Spinach is a cool season plant and the best I have grown has been fall planted and mulched well to protect it from the really cold weather. Some years I have found it again after ther snow melted and then when the weather really warmed up it just died out.
If you want to find them in snow (if you have that much) covering with Floating Row Covers can help.

I've been thinking of trying it to mop up extra nitrogen in the fall. edible catch crop... I like multitasking.

Spinach planted in the fall shoots down a tap root, which - if the ground doesn't freeze - will supply the plant with moisture and nutrients throughout the winter. Then, when temps/moisture/sunlight are there in the spring, off they go.

Here, due to the short growing season and irrigation issues, I have the best luck planting spinach seed in late August when the daytime/night time temps are starting to moderate - our first frost is generally Sept 15ish for a guide to other regions/dates. The plant grows well through Sept/early Oct, then sort of sits there, green, but not growing. It will stay flat and green most the winter, with the leaf edges browning in the cold, dry wind, however if mulched about mid-october, that doesn't happen.

In late March, it takes off - even though nightime temps are still in the 20's. Start harvesting significant quantities of leaves in April/May/June until it starts getting hot, and they bolt.

Anyway, the trick seems to be to keep the ground moist, and a mulch helps - pull the mulch off when the temps start to climb in the spring. And protect them from deer - but thats another story.

The best, sweetest spinach we eat is the overwintered crop. I have planted as late as the garlic planting and had good luck with over-wintering. I find that leaves, and there should be at least 2-3, should get to thumbsize at least. If they get there before any hard freezes, then I cover them up and uncover them in the spring. Good luck and plant alot, because once you've tasted it, you'll wish you had more!

I did mine in spring....was eating all the way up until summer almost.....germinated quick, but then took ages to actually grow. Once it got going I was eating spinach salads like Popeye. But, that is in a coldish Australian Spring climate....half shade patch too

Thanks for the tips! I ended up putting lots of hay on top just before the hard frosts this week. Most of the spinach only had their first true leaves, so we'll see if they survive till spring. Good experiment though!

I have been reading about winter sowing here on GardenWeb. I may do another experiment and put some seed down with compost and more hay at the end of the month to see if that works too--just for comparison. Since my true intention was to have the seed sprout in March for an early spring crop.