i have mature silver maples in my yard and almost none of my grass is outside the root zone. these trees have some type of disease, and i'm afraid the weed/feed i was using might have weakened them, causing them to be more suceptible to the disease. i quit using the stuff last year. now my st. augustine grass is beautiful in mid summer, but full of weeds in the spring/fall. is there anything i can put out for the weeds that won't hurt my trees?