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
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 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 www.specmeters.com.)
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.
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.
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.