It depends on the situation. If you have lots of mulch and it's all bumpy or lumpy or uneven, then yes I would smooth things out first or you will have a high level of undulation, similar to the Masters golf tournament greens. But if you lay this down, it has drain holes in it and so it doesn't really matter what you put it on. I have mine on the painted wood deck on our balcony of our apartment. So I don't think the grass can be ruined by the mulch, or by any other common landscape surfaces. ALSO...if you live in an area that has high winds, you may want to pay attention to the anchoring. I'm not sure how securely you could anchor this thing into mulch. Although if the wind is not a problem, then go ahead. I hope this helps!