Answer: When faced with the same dilemma in my own yard (only I was trying to cover massive Silver Maple roots) I chose Pachysandra (also known as Japanese Spurge). Don't let the name scare you - it's a great ground cover and did exactly what I wanted it to. I found that it does best in the shade, it will not spread well in the sun. I have seen it available in just about every nursery catalog I have ever looked at. Mine came from a fellow gardener and she obtained hers from Wayside Gardens in Hodges, SC. Their phone number is 1-800-845-1124. I don't think the Spearmint would spread vigorously enough in the shade to meet your expectations.
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