The Unfrozen Tundra: Extending the Growing Season in Green Bay
By Bill Wright
This article was first printed in the November - December 2005 issue of the Organic Broadcaster, published by the Midwest Organic and Sustainable Education Service.
During the winter of 2002/2003 I read Eliott Coleman's book Four
Season Harvest, and became aware of several growers in southern
Wisconsin who were using high tunnels for season extension. This
intrigued me, but the winter climate in Green Bay, Wisconsin is
different than Madison or coastal Maine. In January 2004 the average
high temperature for Green Bay was 20 degrees, and the average low
4 degrees. During that month we had 11 nights of below zero temperatures,
with the bottom dropping out at -14 degrees. We also frequently
see cold temperatures fairly early in the season, as was the case
in November of 2003 when the temp. dipped to 11 degrees on November
8th and 13 degrees on November 9th. Not exactly ideal growing conditions
for tender green things.
I decided to give it a try, however, and with financial assistance
from UW-Extension Northeast District Fund for Innovative Programs,
and space provided by Green Bay Botanical Garden, constructed a
12'x 44' high tunnel in the summer of 2003.
In planning I felt it important to impose some restrictions on
the project so that the system being trialed would be strongly appealing
to growers in our area . First, I decided to not use any supplemental
heat, thus eliminating the extra expense of heaters and fossil fuel.
I did not want to use any electricity; therefore ventilation would
need to be accomplished without electric fans. This would give the
grower more flexibility in placement of a high tunnel. A system
that required minimum human intervention was needed since we were
not going to be there on a daily basis to monitor the tunnel. Likewise,
many growers have off-farm jobs and are not able to constantly monitor
conditions. Also, being unsure if the concept would work, I did
not want to invest a large amount of money and decided to design
and build the tunnel myself. From the grower's standpoint, this
would also help to maximize the return on investment.
With the assistance of a summer intern, the site was first leveled
and the four corners of the structure were marked, making sure everything
was square. We used ¾"x 30" black pipe driven 24"
into the ground for our anchor posts. The corner posts were set
first, and then using a string stretched between the outside corner
posts to keep all posts perfectly aligned, we set the ground posts
at 4' intervals. I highly recommend having two people for this operation;
one to hold the pipe and one to swing the sledgehammer, since you
need to drive the anchor post straight into the ground. We used
a level to check our progress when half of the post was in. A large
machine bolt placed in the end of the pipe can protect the end of
the pipe from being smashed, since a smashed pipe can make attachment
of the ribs very difficult.
We bolted 2"x 6" cedar boards to the anchor posts to form
a kickboard. While treated lumber is less expensive, we chose not
to use it because of the direct contact with the soil and restrictions
for use in organic systems. A small trench was dug around the outside
of the kickboard and using 8" wide flashing, a varmint guard
was placed on the bottom of the kick board. The thought was to keep
voles from tunneling under the kickboard. However, it didn't work
Our rib design was a gothic arch with a 4' sidewall. In order to
create a form to bend the ribs, we hammered pipes into the ground
at the ends and corners of the pattern. We used this form to bend
5/8" rebar to the desired shape (3-10' pieces were joined together
with ends overlapped 3' to form the 24' pieces we needed for the
rib). We then covered the rebar with 23' pieces of 1 1/2" plastic
irrigation pipe so that there would not be any abrasion on the poly
cover. We also installed 3 purlins using PVC pipe. While these ribs
were economical, they were too flexible and after 2 years the structure
shows obvious signs of wear. I will purchase a commercial structure
for our next project. I think the extra cost is well worth the investment.
The end walls were framed using 2 x 4's just as you would frame
a wall in a house. Each end wall had a door with a vent window near
the peak. The south end wall and the door were covered with the
same poly material used to cover the structure. The north end wall
was covered with used wet felt from a paper machine. This material
was free and held up well even when exposed to the elements.
The poly cover used was standard, 6 mil, 4-year green house poly.
The poly was fastened to the kickboard by "sandwiching"
the edge between lathe strips and then screwing the lathe/poly sandwich
to the kickboard. We started early one morning to apply the poly
cover but it seemed that as soon as we unrolled the plastic a breeze
appeared. Since there were only two of us to handle this 48'x 24'
sheet of poly, which acted like a sail with the slightest breeze,
we started by fastening the plastic to the west side of the structure.
We then used tennis balls to fasten 25-foot ropes at 8' intervals
to the other side of the poly. We would then start at the north
end, throw the first rope over the top and pull the poly as far
as possible. We then tied this rope off at the base and repeated
the steps with each rope until we reached the south end of the tunnel.
We then went back to the first rope at the north end and repeated
the process until the poly was in place. While I was pulling the
ropes, my helper was inside the tunnel with a broom to help the
rope and poly over the purlins. The plastic was then fastened to
the kickboard on the east side in the same manner as described earlier.
We achieved our "automatic" ventilation by using a product
called Univent, which we were very pleased with (though, in my role
as Extension Agent I want to be careful to not endorse any product
or manufacturer). This unit consists of a wax- filled cylinder with
a metal rod. When the wax inside the cylinder heats up, it expands,
forcing the rod forward and opening the vent. When the wax cools,
the rod can retreat and the window is closed by a spring. (The manufacturer
recommends that the wax cylinder not be left outdoors during extremely
cold weather.) In our next high tunnel, the only change we will
make to this set-up is to have larger vents. This system has allowed
excess heat to escape from the high tunnel without the necessity
of being present on a daily basis to open or close doors or to roll
During the two years our high tunnel has been in operation, we have
conducted a variety of trials. Some have been successful, some have
been labeled "learning experiences." Here is a brief rundown
of the items we have planted and results we have seen in the unheated
Fall Salad Greens
We planted a variety of greens including spinach, lettuce, mustard,
etc. and have had our greatest success with these greens in the
fall of the year. We used #9 galvanized wire to form loops over
the beds. These hoops were used to hold floating row covers above
the crops so that the row cover and the greens would not come into
contact and freeze together. With this set up we have successfully
grown salad greens into January each year. We are still experimenting
with planting times for late season harvest. In our region, the
amount of available daylight diminishes rapidly throughout October
and drops below 10 hours per day in early November. At this low
light level, plants begin to go into a dormant state. Therefore,
planting times must take this into account so that plants are not
too mature (I'm not sure I understand what this phrase means) or
fail to grow properly.
Fall Root Crops
Carrots and beets were planted in the fall of 2003 and did well
(the carrots were not planted until September 1 and should have
been planted earlier). The soil within two feet of the sidewall
froze but in the center of the high tunnel only the top inch or
two of soil froze and only for three weeks in late January and early
February. Therefore, the high tunnel acted as a "living root
cellar" and allowed for carrots and beets to be harvested throughout
Spring Salad Greens
After our success in the fall of 2003 with salad greens, I thought
spring greens would be easy. However, I soon discovered that the
opposite was true. The first problem was the wildly fluctuating
temperatures. Daytime temperatures would climb to 100º or more
on sunny days and the upper layer of soil would quickly dry out.
Since we had direct seeded our greens, this lead to poor germination.
(Of course, if we would have been on site on a daily basis we could
have avoided this problem). The next problem we encountered was
voles. Voles are vegetarians and really enjoyed the tasty greens.
We tried to trap them but had little success. We later experienced
aphids and attacked them with an insecticidal soap solution. We
did manage to grow spring greens; however, it required a lot more
effort than the fall greens.
We planted 48 Chandler strawberry plugs in September of 2003 and
picked our first berry on May 6, 2004. That is about 6 weeks earlier
than berries grown outdoors. We picked 35 quarts of berries over
the next 5½ weeks. The berries had an outstanding flavor,
were firm, and the centers were not hollow. Once in blossom, we
covered the berries with floating row cover whenever temperatures
in the low 30's were predicted. We pollinated the blossoms with
a small artist's brush since we did not have any natural pollinators
available during April. Lack of pollinators would seem to be the
biggest problem facing growers and purchased bumblebees may be one
solution. I have read of leaf blowers being used to move the pollen
around but have never tried this myself.
Russian Fingerling Potatoes
We planted one pound of Russian Fingerling seed potatoes in April
2004. The emerging potato plants were not covered by floating row
cover, and one night, with temperatures in the 20's, the emerging
plants froze. (Only the portion above the ground froze). The plants
recovered and we later harvested 43.74 pounds of fingerlings from
that one-pound of seed potatoes. I checked with the dealer from
whom we purchased the seed and he stated that under good growing
conditions, we should expect up to 20 pounds from one pound of seed
potatoes. We certainly exceeded our expectations on this one!
Sugar Baby Watermelon
We direct seeded Sugar Baby watermelons in May but faced several
problems. First, under the warm conditions of the high tunnel the
plants produced additional and longer vines than I had ever seen
on watermelon in our area. This created very crowded conditions
and when combined with the fact that we did not have roll up sides,
led to powdery mildew problems and dying vines. I still believe
that the high tunnel is an excellent place to grow watermelons and
plan to try them again with greater spacing and more ventilation.
We transplanted striped German and Nebraska Wedding tomatoes in
late May (after danger of frost) both inside the tunnel and outside.
The tomatoes inside were watered with a soaker hose and showed no
sign of disease. The yields inside and outside during most of the
season were comparable, but the total yields for those inside the
high tunnel were greater since the plants escaped the early frost
and were able to produce for a few more weeks.
Asparagus (or yard long) beans are native to Southeast Asia and
are one of my favorite beans. We planted a ten-foot row and set
up a trellis. Under the warm conditions the vines climbed up, over
the trellis, then kept on growing. We harvested 15 pounds of 12-14"
beans from that single row. Other than the problems with all of
the vines, these beans could create a shading issue for vegetables
in the immediate vicinity.
This is another Southeast Asian vegetable that we planted but did
not trellis. The vines soon overtook the floor area. We did not
weigh the fruits, but the plants produced a large quantity of bitter
On April 15, 2005 we started transplanting tomatoes into the high
tunnel. We used Early Girl and Stupice and found that Stupice withstood
the cold much better than the Early Girl. The plants were covered
with floating row covers since 10 of the last 15 nights in April
saw outside temperatures in the low 30's. The Early Girl's did not
die, but turned a gray-green color and growth was stunted. We picked
our first tomato on June 9th and full production kicked in around
July 1. The tomatoes continued to produce abundantly throughout
July and August.
We planted sweet potato slips and the vines spread rapidly. The
only problem we experienced was that the grasshoppers quickly chewed
off all young leaves. A layer of floating row cover solved that
We planted jicama in the spring of 2005 and as of this time (October
2005) the plants are still climbing the ribs of the tunnel. I'll
be anxious to see the size once the roots are harvested.
As you can see, the high tunnel has worked well in Green Bay, even
with the self imposed restrictions mentioned earlier. We will continue
to conduct trials in our present unit and would welcome any suggestions
for future trials. Our next major project is to erect a large high
tunnel in cooperation with Green Bay Area Public School Food Service
Department. This high tunnel will be used for gardening and nutrition
education each month of the school year except January and February.
This will be an exciting adaptation for utilizing a high tunnel
I would also like to experiment with peach trees in a high tunnel.
There are many varieties of peaches that are only hardy to zone
6 that have outstanding flavor when tree ripened. If you bite into
a tree ripened Sun High peach like you would an apple, the sweet,
warm juice will run down your wrist. There is nothing like this
available in Wisconsin and we need to bring this experience to our
customers. If anyone has tried to grow zone 6 peach varieties in
a high tunnel in the upper Midwest, or know of anyone who has, please
let me know. I have some ideas about how to trial this concept and
would enjoy comparing notes with others.
Bill Wright is Community Garden Coordinator for Brown County UW-Extension.
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