You will get about 75% control with corn gluten. Repeated use will eventually suppress the weeds in an ornamental setting. Corn gluten should give you about the same length of 75% control as the Preen. I have commercial applicators who have used corn gluten with good success. One advantage of corn gluten is it is also a good long lasting fertilizer and will give your desired plants a good shot of nutrients. It is such a short lived plant with a possible 3 crops per year I would apply a couple of apps 8 weeks apart. Should give your desirables a good slow release fertilizer feeding.