I'm not a chemical expert, but have sprayed lots of GLY, because you can spray around 1000's of trees, and only put weed mats around a few.
I hope experts will join in and offer additional advice. I've learned from trial and error, and was initially afraid to spray around seedlings so I posted this basic tutorial. Questions and advice are encouraged.
I'll add that mistakes happen, so I always carry a pruner to cut off leaves and branches that come in contact with GLY. It's better to lose a branch than risk a seedling dying.