Apparently flowers will not spread onto custom patterns. You can make 100% transparent custom texture and display it on the ground around your flowers to create an invisible barrier to prevent them from spreading further once you're satisfied with what you've got. This is pretty awesome if you don't want a brick or stone or fence barriers around all your flowers or want to stop them from spreading on to dirt paths. Speaking of which, you can also inlay custom patterns into paths too. I made little pebbles to scatter randomly into my dirt paths to make them more fancy looking. I love this game for that. ^^