Modeling Ivy on a Stone Wall

I have a few stone retaining walls on my layout and I think they could use some ivy growing on them. What are some easy ways to model this?

topbaum.de in Germany makes ivy you can use to cover stone walls.

Take a look here.

Take some glue, white glue works OK, and with a small brush, paint the glue where you want the ivy. Then put some fine ground foam in a 3x5 card that has been folded in half. Gently blow the ground foam from the card to the wall. Sometimes using a straw works for the air flow. Start at the far end of the card and work back. Of course you don’t want to let the glue dry before applying the ground foam.

This is MiniNatur ivy on my HO scale station.