If the nylon grass mentioned yesterday isn't enough, there are other tricks to produce a suitably untidy example of nature on a model railway. Over the years I've accumulated a couple of shoe boxes containing various scenic stuff.
One packet contains a sort of netting that you use to support leaves on tree armatures. I'm not sure what it is made of but the consistency is like non-sticky candyfloss, just not as tasty. In use it's teased out and stuck in place with some PVA. Then some puffer grass is fired it at. To finish the job it gets a coat of hairspray and a sprinkle of various flocks.
I'm not actually sure what I'm modelling here - probably a sort of low lying bush thing. What I am sure is that I've tried to walk through these in the past so they do exist. Botanists can fill in the details via the comments page please.