Chamaerops humilis seeds
 

Two of my European fan palms have made some seed. A
couple of years ago I managed to germinate some of them
even though they were not still green and not ripe.
This year's crop is still green. Would it be OK to
leave them on the palms and see if they ripen, or
should I pick them now? And if I picked them now, how
long would they remain viable?
I live in NE London and it rarely gets colder than a
couple of degrees below zero.