Changing a vanilla block feature

Well, yes and no.

This can be done but only with coding(as I know).

Just insert this:

@Override
		protected boolean isValidGround(BlockState state, IBlockReader worldIn, BlockPos pos) {
     		Block block = state.getBlock();
     		return block == Blocks.END_STONE;
		}

in the class of your block and that's it.(Also press ctrl+w, it will add all required imports)

But I think you can also try making a block that looks like a plant, then you need to create a procedure with a global trigger "a block is placed", and then

If get block at X Y Z == *your flower block*

do: if  not get block at X Y-1 Z == End Stone

      do: Cancel event that triggered global trigger