Hello, 
It is always best to avoid contact of Roundup with non-target plants. That being said, often large, mature trees will be undamaged if a little bit of herbicide touches the bark, however, it is also species dependent as some types of trees are more tolerant than others. In addition time of year and other specifics can also affect potential damage.
https://extension.psu.edu/use-glyphosate-with-care-near-trees

Thank you for using Ask Extension!