It is not necessary to have the Roselle/Jamaice/red hibiscus to make Kombucha. My DD uses good quality black tea and sugar and adds the SCOBY.
Here is a helpful article.
We can grow the Roselle here in Savannah, Georgia, zone 8b.
In 2013 I got a late start with my seeds; here they are on June 15th
but they still had enough time to mature.
Here are the same plants Sept 2013:
and on Nov 12 2013 ready to harvest
My friend Jane grows the Roselle. Here it is in her garden Oct 3 2015
Since you are a 1/2 zone cooler I would suggest starting the seeds indoors under lights to get ahead start. You should enjoy a good harvest.
This is as easy to grow as Okra. Good luck.