If I understand the literature correctly, 100% organic Preen is simply corn gluten which binds with and coats seeds to prevent them from sprouting. Most other Preen products contain trifluralin, an herbicide that is highly toxic to aquatic wildlife. Some contain isoxaben, 2-4-D, mecoprop-p, dicamba, and/or dithiopyr. You'd have to research each of these chemicals to determine your comfort level using them. I've never used Preen, so can't speak to its effectiveness.