expression, noun
too late to be useful; after the fact; a day late and a dollar short
Idiomatic expression literally meaning 'flowers that don't arrive in time for a Buddhist service'. Used for something that comes too late to be of any help, like an offer or assistance after the need has passed.
彼の助言は会に合わぬ花だった。
His advice was too late to be of any use.
事故が起きてから安全対策をしても会に合わぬ花だ。
Implementing safety measures after the accident is like closing the barn door after the horse has bolted.
From the literal meaning 'flowers that don't arrive in time for a Buddhist service (会)'. The expression uses the archaic negative ぬ and refers to offerings that miss the memorial occasion, hence something that comes too late.