It seems that lemon trees are bugged.
In a planter they are already 'Potted Lemon Tree's the moment I craft them (not the usual small version) and even after several weeks they don't change at all. Using cheatmenu I discovered that there aren't a small, nor a ready variante.
If I try to plant them in a furrow nothing will work as a seed.
I haven't tested it with candleberries and mulberries but they seem similar to lemon trees. Meaning they only have the normal size and can't be planted in the ground.