from the link below: the more reliable method of propagating heavenly bamboo is through semihardwood cuttings taken from mid-July to early September.

https://homeguides.sfgate.com/propagate-nandina-domestica-22985.html

semihard is the key word ... new growth is lush and green and usually floppy .. the stem need to bulk up enough.. that it can hold itself up ... but also.. wont rot before it roots ... too young they rot.. too old they wont root ... so the middle phase ...

thats the key idea you are asking about ...

ken

ps: the alternative.. might be easier ... take some runners from behind the plant ... which already have some roots ... pot a few... water them in full shade for a few months.. and by fall they ought to have a root mass for fall planting ... or just shove them somewhere else.. you know where to get more.. lol .. the pot just makes you remember to water them while they grow larger roots ... thats how i do it with lilac

as per your title.. so its not a zone thing per se ... no matter where you live.. its all about the wood ... that is why you never found a definitive answer for z8 ...

Ken, thanks for finding that link for me. Sounds easy enough to do, just requires some patience for start time and results!

Hello KW.

I'm in your zone in King county, a few years ago I've used tip cuttings from the current year's growth around June'ish I think, and kept under a pop bottle cloche. I used 0.1% IBA dust. Works well, and rooted in the shade unattended within about 3 weeks.

Semi-hardwood cuttings from last year's wood should do fine if you want to prune things up now and use the prunings. You may have to up the IBA to about 2000 ppm (0.2%) for semi-hardwood.

This is what Hartmann-Kester say:

Cuttings. Dwarf nandina are easy to root.

Shoot tips (without brown wood) are trimmed to 2-2.5 in lengths, stripped of bottom leaves, quick-dipped in 1,250 ppm IBA and 500 ppm NAA, and stuck directly into liner pots.

Cuttings can be rooted any time of the year, except during the spring growth flush.

Winter rooting requires bottom heat. <---this suggests semi-hardwood is ok to use

‘Harbour Dwarf’ and ‘Compacta’ can be propagated by separation of suckers at the base.

Good luck.