It is the fern, or in this case a large number of these ferns and other plants and things that you find on a forest floor. Wombles live underground. When the wind does not blow, the smoke goes up the chimney and appears to rise from the ground through the bracken (the ferns that cover the ground) and makes the men believe that the ground is on fire. The men stomp the ground to put out the fire. This causes mud and dirt to fall down into the Womble's house.

Hope this helps!