Yes, if your soil temp is at the minimum to germinate; otherwise, they rot in cold, wet soils.

My soils are all above 40F now & with a covering even warmer, so are ready to plant greens. Take advantage of the nice weather to plant & the rain that follows to keep the seed bed moist.

Corrine

mustard is already growing in my garden here, it is very cool weather tolerant.

In fact it overwinters well for me, here in Seattle--the Asian mustard greens growing in my raised beds (albeit sheltered from the wet by plastic covers) are looking pretty and robust after even this year's unusual chilliness.

Germination, too, of cold-hardy greens has not been a problem for me in our mild-ish winter/early spring months. What has seemed the main hindrance is wetness--all that rain that we get makes for water-logged soil that, as corrine1 suggests, leads to rot.