By John Mattingly Ask not what you can do to weeds, ask what weeds can do for you. Weeds can tell you a lot about the tilth, fertility and structure of your soil. Some weeds can be used to improve the soil for garden crops. If, for example, the ground has a drainage problem or a hardpan, weeds like cocklebur, thistle and quack grass, if managed properly, can yield good ground from wasteland because their tough and extensive root systems break through the hardpan and, in some cases, absorb salts. World War Weeds. Waging war on weeds shows a lack ...