I have bedstraw growing in a large area of lilies of the valley. How do I...
Bedstraw is an annual weed. If it has been there before this year, there are probably seeds still not germinated in the ground (waiting to come up next year).
Pull out what you see---it is easy because it is shallow rooted. What you need to do is put two to three inches of mulch around the plants to prevent the bedstraw from coming up again. A layer of newspapers, covered by mulch will stop the weed from germinating. Next year be vigilant about promptly pulling out any bedstraw you see.
It is important to put down the newspaper (or cardboard) to prevent germination of the seeds. Just mulch might not work.