Interesting. I grow Silver Mound and it does very well. I have split one a couple of times to put it in other places. My soil is decidedly acidic and perhaps a tad heavy. I don't do anything for them at all other than cutting back in the spring as they tend to grow to huge mounds that sprawl over fall and winter. I also grow Silver Brocade and Valerie Finnes. VF was positively invasive and I began digging it up. I don't think I have any left. SB can be broken off and planted anywhere and it grows wonderfully. When it 'flowers' I just give it a good haircut and also cut back straggling growth as it would cover anything short around it.
Our average temps are in the 50-60 range, not a lot of rain but fairly cloudy quite often.
My soil is a clay base with organics I guess. I have amended with compost and usually grind up garden detritus and leaves in the fall to spread on the garden. Really over did it last year. I must have 3-4" on some beds. Should I rake it back or will my perennials just grow through it. Am a bit worried about that. Where I have raked back I see nice green growth.