Timing for a trunk chop.
 

I've been offered a Laburnum (golden chain tree) to bonsai. Right now
it's got about a 4 inch trunk and is around 15 feet tall. I'm planning
to do a trunk chop on it and dig it up, but I need to do it somewhat
soonish or it's going to get removed and chipped. Is it better to chop
it now, or should I try and wait until the leaves fall off for the
winter?

If it matters, I live in Seattle (zone 5)

Thanks!