Q: My son has a house in the woods that's mostly in shade. He wants to replace the vinca groundcover that he has now with another single-plant groundcover that would look good among a few boulders and a backdrop of ferns that's already there. Could you suggest something?
A: The least expensive and most obvious choice would be pachysandra. It grows about 6 inches tall, gets little white flowers in spring, stays evergreen in winter and spreads fast but doesn't climb like ivy or spread uncontrollably like some groundcovers. On the down side, it doesn't like direct sun or above-100 temperatures, occasionally gets scale (bugs) and isn't a native. There is a native American pachysandra, but it's much slower to spread, harder to find and significantly more expensive.
One of my favorite shady groundcovers is foamflowers. These grow about a foot tall, have burgundy-tinged foliage and get pinkish-white bottle-brush flowers in April. They're trouble-free, moderately fast to spread and native to Pennsylvania, but they sell as perennial flowers and so aren't as cheap as groundcover-sized plants like vinca, ivy and pachysandra.
A third good choice is barrenwort. These are heart-shaped perennials that grow about a foot tall and get hanging flowers of pink or yellow in April. They're somewhat slow to get started, but once they fill in, they make a beautiful, trouble-free mass.