Answer: You might have success using mint, if the area is truly contained. The caution with mint is that it may be truly invasive if allowed free rein. Another possible plant would be the ground covering Houttuynia cordata although it may not be winter hardy for you. If you do not mind the idea of replacing the plant each spring -- or bringing it indoors for the winter -- you could look at tropical bog plants such as papyrus and alocasia and so on. Although they are not really ground covering plants they would not mind the constantly damp soil. Yet another option might be to plant English ivy (Hedera helix) nearby and train it to grow along the wet edge. I hope this gives you some ideas.
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