Eliminating established weeds in pots

[mydogmimi]

Hi Everyone:
I have a large greenhouse collection of Cattleyas, mainly in a bark mix that have established oxalis and another broadleaf nuisance that I need to eliminate. Any ideas on how to do this without disturbing the established orchid? As you know pulling without getting the root is futile as they come back. I was thinking of mixing up a solution of round-up (no lectures please) and lightly applying to a few leaves of the weeds with a small soft brush and see if that works. Any other ideas would be appreciated.