Block that spreads over other blocks

I'm making a mod that adds Kudzu into the game (to put it briefly it's a sort of ivy that covers plants and grows really really fast, look it up if you want more info), and I wanna make it so it spreads on top of all other blocks (not replacing the blocks but completely covering the blocks). However with each attempt it either stops spreading after a certain amount, or piles on top of each other until it forms a giant blob (I want the layer to be 3 blocks thick at most). Is there any way to make it work the way I want it to?