400 feet is a long, long hedge. Why not a mixed shrub planting? Even if you love a shrub, 400 feet of it will be interesting for only one season (two if it has berries or fall color). I'd most definitely do a mixed planting that would include lilacs, viburnums, hydrangeas, and a few conifers.

i agree .. too much of one thing.. will lead to nightmares ....

10 years ago ... i planted a plethora of flowering shrubs ... i was young.. i was stupid ...

10 years later.. they all need RENOVATION PRUNING OF FLOWERING SHRUBS [google that] ...

what was easily dont 10 years ago.. is seeming incredibly overwhelming now ...

you mention showing their knees .. i dont know what that means .. unless you simply mean that an overly mature flowering shrub needs its maintenance pruning..

and to contemplate 400 feet of hard work blisters my mind ...

diversify ... add many things that will need no care EVER .. and throw in a few of the things that you like and wouldnt mind taking on the maintenance ..

good luck

ken

Take a look at Syringa microphylla 'Superba'. It's grown reasonably fast for me, requires no pruning, and does a good job of stinking up the yard when in bloom. I've managed to tip root several, so it can be propagated, but that isn't a fast method or a method that produces a lot of plants.

hey mad.. you made me chuckle ... a lilac that requires no pruning.. lol ...

what is your oldest one???

out here in the country .. i have seen generic historical lilac.. bigger than the 100 year old farmhouses they were planted in front of ... and some of them are 2 story houses ...

that would be one magical named cultivar to never need pruning ...

i reject your reality... and substitute my own ... lol ..

i have gotten many types of lilac from gardening friends... since they put up babe plants as runners/suckers ... cant think of the term.. and are easily transplanted in spring.. talk about cheap .. free ..

ken

There is a lilac nursery on-line and they list ultimate height/spread for many cultivars. Maybe you'll find a couple you like.

I have six s. microphylla Superba, which received their first rejuvenation pruning today after 23 years. If I deadhead them after flowering, I usually get a respectable second flush later in the season. If I don't prune them---which is the case most of the time---I get a much smaller second set of blooms, if any.

I estimate these guys to be no taller than eight feet after all these years. However, they don't fill in a hedgerow the way vulgaris might---with lots of suckers. Superba has a habit more like weigela in my yard---a vase shaped shrub, 6'-8' high.

Just make it a point to buy your lilacs on their own roots---NOT grafted.

Here is a link that might be useful: lilacs.com