From the ArcaMax Publishing, Gardening Newsletter:
http://www.arcamax.com/news/gardening/s-156670-819154
Question: How can I get rid of weeds in my ground cover without
damaging the ground cover plants?
Answer: It's amazing how annoyingly tenacious weeds can be! If the
weeds you're dealing with are perennials that spread from runners or
stolons along or below the ground (e.g., Bermuda grass or nut grass),
you'll need to pull them out and mulch heavily under the ground cover
with organic matter such as wood chips. You can repeatedly cut them
off to keep them under control, but, unfortunately, you'll need to be
very diligent because they usually pop up elsewhere. Herbicides
probably won't work in your situation, because the weeds are
intermixed with your plants. The herbicide won't distinguish between
the plants you want vs. the plants you don't! If the weeds in your
ground covers are coming up from seed, then you might want to try a
pre-emergent herbicide made from corn gluten. Corn gluten produces an
enzyme that kills germinating seedlings.