Should be OK as long as you cover the trunck while you apply it.
Glyphosphate stops working once it hits the soil so shouldn't get into the
tree roots. You may find it worthwile putting down some weed-suppressing
membrane and mulching the area for the future.

Martin

Glyphosate won't work on tree trunks, only green leaves. All you need to watch
is any spray drift onto plants you don't want to lose. Personally, I'd spray,
then mulch. Membrane should be water permeable, so has to have holes in it.
Any membrane with holes in will let roots grow through. When you go to pull up
weeds the membrane has a tendancy to come up with them, and it looks awful when
it starts to show through the mulch. If you mulch sufficiently the weeding
will be really easy. The roots of weeds don't get a good purchase on the mulch
and it's easy to tease the weeds out. Use the right mulch and you'll also be
feeding the trees in the long term and improving the soil in general. Let the
worms work the mulch in and just keep replacing it as necessary. They do a
better job of working in than you can do with and tools, and they won't disturb
the roots of your trees.

Hope this helps.

Dave.