That's what they do. Form a hedge.

yes.. they are huge uncontrollable plants ... you probably should start thinking about removal ...

there are some dwarf varieties.. that still can get rather wide .. but not as high ...

if you are dealing with what one might call.. just the old fashioned run of the mill Ls ... well.. i have seen them at centennial farms.. nearly as big as the farm house ... given apparently.. a hundred years.. lol ..

ken

Sucker-Stopper is a product which could be of use here, although I think it a bit of overkill perhaps, in that this is a highly toxic material, expensive, and probably better used in a more high-impact situation. Just throwing it out there for your perusal.

There are non-suckering "regular" lilacs, but I think that it may depend on if the shrubs are grafted or own-root. I got lucky with a President Lincoln (I think that's the name) that I got, but if I got another, I'd probably go with a reputable mail order place that would have suckering/non-suckering information.

There are a whole line of lilacs bred by Father Fiala, who, I understand, focused on breeding non suckering shrubs.

I wouldn't consider replacing the lilacs in question with another. Now that you've confirmed my suspicions that they will present an on-going maintenance headache (behind rose bushes), I will eliminate them. That will relieve me of pruning them and the neighboring spirea to make them all fit. The tree form idea was their last chance, and it didn't work. It also wouldn't have made sense since the bare legs wouldn't have shown anyway. I will transplant a sucker to an open space in the back forty and let it grow into a centennial house. Thanks all.