My friend Mike Nease and I refurbed my old layout to donate it to the Western Maryland Scenic Railroad. For Autumn background trees, we made the classic "puffball" trees. These work well for distant trees. We used Woodland Scenics coarse turf in these colors: Fall yellow, Fall rust, Fall red, Fall orange, Light Green, and Conifer.
Tear off pieces of polyfiber, roll them into 3" balls, and secure them with a toothpick coated with white glue.
Spray with black flat primer.
Spray with hair spray and roll in ground foam. Repeat. (you can mix colors like yellow and orange for more variation). Secure with additional hairspray if needed.
Set aside to dry.
To install on the layout, it was simply a matter of poking the toothpicks into the foam.
For foreground trees, we used the Woodland Scenics plastic trees that you twist into a 3D shape. We enhanced them by spraying with hairspray and sprinkling on 3/16-1/4" pieces of sisal rope fibers. Spray them with Rustoleum camo brown.
Spray with hairspray and spinkle on coarse foam. Repeat.
Bob