This is a pretty much a lost cause I think, but just in case there is
any chance at all of rescuing the disastrous winter losses...

I previously said my bamboos were all stone dead after last winter.
There is absolutely no signs at all of any regrowth so today I bit the
bullet and dug them up. The bad news is that the roots really are
completely dead and rotting away. But amongst the above ground stems on
two clumps I have found a total of five stems in the middle that are
obviously still alive (but with no roots). Phyllostachys nigra & aurea
respectively so both have issues with photosynthesis in the absence of
any green leaves.

I have cut them one node clear of the basal rot and placed them
horizontal on a tray full of peat based compost. I have no idea if you
can take cuttings from bamboo this way, but it seems like my best chance
of salvaging something from the situation. All the outer stems were
completely dead and dehydrated it was just a few at the centre of the
clump that were "alive" and were dying back from the ground up and top
down. There are no leaves at all.

My reasoning is that a snapped off piece of bamboo will lie on the
ground and might if conditions are right root from the nodes. The big
question here is what are the right conditions?

Being about 10 degrees of latitude further south might help...

Any suggestions or tips for making bamboo shoots root down?

Thanks,
Martin Brown

PS Looks like my fig tree is also stone dead and I thought they were
hardy once they had established. The winter was obviously pretty bad.

My understanding is that propagating hardy bamboos from culm cuttings is difficult, and doesn't work at all for running (leptomorphic) bamboos such as Phyllostachys. It seems to be mostly used for tropical clumping (pachymorphic) bamboos. The cuttings, as I understand, are usually stuck fairly deep vertically into a pot, or at an angle, with about 2 or 3 nodes under the soil, and the top cut off. Keeping them alive after they have rooted seems to be challenging. But you can google "bamboo culm cuttings" and "bamboo propagation cuttings" just like me and see what the sources that come up say.

I'm very surprised you have lost Phyllostachys bamboos such as P aurea and P nigra, because, planted in the ground rather than a container, both are supposed to be hardy to around -25C in this country, and I've heard of them surviving lower in the US. I've not heard of other people losing their bamboos because of cold, apart from known tender things, and container grown specimens.

I wonder if the real problem in your case is ground waterlogging. Apart from a couple of waterlogged specialist species, they don't like it soggy.