"I think everyone knows to not spill it on your tree."
Yeah, BUT inadvertent spills, splashes can ruin your day...Glysophosphate is NOT something you want on your tree. Although it's apparently not really dangerous in the soil, it IS very potent when it comes in contact with foliage. If you apply just before a rainstorm, for instance, the stuff would splash onto the tree...
I get irish moss on just about all of my trees--all two dozen of them. I remove it by hand. Doesn't take that long. The trick is to get it as soon as it appears, allowing it to get past one plant results in heavy matting (which still isn't all that hard to remove by hand).