Beginner Gardening: Corsican Mint issues, 0 by Birdwood

Birdwood wrote:
We recently landscaped our backyard in Venice, S, California (Zone 6) and planted Corsican Mint between the large paving stones. It was doing really well for several weeks but now it's going brown and wooly ( see attached photo) in some areas and has pretty much died off in other parts. Our landscaper thinks it may have been getting too much water and has decreased that this past week. However, our local nursery (Marina Garden Center) thinks we may have a bug that's killing it; another nursery (Armstrong Garden Center) thought it was because the stones were getting hot and 'burning the mint' in the sunny areas and getting too much water in the shady areas. Don't know what to think but concerned that the problem will recur if we plant more Corsican Mint without determining the cause of this problem first. Armstrong's suggested we plant Blue Star Creeper (Isotoma fluviatilis) but we're getting a puppy later this year and I read that it's toxic. Any suggestions?