Depends on when you're harvesting. If you're wintering over the spinach for spring harvest, and don't care about getting any more growth this season, you can cover them with a heavy mulch of straw or shredded leaves to protect the plants from desicating winds and to prevent the soil from freeze and thaw action. (If the plants are real small, though, I'm not sure how well they'll do under thick mulch.)

If you're aiming for winter harvests, I would avoid mulch and use floating row covers that protect the plants, let in light and rain and hold in the heat.

For any plants that are protected in a cold frame, hoop house or floating row cover at this time of year, I avoid mulch. Mulch prevents the sun from warming the soil and insulates the soil, which blocks warmth from radiating from the soil to warm the air around the leaves at night.

I had a similar question. I planted my spinach (and broccoli raab) for early spring harvest a few weeks ago. I covered them with floating row cover (they aren't super big right now - the b. raab is maybe 2 " high). Last year I tried covering the spinach with thick mulch and nothing lived. I think maybe the spinach was too small? That is why I'm trying the floating row cover.

Sorry I didn't really answer your question - but by next spring we will know if this will work :)

Irene

I have read about using evergreen boughs to keep the wind off spinach but still allow it to get some light and rain easily when overwintering. I've never tried this but thought I'd pass it along.

I have used coldframes and FRC with good luck on such things with very good luck. When I want to kill something I mulch it--never fails

I've lived three years now in zone 6a and my Spinach ( fall planted) has survived to be the first producing vegie come Spring each year! I have made berm-type raised beds and lightly mulch around the plants with leaves or straw. They do bounce back in the change of seasons thus far! We ate our last salad of these plants a couple of days back....plants are now too picked from and small, so will mulch in the next few days.
Toni