How do you convince someone to stop using Round-Up

Someone I love and respect is certain that there's no real reason to stop using Round-Up because "the weeds come back every year, so it's not doing any permanent damage."  It's not my property at stake, but I'd love to see less toxins sprayed around if possible. He put down inches of tree trimmings...and then sprayed them all with Round-Up because he doesn't think they hurt the environment at all...

Where is this person coming from? That is does this person in general consider himself to care about the natural environment? If he does then you could try talking about the damage it can have to the surrounding ecosystem. Does he watch what he eats and only buy organics, or is careful about his health, etc.? Then you might come at it from the perspective of food and impact to human health. Is he out doing a fair bit of yard work and complaining about how much time it all takes? You might try suggesting alternative methods that could actually save him time and money. Is he a family guy that values family above almost all? You might try gently talking about the potential impacts to kids and their health.

So really what I'm trying to say is put yourself in his mind and try to figure out what would motivate him - what his values are, what he cares about, etc. Don't try to convince him based on your values or your concerns. This means you might need to talk from a perspective your not comfortable with but if it reaches him then all the better. Though some people just want to do things the way they have always done it and it can be fairly difficult to get these people to change. But if you connect the change you want them to make to a set of values they already have and actively follow then you can sometimes get them to see the issue in a new light. Preaching at them based on your values though will rarely get them to change.

Good luck and I hope that helps!

If Roundup doesn't hurt anything at all, ask him if he would like a big glass of it with his dinner.

If the weeds come back, then the round up doesn't really work. So why use it?