I know that's not exactly the input you were seeking, but having a nice, impermeable floor in your greenhouse may be a continuing source of unhappiness, and a possible slip hazard.

But why are we assuming you don't? Surely you did have a floor drain installed before the concrete was poured?

As for the paint: I know several people who did their garage floors with the epoxy and for the mess and expense, both have now regretted they didn't just lay down the same commercial dry back asphalt floor tiles used in stores. If they can take being abused by fork lifts being run on them in Home Depot, they will hold up even better in any home garage or GH and compared to the fancy vinyl's are reasonably cheap too.

I currently have a good Epoxy floor in my garage and I love it. If done well with good prep and materials they come out indestructible...and I have seen terrible Epoxy floors too so I'm not surprised to hear about your friends.

My concern with tile is dirt & debris getting down in the cracks making it difficult to keep clean, and the transition to the floor drains (although I think this could be worked out). I've looked at the commercial VHT tile (like supermarkets use). It requires adhesives that I wasn't crazy about using in a greenhouse and I was concerned that perhaps the chemicals in the fertilizers and long term water exposure would cause the adhesive to fail. They're also not the easiest to install well on a floor sloped for drainage.

This is why I was leaning toward Epoxy...durable, chemical resistent, easy to clean, looks good and won't stain...it's just the UV issue that I'm strugggling with. I was hoping to find someone that had treid it already in their greenhouse.

Do it, but then not have to deal with it again, but have it look nice too.

So..... Have you thought about the matte glazed ceramic tiles made for bathroom floors? Yes, it takes more time and labor to install them, but once done, they would be impervious to anything you spill on them, never fade, last pretty much forever and with some spares on hand, an occassional cracked one would be a quick & easy replacement. Just a thought.

And we DID do the Epoxy floor with great results.

I'm happy to hear that you finished your flooring, and that you chose to do what YOU liked. I know you will love it.
We need to take our own thoughts more seriously sometimes, and just do what WE think is best. It works best for me, and even if I mess up, I will learn from it anyway, so no loss at all.
I love to toss out questions for others to give their thoughts on, but in the end, it's up to us to use our own "Guts" to determine what is the best.
Well Done! and good luck!