I experimented with using corn salad, Valerianella locusta, in some rose and perennial beds for a ground cover this year. I am pleased with how well it covers the ground and therefore crowds out some of the weeds that also grow in winter when it's hard to get out to weed.

It is planted in the late summer or early fall, or allowed to self-sow, or scatter seed when it ripens in early summer. It grows during the winter and can be harvested as individual leaves, which are small, or the whole plant can be cut off and used. It flowers, tiny white flowers, in late spring then goes to seed in early summer and will self-sow freely in my zone. There are other varieties with larger leaves and one that is golden. It has a delicate flowery taste. I found reports of it being hardy in winter to zone 6 or possibly 5. Here is a photo from my garden, growing with Ajuga-