This is MiniNatur ivy on my HO scale station.
Swedish Custom painter and model maker. My Website:
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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.
Elmer.
The above is my opinion, from an active and experienced Model Railroader in N scale and HO since 1961.
(Modeling Freelance, Eastern US, HO scale, in 1962, with NCE DCC for locomotive control and a stand alone LocoNet for block detection and signals.) http://waynes-trains.com/ at home, and N scale at the Club.
topbaum.de in Germany makes ivy you can use to cover stone walls.
Take a look here.
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?
Modeling the Pennsylvania Railroad in N Scale.
www.prr-nscale.blogspot.com