@Liteblob I can say for sure that if you harvest from an evolved plant, the harvest retains the quality. So if you plant a nice quality seed, it should start as a nice quality plant.
I did some more testing and found that I wasn't able to evolve any veggies in plant boxes. However, I did get the option when I moved them to the ground. I still was not able to evolve any trees including apple, plaintain, avocado, coconut and plasma fruit. I also was not able to evolve the bluebell for some reason so I tried some other flowers. I was not able to evolve daisy or rose either but I was able to evolve a begonia.
Even though I can't be sure, I don't think there's a bug here. It seems we have the ability to evolve some plants in a planted state but not others. I have no idea why though.
When you mentioned the difference between the ground vs the plant boxes, it might be an important detail that these particular plants AND trees are in the new ... 'gardening plots' that came with the new expansion Cottage Living.
