I've been working on the foliage overlay generation, which has been giving me some issues. However, the basic algorithm is there - next is the code to cut the rendered image up so it can be spliced together with end versions of that foliage cell. This way the foliage can be extended arbitrarily far and will end gracefully - including producing the desired overdraw at the edge of tiles.
This is an example of the leaf overlay algorithm.
This is an example of the mold algorithm.
Additional algorithms are coming, but I'm rolling ideas around for mushroom/fungus algorithms and a crystal growth algorithm.