A gardener asked the following question about building small greenhouses and growing in cold weather using information from the Mittleider Grow-Box Gardens book: “Since I am in zone 3 it would be very helpful if I could have the rest of the information the article referred to on “cold weather gardening” in chapter 12 if you think I need it. Is that the information that will tell me how to double-layer the greenhouse to have a 3-to-4 inch dead air space?”
Here’s my answer, which will be helpful to anyone who is building a greenhouse to grow in cold weather.
Chapter 12 of Grow-Box Gardens does indeed show and tell you how to double cover the greenhouse. I am sorry that Grow-Box Gardens is currently out of print and available only as a digital download at www.growfood.com/shop. Let me tell you a few things the book says about winter gardening in cold climates.
Seedlings should be started in warmer weather and transplanted into the greenhouse by early fall if possible, so that much of the vegetative growth takes place before it gets cold.
During the cold months, plants can be maintained and harvested at lower temperatures. It is important, however, to maintain soil temperatures above 50 degrees as long as possible, otherwise, the plants will go dormant.
A greenhouse is important, and it should be double-covered, with a dead-air space of 2-4″. Building the greenhouse east to west, with the north-side wall built into a hillside or against an insulated wall, can reduce heating costs significantly, and even provide some heat from the mass of the north wall.
If you’re really serious about growing in cold weather, hot water pipes buried 4-6″ deep inside the greenhouse near the outside edges will provide some heat and ward off cold from the frozen ground outside.
If it is too cold to keep the entire greenhouse from freezing, consider a greenhouse within the greenhouse to protect valuable crops.
Arched PVC frames covered with 6 mil greenhouse plastic, with a small space heater inside, can keep a row or two of plants warm enough to save them even on very cold nights if it is inside a greenhouse already.
If daytime outside temperatures rise above 65 degrees some ventilation should be provided in the greenhouse.