Actually, both should germinate in the fall. I would sow in late Sept in your region, germination should occur with cooler October weather. Both should overwinter in your region -- mache is fully hardy, the Claytonia I'm not sure about, only tried it once. I believe the cold isn't an issue for it, but its from a much drier climate, so may have problems with winter wetness.

Also, be sure to plant some spinach and romaine lettuce to overwinter if you like those -- I get the best results on spinach that way. And, for the lettuce, while it doesn't yield a superior result to spring sown, it certain yields an earlier crop by a good 3 weeks on average. Leaf lettuce works, too.

Great idea. My garden's in 3' x 70' beds. I think I'll set aside one for fall-sown, overwintered greens.

I agree with the general considerations Dennis offers, as they closely match my winter gardening experience. IME claytonia prefers spring seeding. Both it and mache are top notch when it comes to cold hardiness, claytonia being probably the single cold hardiest vegetable I grow. But I do not plant it anymore because it has become feral in several beds.

Mache germination depends on the variety (I have two). True valerianella (a relatively large plant) germinates more slowly and irregularly than a smaller, wilder variety which also has naturalized in a few beds. The naturalized one does prefer fall germination, with overwintering and a crop that is available in late March- early April. Even as it bolts the leaves remain neutral-flavored.