London plane seeding pattern / rhythm?

[Sheal]

Presumably, that they remain 'viable' for some time, maybe years, 'waiting' for better (wetter?)

Some seeds can remain viable for hundreds of years. Even our every day tomato seeds can be stored for decades and will still sprout given the conditions to do so. Many plants have amazing ways of keeping themselves from extinction.

That is a really good read. Many thanks.

You're welcome.