Watch the grass grow

You remember my underground garden? It's coming along nicely:

The problem is, though, that trees growing on plain dirt don't look very foresty. I'll need grass, but here's the catch: while grass is generated whenever a new chunk is generated, apart from that, the only way to get grass is by making it spread from one tile to another. In other words:

And before long:

Until finally:

Did it take long? Yes.