Lilac tree pruning - SoS

I have 2 lilac trees (at least that’s what I think they are!) which aren’t showing much growth, and haven’t performed well over the last few summers. But they do have buds appearing in parts. Pictures attached. How do I revive these? Any ideas on how best to care for these through the season?

 

I have cut lilacs back severely to within 24 in of the ground and they shot back up to 6 ft by the end of the season. Admittedly I lost a seasons flowers.
Common lilac can sucker quite badly, pull these off when you see them.

 

We have several lilac trees, and all did well last year. None of ours have buds yet this year, so yours are doing well @Greenfield81

 

OP could be in a warmer part of the country or their lilac is in a more sheltered spot.
My Syringa pinnatifolia has buds some of which are just bursting.

 

Mine is hardly even showing any leaf yet, give them a bit more time. The new leaves coming on the bit that you're holding look healthy enough to me.

The one near the big laurel looks as if it is leaning towards the light, which is inevitable with the laurel casting shade - I don't think there's anything much you can do about that (other than remove the laurel which would be a bit drastic).

The one with the thick knarly trunks is probably just old. You can cut the whole lot back down low as @NigelJ said, or for a less drastic renovation take out just the oldest branch right down, then another one each year.

 

Thanks very much for the advice. I’ll give them the season and see how they perform - patience is a virtue. Fingers crossed. Out of interest, if drastic pruning is required as suggested, when should this be done? After flowering in winter?

 

In general to maintain flowering the next year you trim back just after they have flowered but drastic cutting back should be done when they are dormant. You can get away with hard prunning when they are in growth but it will slow the rate they recover, whilst winter pruning will promote stronger growth.

They look quite congested as they are and I think I can see some dead wood. If it is obviously dead you could remove that now.