It's snowing here! Ack!

I have a heavily shaded garden canopied by oaks and hickory and maple. We've shredded all those leaves, and I put them on my gardens to condition the heavily-clay soil, which seems to make my azalea, hosta and annuals very happy. This year, I planted primroses, and they still have tiny green leaves. Should I cover them up completely, or should I keep them exposed? Also, even though my plants seem happy enough, should I add something else to the soil to counteract the acidity, or should I leave well enough alone?

One more thing: can anyone recommend a plant other than hosta which will provide perennail foliage and then die back, eventhough my zone can be quite cold in winter? I'd like more plants, but ground cover makes gathering and mowing leaves hard, unless the plant has diappeared.