By the way you describe the playing with grass, it sounds like she is creating toys/play for herself. The best way to stop this, besides maturity which you can do little about, is to create an acceptable alternative and reward her for engaging in it. When you see her digging up patches of grass, grab a ball/stick/toy and get her excited about it, then reward her with a treat (having a lab, I assume that yours is equally treat motivated). Then reward her with each step toward playing with the appropriate toy: throw it, bring her over, and give her a treat; put in in her mouth, give her a treat; if she fetches, give her a treat. Pretty soon you can ditch the treat because the play will be reward enough. Then you'll be encouraging her to go back to self-entertaining because she won't give you a moments rest from throwing her toy! Good luck.