Whether you mound mulch now or later will depend on which will keep the ground colder. I would suspect that mulching now would keep the ground warmer and spur growth, rather than retard it. If threatening frosts are predicted, then I would mulch, or perhaps better, put a bucket upside-down over the sprout(s) and secure. The ground warmth will keep the inside warm enough, but still allow the surrounding soil to stay cool with the cold weather. Cold soil and lack of sufficient moisture are the only things that are going to retard growth. (Obviously then: don't water!)
