Hi Guest. I don't have a numerical answer to your question, but I can assure you that it is perfectly hardy throughout Britain, so I'm sure it will be fine where you live.

Do you know about the US hardiness zones?
http://www.usna.usda.gov/Hardzone/ushzmap.html

http://www.usna.usda.gov/Hardzone/hzm-nw1.html (this page has a temperature conversion as well as examples)

You could find the zone with the range of temperatures (don't forget they use the Farenheit scale) you'd expect where you live, then check on an American website to see if the plants you want to grow are likely to survive.

We live in what equates to zone 9a here in Cornwall. I see that a US nursery in zone 4 finds that Sambucus is hardy there.
http://www.stargazerperennials.com/BLACK_BEAUTY_SAMBUCUS.html

My info is that it is hardy down to -37.2 deg. C.

That's REALLY hardy! No worries in France then.