Dig a hole in the dirt, drop/plant assorted seeds, put the dirt back into the tile and then wait.
Wildflowers grow and then spread the same as other terrains do to add a little more scenery to the game world. Can only be planted by players and spread can be increased based on fodder you give it. Starting being 2-3 tiles from the intial planted flower.
Secondly, planting grass seed to get rid of ****** forest tile and other unwanted floor tile that you may be unfortunate enough to have your plot on.