I've seen this
tutorial vid floating around, using wavy craft pipe cleaners.
There also wire tree method, only instead of flock you use small tufts of glue-hardened cotton balls (alternatively wirewool or chipfoam).
Or skip the cotton ball and just paint the wire trees dark colors like they've been fire scorched.
Otherwise there are plastic aquarium plants or fake foliage for making wreathes from craft stores that can be cut to size.
There's another technique I've been meaning to experiment with using actual plant clippings preserved with a clear coat spray.