Hi Marijn, Thanks for the kind words.
To answer your question, all I did was to spray the trees, once finished, with floor polish (Future or whatever) and sprinkle the grass over and around, very sparingly...I din't want to overdo it, just to suggest some finer 'needles'. The static grass tends to clump up a bit, so I held the tree in one hand, and, spinning the tree slowly, sprinkled from about 30-40cm above, so it would fall evenly.
I'd already painted the trees with a few shades of green, so I then sprayed a thin coat of light green, then a couple of shades of brown just on the tips of the branches, to stop the tree looking too monochrome, if you know what I mean.
Then finally, a couple more coats of floor polish, to give the whole thing a subtle sheen, because the paint (Tamiya acrylic, mostly) turns them very flat looking.