The VERY approximate rule of thumb is one zone per layer of protection, but this is pretty variable as you said. Besides, zones are not that important unless you have perennials or plan to grow vegetables through the winter. Even in this case, it's complicated by the issue that a hoophouse won't have protective snow cover inside, therefore cancelling out zone benefits in mid-winter.

If you want to maximize your warmth at night as well as winter protection, use heavy row cover (winter) or light row cover (spring/fall) for the low tunnels, instead of regular plastic. Optionally, plastic can go on over the row cover (only in winter, otherwise you can cook the plants).

In a hoop house, water drums are fairly insignificant. Much better to rely on the ground as a heat sink and insulate the ground + plant system from outside.

I have a single layer PC GH (behaves like a hoophouse) with low tunnels made up of double layer light row cover and 4 mil poly. Kale, radishes and turnips planted in there.

Lowest temp. outside this winter was 15 below zero F. Lowest inside was +19 F. Daytime temps. are 10 to 40 degrees F above outside, depending on how sunny it is and how strong the sun is.

Beware of bugs (aphids, scale, mealybugs) that thrive in the warm damp environment in a hoophouse in fall/early winter.

I insulate the ground in my coldframe and low tunnels over here and the ground does not freeze and I use water as a buffer (may need to go out soon as temps may not get above freezing today). If I had the room to do a high tunnel over here I'd insulate ground and use cheap CMU 2 high and fill with preferred insulative material, similar to what you see with the superinsulated houses like the Durisol-types. I'd also do 2 layers of good IR-anti condensate greenhouse film and use a squirrel blower. Coleman has a nice design on his where he has a hardscape portion to work on and this is a bit of insulation for part of the soil as well.

My 2â

Dan

Dan,

When you say "insulate the ground" I assume you are talking about the perimeter?

I'm thinking of wood, 2x6 @ 16" o.c. I know that would block some radiation, but I have plenty to spare. Also, I could double the plastic without a blower. One on top, one underneath like I did in the GH. I have significant snow loading to deal with (I build at 74 psf live load here). Of course, snow wouldn't build up as much on a hoop house. We can get heavy dumps from the Baja from December through early May, when hoop house would be in full use.

Also, I have access to free blue plastic drums, so water as thermal storage couldn't hurt. The idea is to extend the season by 6-8 weeks each way, especially in the spring.

Mark

Yes, Mark, my perimeters are insulated and the ground within does not freeze, but this is because of course we get tremendous solar radiation here; I suspect somewhere like New England-Midwest would have much trouble after 4 straight days of gray. One year I buried spare sheathing insulation horizontally as well, but too much work to remove in spring. I have another low tunnel almost ready to go that has two layers of GH film without a blower, using lath to keep separation. I use water within my two tunnels right now as well.

Dan

I'm leaning more and more to undertaking this small project. Maybe 20' x 40'. Maybe bigger. I'm interested in growing edible pod peas. Anyone grown them in a hoophouse or is powdery mildew too difficult an issue?

Thanks,

Mark

Mark... I grow peas overwinter but they stop growing early december and stay as they were until mid-january. I should add that my GH is unheated. So far we have had overnight temps as low as 8 degrees and they are taking it like champs.

Mine are about 18" now and i see no signs of mildew.

Last year was my first with the GH and i planted peas and beets when i finished the GH in november , just to see what would happen. I did not prepare a bed and i basically planted them in soil that isn't too far from beach sand. The peas grew until June and were over 11 feet tall, but they produced little in the way of pods. I think it was mainly due to the poor soil. I planted more in march in a well prepared bed and had the most amazing peas by early May. It was well worth the effort!!!

Poppa

One note... I wasn't able to afford to insulate the ground and i have had no frost inside the GH (so far). I still plan to do it (our frost line extends to 42" below grade here) and as Dan suggests, I am fearful of a cold week (highs in the teens) while the sky is overcast. The insolation on a sunny day is critical. Luckily cold days tend to be dry and sunny.

Poppa

i am running an unheated hoophouse that has lettuce, spinach, and various other greens growing. i throw a row cover over everything and it has kept everything alive with temps below 10 and high winds. but there is not much growth with these short days. the house is 14x72.