Look for some of the steel wool and cut into it randomly before breaking it up from the central mass almost like you can make spider webs out of cotton balls, but no where near as thin at the end, you want a tangled mess to remain, but to look like a group of intergrown bushes.. Take a heavier wire and use that to attach it to the base by threading it through. Mix up some extremely thin plaster to something like cream texture and dip it in and let it drip off, then repeat a few times after a few minutes of time to set. Brush it with a layer of thinned pva, and then paint it as you would a tree trunk after the glue dries.
Then hit the tips with a dark green and add flock in several, heavy layers before sealing.
A friend of mine did that for a nativity set, and it looks rather good.