Kate, an issue that I have with ground cover (of any type) is that the weeds that are around my area can find a home in the bed, and they're harder for me to see...its an ongoing problem. For pots I have that have semps with sedums I have to redo them once a year or so. I don't use weedkiller, so maybe that's an option that other people use?? But I can't imagine having ground cover growing in a broad area with the number of weeds we have around here!
