Hi
I personally have had the best luck with Vandas,Epicats ,epidendrums though I also grow Catts and phals with much less vigor though. The main problem over the years has been storms , extreme cold. squirrels
At one time had around 50 attached to an orange but was cut down by the state during the citrus scab scare lol Most of those went back to pots into the shadehouse though I'm now using a carambola and a cassia roxburghii as well as a couple of queen palms
I put them on the tree in the natural growing position some times with a pad of sphagnum trying to locate them in groups with similar needs so I can provide temporay shelter as dictated by the weather as well as light requirements.. Be sure they are secured to the tree well
as movement will damage developing roots
the really tough part is finding a location that is stable ,provides the necessary amount of light as often you get good growth But poor flowering
The specie of tree doesn't seem to matter but rough barked types allow MUCH stroger attachment in a shorter period.
It's an adventure for sure lol Good luck gary

I've got a few orchids in trees. Dendrobiums affine, canaliculatum, hybrid bigibbum, some Vandas (forgotten names), Bulbophyllum, Dockrillia, Arachnis, and of course Vanilla which just runs rampant. And a Grammatophyllum speciosum which is actually on a tall (dead) tree stump.

Thanks for all the advice! I'm going to experiment with a few positions. I don't know what kind of tree I have (it was there when we bought the house) but it has somewhat rough bark and lots of branch nooks to tie them into.

Cearbhaill - gorgeous! How long have you had them growing into the tree to get such established roots?

I wouldn't try phals on trees. Vandas, hard-cane dendrobiums and epidendrums might be more suitable. Maybe even some catts.

Julien, I grow phals directly on to the tree and some mounted on a bark and hang it on the branches, in case it gets too cold I can move them in. I used to send all phals to orchid heaven till I started doing this. Have had good success with phals Oncids and catts. Here is my phal tree, I mount them crown facing down.
{{gwi:168095}}

{{gwi:168096}}

Phals are fine on trees with a good amount of shade, but if you want those huge, long flowering spikes you may need to fertilize and water a bit. - Ian.