As the summer draws to a close, there are one or two tree species that actually start flowering.
One of these is the Japanese Pagoda Tree which comes into flower in late August and can keep going until the start of October. The trouble with this species is that it needs a warm summer to ensure good flowering, and this year, in London at least, it's been a bit disappointing. So while it's warmed up in September, it might not be enough to ensure we see many flowers this year. The best thing to do then, is make a note of where your nearest tree is, hope for a warmer summer in 2022, and come back next September!