Hi Cynthia and Penny, Roundup is so effective in killing unwanted plants we use it to kill poison ivy. It takes about six weeks for the poison ivy to die completly. The problem you are having with the roots on plants that roundup has killed is that the roots are usually the strongest part of plants. The roots are usually the last thing to break down unless affected by disease. I grew garlic the size of coffee cups this year. When I tried to pull them up I couldn't even though I have a sandy loam soil. I had to use a digging fork to loosen the soil so I could remove the bulbs. Happy Gardening, Allen Bastrop Co.,Tx.