Update.
2 of these Borys are now shooting !
One has 2 medium shoots showing and the other has what I would estimate to be ( in relation to the existing culms) 1 large shoot.
This seems to be very late in the season.
Do these shoots have a chance to attain full height, branch and leaf ?

Good question. I have some shooting now too which have even less of a chance here of making it into winter. My problem is different tho since my culms won't make it thru winter anyway unless I bury them. Anyway, I think if you give your plants time to grow out some new rhizomes then their food storage will assure you get new growth in the spring. This time of year I begin thinking more about what's happening underground for winter survival.

In my experience, if the shoot has around 60 days before a hard freeze, it has a decent chance of surviving. Differences among species, for sure, I have seen a very late shoot on our native Gigantea reach about 4 feet, stop for the winter, and start up again in the spring, fully maturing. This has happened the past 2 winters, so these guys survived -12F & -19F without even blinking! Thuja if you can get yours to overwinter you should have the hardiest bamboo that I've ever seen including the Fargesias in my area.

Hi Thuja & Hoosier52, thanks for yr input.
It now seems that 3 of my 4 borys are shooting so it will be interesting to see if these culms manage to manage to make it to maturity. I would not expect to encounter any hard frost until later in December if at all. In fact last Winter was almost frost free so it is as though frost is not really an issue for me.
I suppose that my question re the development of these culms was borne with the thought in mind that I normally never have to mow the lawn much after the beginning of October. The grass seems to more or less stop growing then so I associated this with being general halt to all growth. This is my first season with Bamboo so I have no experience to draw on.
I have a Vivax which put up 3 really nice culms this Summer. I first noticed these culms emerge during early May and only now have they nearly fully leafed out. I suppose that this process has taken more than 3 months so once again, give the timescale, I was curious as to whether this late season growth had a chance to make it.
I guess that I will just have to be patient and wait and see!