If plant growth is case in an area with minimal plants (such as inside a castle keep) does it have an effect? How much plant life is necessary? I can make a bit of lichen fill 100′ radius? There needs to be plants present for plant growth to affect them. Inside a castle might very well have no plants, that's up to the DM's adjudication and description.
If we want to get super technical, lichen isn't a plant. Again, DM's call if they want to count it.
— Dan Dillon 👥 (@Dan_Dillon_1) January 27, 2020