Organic University: Greenhouses for year-round food and farming
Part 3: Environmental management
Our first installment covered site selection and soil preparation. The second covered construction. Now that your greenhouse is up and running, we’ll learn how to manage the darn thing.

By Katie Olender
April 19, 2005

Editor’s note: This is part three of a four-part series on greenhouse operation based on the author’s participation in a daylong Organic University workshop that was part of the Upper Midwest Organic Farming Conference, held in late February in LaCrosse, Wisconsin.

Before instructor John Biernbaum even begins his discussion on environmental management, he asks everyone in the room what we want to learn. Several people answer that they are interested in season extension, a common concern of many Midwestern farmers. One audience member reminds us all that each farmer has individual needs when he exclaims, “I’ve had two killing frosts in June and two killing frosts in August, and I don’t care about season extension; I’m interested in season retention!”

I am at Organic University, a pre-conference program of the Upper Midwest Organic Farming Conference in La Crosse, Wisconsin, and I am attending the workshop entitled “Greenhouses for Year-Round Food and Farming.” Biernbaum is a professor of horticulture at Michigan State University and faculty advisor to the school’s Student Organic Farm, a thriving 48-week CSA that operates, in no small part, thanks to a number of greenhouses. That is to say, Biernbaum comes to his knowledge through practical experience.

As the laughter dies down, our instructor assures us that, with an understanding of seasonal changes in light, the effect of temperature on plants and some greenhouse management considerations, both season retention and season extension can be profitably achieved, even in the mercurial Midwest.


Here in the “Mitten State,” the first thing we associate with winter is that gosh-darn cold weather! But to a Midwestern farmer, the cold can mean more than just chattering teeth. We are reminded of the USDA Cold Hardiness Zone system, that many of the farmers currently doing winter greenhouse production are growing in zone 5 or higher, and that zone 4 or zone 3 each provide new low temperature challenges and risks.

For winter production and harvesting, Biernbaum says, a key goal is to protect the crop and trap heat so the soil freezes minimally and the air temperature does not drop below 15°F. Many cold-tolerant crops can handle frozen foliage when protected from excess soil moisture and wind, he says, and greenhouse and internal crop covers--either frost fabric or polyethylene--will provide the necessary protection if there is enough light (more on that subject later).

Biernbaum recommends Agribon + AG-19 floating row cover or something similar that allows for gas exchange, moisture loss and light transmission (Agribon allows for 85 percent light transmission). The second less-common but promising option, he says, is greenhouse polyethylene, which traps more heat and moisture than frost fabric.

Both cover options help retain heat by creating an “igloo” or “cloud” effect, partially due to the moisture that collects as condensation when the temperature falls. When the ground radiates heat, it cannot escape through the moisture (or ice when it gets colder), so the plants underneath stay nice and toasty. Biernbaum stresses that it is important that the cover not lie directly on the plants. For this reason, he recommends no. 9 galvanized wire wickets (see below), water or irrigation pipes, or ½- or ¾-inch EMT conduit as a frame over the row, with the cover draped over the top.

Cover the plants when the temperature is dropping near freezing, he says, adding that it is usually best to leave the crops uncovered during the day when it is sunny, since this helps to increase plant growth and the soil temperature. (Sun is critical to maintaining temperature, as we see below.)

"The key is to plant early so the crops will grow during the fall and survive (with very little growth) during the coldest months."
The second key factor related to temperature is that how fast plants grow during the fall--and whether they grow at all in the winter--is determined by temperature, and more exactly the Average Daily Temperature (ADT), Biernbaum says. “It impacts the leaf formation and unfolding rate, photosynthesis and respiration,” he says. ADT is similar to growing degree days.
We calculate the ADT, he tells us, by recording the temperature electronically at each hour of the day and night, adding the numbers together and dividing the total by 24. The higher the ADT, the faster plants will grow. Therefore, Biernbaum says, the best strategy is to maintain elevated temperatures both day and night. Check out agriculture weather websites for estimates of the ADT in your region. (Electronic equipment for measuring the ADT is available through and other sources.)


Light also greatly influences fall and winter production, Biernbaum tells us. In case the season change alone did not lower the temperature enough to make farming a challenge, winter’s low light also works to lower temperatures. At latitudes of 40°45°, he says, there is enough light for leafy greens to grow through the winter, if there is enough warmth.

While we average the daily temperature, we total the amount of light available each day. In the fall, the days get shorter and the intensity decreases, Biernbaum says, so total light falls from as much as 45 to 50 units in the summer to less than 5 units on the shortest, darkest days. Ideally, you want a daily light integral (DLI) of 10 units or more, he says. In the Midwest, where values less than 10 are common in December and January, Biernbaum says, growth is going to be very slow. (Light data is also available on agricultural weather web sites or can be collected on-farm with equipment available at

The key is to plant early so the crops will grow during the fall when there is more light and survive (with very little growth) during the coldest months.

Soil Moisture

With the outside factors under control, it is time to move to the inside of your greenhouse and consider soil moisture. To prevent flooding, make sure you drain excess water from the perimeter. I relayed Biernbaum’s drainage advice in part one of this series, so revisit “Greenhouse Site Selection” if you need a reminder.

It is best to keep the soil moisture lower during the coldest periods of the year (when temperatures fall below 25°F), Biernbaum says. If the moisture content is too high in winter, the bottoms of the plants are susceptible to rotting from the excess water. Some watering in the winter is required. Ventilation or air leaks will also influence moisture and humidity in the greenhouse and on the covers.

Scheduling and Timing

One of the most important steps you can take to ensure a bountiful winter harvest is to plan ahead. Regardless of your efforts, you probably will not see peppers in November, so push that idea out of your head and choose only cold-tolerant crops. The Student Organic Farm chooses spinach, bok choi, carrots, turnips and beets, and 10-12 baby leaf salad greens among other cool-weather crops. Planning ahead means getting your crops in the ground early. The Student Organic Farm has two crops of cool-season vegetables.

The first crop is mostly planted in September and early October and is harvested throughout winter. “As you move into the late September, timing is critical,” Biernbaum stresses. “Even a one- to three-day delay at getting seeds or transplants in the ground can delay your harvest a week or more. We follow soil temperature with a simple metal oven thermometer in the top 6 inches and know things will slow significantly when it gets below 60° to 65°F. When we sowed seeds in November, they germinate but do not grow much at all until February – so it is probably better to wait until February and sow.”

If you plant early enough in the fall, your crops will mature before it gets too cold, Biernbaum says. “Then your greenhouse will act as a refrigerator during the coldest months, keeping your mature crops alive even if they do not grow much.” Young or small plants used for multiple harvests (cut-and-come-again) are more cold tolerant and grow back faster than if you try to seed in winter, he says. Unless you are heating your greenhouse to 25°F-32°F—an idea some growers are trying), Biernbaum says—the larger plants from this first planting may need to be harvested before the temperatures get too low (below 0°F outside and below 20°F inside the greenhouse). A number of student farm plants survived better than expected this past winter, he says (particularly under polyethylene row cover), including several -10°F outside nights.

"Timing is critical. Even a one- to three-day delay at getting the transplants in the ground can delay your harvest a week or more."

To prepare for the second crop of cool-season vegetables, Biernbaum counsels, make sure your transplants are ready. Then, he says, plant them along with your direct-seeded crops (more about crops next time) once the light levels and soil temperatures start increasing in mid to late January or early February and growth starts up again.

The Student Organic Farm at Michigan State University is the perfect example of a farm that is successfully using winter production, offering three 16-week subscription options to students, faculty and staff. The farm could operate all 52 weeks of the year, but the student farmers take a well-deserved break from harvest and distribution during the darkest days of the year (Dec 15- Jan 15).

Katie Olender is a senior majoring in agricultural communications at Michigan State University.