anyway. In the leaves picture, it's the lighter green leaves in front. The darker maple leaves on top are from the giant maple tree dangling over.

Just going by the height you described and the similarity of the flowers to lilacs, you most likely have Viburnum. They will grow to 20 ft. or so.

Viburnums are also full sun plants so you might not want to cut them down too far if the height helps them get more sun.

You can Google pruning of Viburnums, yours may be a unique situation with them growing is such a tight space, so you might have to 'wing it' but I would keep as much of the height at you can, again if it makes a difference in sun exposure.

They too, like lilacs, should be done after this seasons bloom so they can make flower buds for next year.

The last two shots do show a lilac. It's just a different kind than Syringa vulgaris. One like S. x prestoniae for instance.

Anything over ~10-12ft would not be common lilac (Syringa vulgaris) but Syringa reticulata (Tree lilac) has similar looks like vulgaris when bud breaking (last picture).

But those leaves in the forefront of the shrubs from 2 years ago is definitely Not lilac.

Oh?

syringa prestoniae leaves - Bing images

Embo,

The OA said his plant was 2 story high.

So?

So what syringa prestoniae do you know of that has height like that?

I've seen lots of tall lilacs of various kinds. And the original post indicates these plants were drawn up by crowding. And that they are approaching 2 stories - in one size categorization system I think is pretty apt tall in shrubs starts at over 10 ft.

And this is all side issues anyway - there is no question that the last two pictures show lilac features. Whether the plants are x prestoniae cultivars specifically or some other kind.

Could these trees be forced to grow a bit taller than normally recognized? The area is essentially like a 3-sided trap. Left side is covered and over-hang by a 4-story high maple; back-side is the giant 3-story high cedar hedge; front-side is the deck and house (again, 3 stories due to walk out basement). And there they are 3 of these Lilac-ish trees stuck tight in there. Were actually 5 of them too. I suppose 20-30 years ago the area would have decent sunlight when original owner first planted them, definitely not now.

I think it's a preston lilac too.

If it's was trying to grow out from underneath other plants and reaching for the sun......

Most of the preston lilac I looked up require 'Full Sun'.

Yeah I guess it's likely close to "Full Sun" 20+ years ago, only with the house blocking sun in morning. Even with the hedge and maple tree trimmed now, it still won't be an ideal spot for full-sun plant that's for sure. May look into cutting down the maple tree completely in couple years, which will let in a lot more light. Neighbor said the maple was not even planted, just a "weed" that previous house owner left to grow.

Probably a Norway maple which is in fact a nuisance species in North America that floods wooded sites with its progeny. Despite that still being produced and planted within the realm of ornamental horticulture. With plantings of it in my dry summer area often having nothing growing underneath.

Just to add: The definition of 'Full Sun' is at least 6-8 hours of direct sun each day.

So a 'Full Sun' plant doesn't necessarily need a whole 14 hour day of direct sunshine. You might even get by with somewhat less direct sun in addition to some hours of 'dappled sun' occurring at periods during the day.

Just posting a pic of flowers now as there were some doubt if it's really Lilac...