Watching a few lawn care videos, I got to wondering: what exactly is the mechanism that makes the lawn expand over the side walk? How does all that dirt get there? Sometimes it’s shoveled up and looks just like sod.
Water and wind will move soil. And once vegitation is estabilished, it captures more soil.
Earthworms, and various other soil critters, also move soil, and will move some up to the surface; especially in wet weather, when the surface, including the sidewalk surface, is relatively hospitable to them. For that matter, humans and other larger critters also move soil; when we get mud on our shoes/paws/feet, some will come off on the sidewalks; and people (not to mention dogs, birds, et al) drop various sorts of organic matter. And I expect the growth of roots in the soil raises it a bit; and then the roots themselves will start growing over the edge, taking advantage of all the bits of organic matter that have accumulated there, and start holding those bits in place. At which point it looks just like sod because it is sod.
An unmaintained path will not take long to be covered in dust, leaves, and other debris that decomposes into soil, and unless broomed or blown-off occasionally, will eventually get completely covered. Grass seeds are very easily dispersed if a lawn is adjacent to the walkway or driveway. Provide some regular water, either naturally or from sprinkler over-wash, and there ya go - a layer of grass covering a piece of pavement. Once it gets going, it starts to maintain itself, and even build-up more soil.
It’s the kind of grass that sends out runners that spreads over solid objects fastest. I once went looking for a manhole cover in a park. It has been completely covered with about three inches of grass runners. There wasn’t much dirt, but I imagine that would have built up eventually.
I hate the way that looks. I always wanted an edger that would cut at an angle so the grass would hide the dirt
When it comes right down to it. It’s gravity. But that’s a good thing.
I’m not being dismissive or flippant. Water/dirt will move downhill. That’s the way it is. Plants that can take advantage of that will.