I am not sure there is anything scientific but you can check google scholar. I like this website for good info and they have references if you are so inclined to research

http://www.llifle.com/Encyclopedia/SUCCULENTS/Family/Euphorbiaceae/32500/Euphorbia_trigona_cv._Royal_Red

says they don't flower and is only known in cultivation so most likely a hybrid.

....that was my source..... i was hoping for something different.

Thanks Stefan for now peaking my interest. Great question. Hope some one comes up with your answer. By looks alone, my guess is with E. latea. Nothing scientific there. If it was a 'sport', I believe it would still produce flowers and seeds? I really haven't noticed flowers on my E. latea as well.

Ive seen pictures of lactea. Mostly wild shrubs, and they make seed pods too. But it takes an effort to spot them.

Sorry for the miss spelling. Thanks Stefan.

I dont really care. If remembered every correct species spelling, id be a phd by now.....

"It takes an effort to spot them". Maybe they don't bloom easily in cultivation and if they do, not easy to spot. Rubra maybe a cultivar and E trigona clones that bloom rarely (or not) may have been selected recently for commercial propagation, which are the plant we enjoy today. But E trigona seems to have been around for a while. There is a E trigonum with matching description listed in Miller's "The Gardeners Dictionary" published in the 1700's!! Maybe not the same plant, but many searches link to this reference. Still a possible hybrid but resulting from natural crosspollinization a long time ago.... Could explain why little to no info can be found on this. I only found a couple scientific publications available for a fee (~$15) with possible info on their genetic make up. Sorry, did not purchase them but the answer might be in those.

This the sort of info I am looking for as reading material histories of the Cactaceae, Haworthia, Adromischus and the rest of the Succulents.

Could also be...they want a humid atmosphere. Mine have leafed out..but very small. I've seen some Tree Euphorbs in warm greenhouses with large leafs..nothing like the puny leaves even my tree Euphorbias 15' make outdoors in real sun. They do bloom though. And fruit and set seed.

Never a trigona.

To the point of not blooming in cultivation, here is a site confirming this and with a pic of E trigona bloom.

"...The plant’s flowers are white or a combination of white and light yellow. The flowers appear on spring and summer, but potted versions of the plant may not grow flowers at all...".

According to https://worldofsucculents.com/euphorbia-trigona-african-milk-tree-high-chaparall/

Hard to say since they dont show the whole plant. I Googled,and that is the only photo of one purporting to be the bloom. On the other hand, a few more technical sites describe its bloom pretty much as the photo shows.

As summer goes by I will keep it in mind since the one I have is over 5' and about 17 years old. All outdoors in pots. Funny thing is ..one i had planted in ground had done well until 1998's 28f..killed it. This particular plant I bought a year or two later, has seen 30f and quite a few 32's over that time. Only the occasional nipped tips so far.

Ive heard on other forum, there have been 2 recorded sightings of flowers , ever.

Trigona always looks to me just like E. pseudocactus

They bear some similarities with some cultivars, true....but pseduocactus has more variability(undulation/segmentation , multiple ribs, height....)

http://www.llifle.com/Encyclopedia/SUCCULENTS/Family/Euphorbiaceae/27017/Euphorbia_pseudocactus_subs._lyttoniana

https://davesgarden.com/guides/pf/go/58032/

Some sources even claim pseudocactus is a hybrid too.....

Wow, those pictures of pseudocactus looks so much nicer than mine.

I beheaded the top of one above to keep in size for window and planted below.

Similar to E. buruana and they grow caudex.

Thanks all for all the information. Enjoying this post.

This site is not working right. Tried to post and can't. Not letting me move around as well.

Shutting down for today, try tomorrow.

On my email and not showing up here??

"Stefan said:

Stush, think its a grandicornis instead.
Might i ask, howd you germinate euphorbia?"

Answer; Stefan, I think your right. I purchased it from Bob Smoley a few years ago. I did purchase grandicornis but it rotted before being able to root. Bob told me to plant in pure perlite and not water for months. Which I have a hard time doing. Started to and was successful finally.

I don't plant seeds of euphorbia. I do get some volunteers at times but they just come up on their own. 'Bikerdoc' is very good at seeds and from his own stock. One of the best men on this site. Very helpful and knowable.