I have the other 'tree gardenia' (G. thunbergia), which I grew from seed bought on ebay (still a long way off their first flowering). I would think the Rothmannia would require pretty much the same conditions as a gardenia, but here are some official cultivation notes from one of my refs:

Grow these frost-tender plants in a sheltered, sunny or semi-shaded spot. A well-drained soil enriched with organic matter is ideal, preferably with a neutral to slightly acid pH. Propagate from seed in spring or from cuttings in summer. Zones 10-11.

Hope that helps a little.

hello, I have two growing west of Brisbane. One is covered in flowers and the other has very few so far. They are less than a metre apart and are approximately 2m highbut very narrow. They are growing in very clayey soil with only a superficial layer of topsoil over. They face West and have not had any water except from the sky for the last6 years, were only watered to establish. They should do very well with better conditions as they have been very tough here just to survive in the soil which is not good. They do not have any seed at the moment but did set some last year - if they have any this year it will not be ready for some considerable time but if you wish to send me an email with your address I will send some if and when it is ready. I would think the only possible problem could be with too much water during the wet so place on a slight mound if soil does not have a slope. Robyn

Hi Robyn,
Just commenting on an old post here...

I've a Rothmannia which is struggling with some kind of canker, and never really produced much for flowers -- considering removing it.
How are your trees going now? I also am keen about propagating and I understand the ideal time for taking Rothmannia cuttings is now. Would you be interested in sharing a few slips of the floriferous tree? Perhaps I have something of interest to you?
Regards
Paul

Hi Paul, I just noticed your comments from 2016 regarding a Rothmania, were you succesful in obtaining any cuttings? My plant is out at the moment in all its glory, it does not receive much water at all in the hot W.A. Perth summer, it is in a very hot position facing west, so I can vouch that they are pretty tough, so if you still require cuttings or seed (I did raise mine from seed, but very slow), please feel free to ask.

Regards

Margaret

They took a while to get established but once they found their feet boom blooms galore

tree gardenias continue to remove the seed pods which intern force the plant into 🌸

plenty of good organic fertiliser.