It is now the beginning of the
end of my lecture-style introduction to winter greenhouse production.
Surrounded by other eager farmers, I’ve learned about selecting
a site and preparing the soil, building a greenhouse, and managing
its internal environment. Pen in
hand and a fresh cup of coffee on the table in front of me, I am
finally ready to learn about the most important part of the greenhouse:
I am excited to be here at Organic University, the prequel to the
Upper Midwest Organic Farming Conference in LaCrosse, Wisconsin
and even more excited to be seated in John Biernbaum’s lecture.
If there is one person who knows winter greenhouse production, it
is Biernbaum, who is a professor of horticulture at Michigan State
University and a faculty advisor to the school’s Student Organic
Biernbaum begins the final section of his lecture by discussing
what should be planted to have a bountiful winter harvest. First,
he recommends baby salad greens for several reasons. The smaller,
less-mature leaves are cold tolerant, an important quality when
considering Midwest crops in wintertime. Baby salad greens are a
high-value product that receives much positive attention in winter,
especially if you combine different varieties so the final salad
mix is of multiple colors and flavors. Finally, baby salad greens
give good economic returns per unit area because they can be harvested
“The planting density of your baby greens will have a significant
impact on yield, quality, and sometimes leaf size,” Biernbaum
says. Follow the recommendations of your seed catalog when choosing
seed planting density, he advises, adding that you may adjust the
density down the road based on your own experience. The Student
Organic Farm uses a four-row pinpoint seeder. Student farmers there
began with recommended settings from Johnny's Selected Seeds but
soon devised their own table to better reflect their specific growing
conditions and results.
Most baby salad greens are harvestable when just 3- to 4-inches
tall. Mustard and Mizuna, the main exceptions, are harvestable at
2- and 5-inches tall, respectively. Cut approximately one inch above
the soil, Biernbaum instructs, and be sure not to cut the growing
While baby salad greens can be harvested multiple times, he says,
flavor and quality may change in the later harvests, particularly
with arugula and ‘Red Giant’ mustard. Although some
growers of baby salad mixes marketing to high-end restaurants replant
the entire crop after the second harvest, Biernbaum says, the Student
Organic Farm serves mostly CSA customers and so stretches the number
of harvests to as long as each crop holds up and remains palatable.
‘Bulls Blood’ beet, ‘Tokyo Bekena’ Chinese
cabbage, several varieties of lettuce, ‘Red Giant’ mustard,
sorrel and tatsoi are harvested two to three times. Claytonia, kale
and ‘Green Spray’ mibuna are harvested three to five
times before replanting. The number of quality harvests correlates
to the soil quality and fertility, Biernbaum says; the better the
soil, the more times you can harvest.
Before moving on to the next winter crop type, Biernbaum shares
his experience regarding specific salad greens. Mache (a.k.a. corn
salad) is very hardy in cold temperatures, he offers; it does, however,
have a slower growth rate so is usually only harvested once or twice
before it needs to be replanted. Kale has been a consistently reliable
crop for the Student Organic Farm, Biernbaum says—it offers
a ruffled leaf to add volume—though little weight—to
your salad mix. Claytonia is an excellent cold-weather performer
that adds weight, he says, though it is slow to get started.
Biernbaum warns that the best way to avoid disease—such as
fuzzy growth on leaves, brown or rotting foliage, or spots—is
to not plant too densely and to use good water practices. Good water
practices include watering less often in cool weather (because the
water does not get absorbed or evaporate as quickly as in warm weather)
and watering close to the ground to avoid drenching the foliage.
After baby leaf salad greens, Biernbaum recommends
growing leafy vegetables like spinach, chard and beet greens in
winter. All of these crops are direct seeded and covered with screened
Asian greens, like Chinese cabbage, boc choi, mequing choi and
tatsoi, are a good choice for your third crop type, he says. These
are all seeded in flats about a month before planting them in the
greenhouse. The transplants should be planted at the end of September
or early October, Biernbaum say, and should be placed about 12 inches
apart in each row with 12 inches between rows.
Finally, Biernbaum recommends root vegetables as a fourth crop.
The radishes, beets, turnips, carrots and other root vegetables
grown at the Student Organic Farm are direct seeded. Carrots have
to be seeded by the first week of August, he says; radishes, turnips
and beets can be seeded in September. “Our biggest problem
with these crops is the four leggeds that help themselves to a bite
out of the tasty above-ground roots,” Biernbaum says.
Biernbaum reminds us that with biennial leafy and root vegetables,
spring bolting (flower stalk formation) is to be expected if overwintered
crops are still around in March and April when temperature and daylength
both increase. Bolting ruins root crops, he says, so they need to
be harvested well before they bolt. “We have actually found
that our CSA members are happy to have spring leafy or Asian vegetables,
even if bolting has started,” Biernbaum says.
Last but certainly not least, Biernbaum provides some pointers
about proper watering techniques. First and foremost, he says, is
determining when to water. And the first step in this process, Biernbaum
says, is to check the weather when deciding whether to water on
a particular day. Then, he says, check your plants, the soil surface
and the soil moisture several inches below the surface (you can
just dig your finger in the soil for this part.) Third, check the
thermometer to determine the air temperature inside your high tunnel
“If the sun is shining or is going to shine, and it likely
will reach 60 degrees Fahrenheit or higher in the greenhouse, and
the soil is not already wet, water!” Biernbaum says. He recommends
watering before 11 o’clock in the morning but no later than
two o’clock in the afternoon to allow plenty of time for foliage
to dry. When it is cool, he says, do not water, because the leaves
and the area immediately around the plants do not dry well, creating
a hospitable environment for foliar pathogens. When it is cloudy
in the winter, Biernbaum says, the plants do not grow much and so
need less water. Again, he reiterates, do not water on cold, cloudy
days. “Ideally, we want the plant leaves to be dry and the
top of the soil not to be wet going into the night with freezing
conditions,” Biernbaum says.
Next, he explains how much to water. This varies among greenhouses
because soil type and organic matter impact the rate of water absorption.
I had no idea how much you had to consider before watering!
But finally, just as I am starting to worry we will never
actually learn how to water, Biernbaum launches into just that.
If you use a hose and breaker by hand, he says, make sure the breaker
head is free of solid matter. Biernbaum recommends a quarter-turn
valve on the hose end so you can easily and quickly turn the water
on and off. Keep the breaker moving, he says, to avoid compacting
the soil. The flow rate should either be high with the water shooting
away from you and falling on the soil at a distance, Biernbaum says,
or low with the breaker held near the soil to keep water off the
foliage. Avoid creating puddles, he says, which may require multiple
Finally, he says, drain the hose or water line after each watering
to prevent freezing, remove the breaker and quarter valve, and pay
attention to where the water drains to avoid a puddle in the aisle.
It sounds like a lot to plan between choosing the right plants
and watering effectively, but Biernbaum tells us we will all learn
what works best for us with our own specific crops and conditions
over time. “Learn from experience,” he says. “Keep
records, including the spacing you used for your seeds and the watering
information.” This way, Biernbaum says, you can go back and
adjust your methods as needed.
Closing his lecture, Biernbaum reminds us of what he tells students
to remember while giving tours of the winter greenhouses. “There
are four key things that make it work,” Biernbaum says. Holding
up fingers as he counts, he says “The right plants (cold tolerant),
planted at the right time (before the days get too short and temperatures
too low), using multiple harvest or cut-and-come-again harvesting,
and crop protection from wind and excess moisture. It is all about
understanding the system to allow local food all winter long.”
Thanks to Biernbaum’s presentation at Organic University,
a whole crop of Midwestern farmers now have the basis of that knowledge.
Now go. Pull out that seed catalog and start planning for next
winter. And invite your buddies who helped build the greenhouse
over for a potluck with all the fruits (or shall I say vegetables)
of your labor.
Katie Olender is a senior majoring in agricultural communications
at Michigan State University.