Are you referring to the remay row cover fabric.If yes I'd
put them on now. They're self venting , won't burn the plants.

Swanz

I plan to use 6 mil plastic and 9 gauge metal hoops, with an 18 " radius. I figure I will need to keep the ends open for ventilation any time it gets above 40 degrees or so.

I'd use them now and keep the ends open during the day.

Swanz

I put the grow tunnels on October 2nd and 3rd. I grabbed the wrong roll of plastic and used 3 mil instead. On the bright side it lets more light through (no pun intended :-). I'll see how the plastic holds up this winter. I also set up 3 wireless thermometers on separate frequencies. $14 each from Wallmart. One is outdoor air temperature, one is tunnel air temperature and one is soil temperature. I plan to track the min / max from each thermometer each day. I'm curious how much the grow tunnels help this fall / winter. Right now, I have the ends and sides open as its still getting in the high 60s / low 70s during the day. Even with the open ends and sides, the tunnel air temperature is about 10 degrees F. warmer. Enjoy.

Cody, how is your temp monitoring going? I tried four-season gardening for the first time this year as well. I had basil alive in my unheated greenhouse (in a pot) until last week (Dec 15), and then it froze. I planted somewhat late, so things didn't get up and grown, but I am still monitoring to see what happens. It's all good. I just harvested some of my frozen celery (in polytunnel) for beef vegie soup today. Ha.