I picked up a few plastic plants from Michael's (a craft store). I just bought one "fern" and one viny leafy thingy then snipped all the individual strands/leaves from the stalk. Then laid it out and spray painted over the whole thing. After it dried, I picked them all up and put them in a plastic bag for next time.
A problem I noticed when I use real vegetation for painting was it gets coated fast and then stops being a less defined mark on the rifle. Plastic solved that for me. In my experience, the cut outs provided definition that was TOO sharp though.
However, painting a rifle is an art. You might have better luck at a different technique than I. It's mostly trial and error. Try painting something else first...like a shovel or rake or bike or laptop...