I have had many fortresses with constructed floors that were intermittently splashed with water and coated with mud, both inside and outside. I have never once seen a plant grow on a constructed floor. This is after breaching a cavern, and then seeing underground plants grow on muddied cavern floors that weren't covered by constructed floors. If you are seeing plants grow on constructed floors (NOT smoothed floor, but actual floors built with the construction menu) then something is going on in your fortress that nobody else can reproduce and you should upload a save file so we can examine this apparent obscure bug.
Sphalerite, is that a WILD plant growth versus a FARM? Because I have built successful farms on constructed floor. I've also had underground plants spring up in the extra muddy area that I planned to expand my farm plots into after a few more immigration waves.
I suspect it is either just fun with the RNGods, or biomes may actually affect it, as I prefer to build my forts in highly forested areas, to make it easy to get wood in the early years without having my dwarves scattered around the embarkation area. And it is highly useful when you just can't find the coal.
I'm talking about wild plant growth. I've never tried to put a farm on a constructed floor. Most of my fortresses are in heavily forested areas. I inevitably end up breaching the caverns. I like to build elaborate mechanical devices and water-powered computers, resulting in a lot of tiles that are intermittently covered with water. Tiles I leave with no floors inevitably grow plants, but I have never once seen a plant of any kind grow on a muddied constructed floor. If you have a fortress with plants growing on constructed floors, I'd like to see it. It's more than just curiosity, I need to see what conditions I need to look out for so that I don't get drainage channels blocked by unexpected trees.