Ground foam is a commonly used scenery material. You can use it to simulate grass and other ground foliage, and people often use it to make trees as well.
But there are two problems with it. What are the odds of you running out when working late at night when all of the hobby shops are closed? Too high. And it’s expensive. But I found two explanations how to make your own.You can see them here and here.
I’ve seen a similar method used where someone used cheap kitchen sponges from dollar stores. The source of foam doesn’t seem to matter. The materials you need are pretty much all the same: an old blender from a yard sale or thrift store, about a quarter cup of water, a bottle or two of cheap green acrylic craft paint and another bottle of a darker color to tint it, and some foam to grind up.
Cheap model railroading supplies can be hard to find sometimes. It’s nice to see one.