Hi Pete,
I think no one is responding to your question for no one is sure of the answer. I have never read of any study or even opinion on what triggers flowering in any of the species.

In the northern hemisphere the hellebores flower in the south in zone 6 now and in my zone 5 hellebores will start flowering in about 3 weeks. Hellebores can flower sporadicly during the season most anytime. I have buds on x hybridus as well as niger at the same time.

I do not think that hellebore are sensitive to night length for flower initiation but they seem to be more of a growth cycle that might influence flowering. A nice regular growing season with good moisture levels and normal temperatures will produce abundant flowers. If the hellebore become stressed they flower little. Hellebores do form their flowering buds in their growth, they do not form flowering buds in dormancy or going into dormancy. I would not be surprised if they have already determined the amount of flowers they will produce for the spring in the previous summer. They seem to flower once these buds have formed and the temperature fluctuates.

I do not know if all species behave the same.

I hope this has been of help, what are your observations?

Bruce

Thanks Bruce, and I agree that the flowers are initiated during the current season growth phase. What I have observed here in NZ is that some regions are distinctly ahead or behind others, by up to a month (for the main flowering flush I mean) depending on environmental factors I presume. For example another H. enthusiast not 100 kilometers away but situated in a comparatively cold valley in a lower mountain area flowers about a month later than mine, but so too do some of the spring-type bulbs in that area e.g daffodils. Which is why I was presuming maybe soil temperatures and/or other environmental factors play a significant part in triggering off flowering.
Agree also with the comments here and in other threads re flowering being triggered by stress factors. I've observed in 7 years of H. growing that younger plants tend to do this much more frequently than well established plants, suggesting that the oldies are more tolerant of conditions which are not ideal? Bit like humans I guess!
Thanks again for your interest Bruce
Pete