Everything is going to go slower when it's cold. I'm surprised that 8F didn't kill it. You lucked out. I suspect the number of daylight hours has little to do with it. I can grow greens in places where the sun is more than half blocked, and they grow fine in more temperate conditions.

Agree. It is more soil temps than hours of sunlight. Spinach and leafy greens aren't daylight sensitive as to #of hours but they are quite picky about soil temps. I too am surprised they survived 8 degrees. That is quite unusual.

Would a hoop house help? Sure in that it would keep the ground temps warmer.

Dave

If they were planted on top of a lot of organic matter that was finishing composting, the soil temperature might have been significantly higher than the air temperature.

My question is did they quit growing because of shorter daylight hours
or because of the cold?

A bit of both. Probably mostly the cold because it slows their metabolism.

I wondered if we built a hoop house over them
if they would grow or is the daylight too short?

My leafy greens grew under the frost cloth, but slowly, all winter. I didn't plant them until late October. We have sunny weather most of the winter. When it warmed up a bit, they exploded into growth.

I think they do not grow due to cold temperatures and day length. Even when it is warm in December and January, things such as clover, wheat, and ryegrass grows very little. And it can get pretty warm in the winter in SE Texas.

One odd thing (to me) is that hoop houses get as cold or even colder at night as the temperatures outside the hoop house. It only takes about an hour for the temperatures inside the hoop house to reach the levels as the temperatures outside the hoop house. It only takes a small heat source to get warmer temperatures.

I do think a hoop house would give you a couple of weeks of faster growth on both ends of the winter season.

You might have better luck with planting the spinach in 6 inch trays and bring them inside when the temperatures go below freezing, taking them back out when the temperature warms up. I have grown kale in trays and they seem to grow a little longer in the fall and spring.

Maybe Elliot Coleman explains it in his book- I haven't read it, but it is common among high tunnel growers to harvest spinach all winter long once it has reached a moderate growth stage prior to onset of freezing temperatures. Spinach may even droop from a cold night only to perk up and look great with a few hours of warm sunshine. At 8*F nights the ground inside a high tunnel is still not frozen except for the perimeter.

So the key is both soil and air temperature. Soil temperature change is a gradual occurrence and needs to be adequate to stimulate plants to resume growth. Air temperature results in more rapid change. If you talk to high tunnel winter market spinach growers they say that you wait until mid afternoon on a sunny day before the market day to harvest and then keep harvested spinach in a cooler until sold. That contrasts with Spring or Fall spinach harvests that are best done in early morning before the sun is bright.