Jim Ledford wrote:
make several applications
once a week until you see yellowing in the undesirable grass.
wet a paper or cloth with the RoundUp® and wipe the grass for
a highly controlled application.



And why waste your time with a weak solution of Roundup and several
applications? Roundup at a typical concentration of about 2-3% will
whack the grass in one application without harming the tree. It's
done routinely all the time.