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
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
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.
||"The key is to plant early so
the crops will grow during the fall and survive (with
very little growth) during the coldest months."
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 www.specmeters.com
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
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
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.
|"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.