That is not true. I will spray round up on runners of "good" plants and it will kill the runner, but not harm the plant. Have you ever read the label? Kills down TO the roots it doesn't say anything about killing the roots. This is why on harder plants like Thistle you need to add additional chemicals. You will get a burndown with roundup, but then you will have regrowth. It's not because new seeds germinated 3 weeks later, it's because you didn't kill the roots. Most plants can't have the top burned down and still survive, which eventually starves out the root system. That's why you get a kill on 95% of the plants that are sprayed with Roundup. Round Up kills down to the dirt, not below dirt.