Moving to our new farm in mid-June didn't leave much time for planting a vegetable garden so I decided to extend my growing season into the fall using a make-shift polytunnel. Little did I know that I would be harvesting veggies into mid-January! So what's going on in this mini-green house that allows carrots, kale, and arugula to grow during winter? I deployed some of my favorite science tools, iButton temperature loggers, to compare ground and air temperature outside and inside my polytunnel. Read on to see how I made my polytunnel and to view some colorful temperature results!
Figure 1. My initial 3-hoop polytunnel design almost blew away in a wind storm.
I made these polytunnels using 10 foot sections of 1/2" PVC pipe bent over my summer vegetable plots. I made holders for the PVC hoops by pounding 2 foot pieces of 1" PVC into the ground on either side of my vegetable plot. The 1/2" PVC pipe fit nicely inside the 1" PVC pipe creating arches that hold up plastic sheeting. I then clamped the plastic sheeting in place with some metal spring clamps. After a few wind storms, I added rope tie-downs to keep the hoops from popping out of their holders.
Figure 2. 5-hoop version with added rope tie-downs to keep everything grounded. The iButton temperature loggers are visible on the black stakes placed outside and inside of the garden plot.
Some of the plants that survived into January were cold-hardy species sown directly into the garden in late September, including carrots, kale, and arugula. I also transplanted some kale and rainbow chard from my planters into the polytunnel and added a bed of garlic, too.
Figure 3. Left: Rainbow chard transplanted into the garden. Right: View inside the polytunnel in mid-December.
The polytunnel made it through this snowy dusting (below), but totally collapsed during a mid-December Nor'easter, which dumped over a foot of snow on our farm. I add some garden stakes to support the center of each hoop, which helped reinforce the structure. Gotta love adaptive creative problem solving!
Figure 4. Polytunnel covered in snow. Rain guard (lower right) was added to protect the outside iButton temperature loggers from water damage.
The Polytunnel iButton Experiment
Purpose: To examine temperature variation inside versus outside the polytunnel.
Hypothesis: If temperature remains above freezing inside versus outside the polytunnel, then vegetables grown under polytunnel conditions will survive when outside temperature drops below freezing.
Question 1: Does heat trapped by the polytunnel increase temperature inside the polytunnel and prevent plants from freezing during winter?
Question 2: If temperature increases inside the polytunnel, does the temperature vary from ground-level vertically to the roof of the tunnel, or alternatively is temperature uniform throughout?
Materials and Methods:
I placed five iButton temperature loggers (five, because that's all I had) at various heights (0 m, 0.5 m, and 1 m) above ground level inside and outside of the polytunnel to investigate variation in temperature.
I placed each logger in plastic mesh holders (cut up pieces of onion bag) and fastened them (rubber banded) to stakes (old fiberglass tent poles) in the following configurations:
Stake 1: 0 m, 0.5 m, and 1 m above ground level
Stake 2: 0 m and 1 m above ground level
I placed Stake 1 inside the center of the polytunnel, midpoint from front to back and midpoint from side to side and pounded the stake into the ground until the lowest iButton rested at ground-level, zero meters (0 m) above the ground (Figure 2). I placed Stake 2 outside the polytunnel using the same general technique. The loggers were set to measure temperature at 6 minute intervals starting on December 31st, 2020 and retrieved on Jan 8th, 2021.
A rain guard (tomato cage with plastic over top) was placed over Stake 2 (outside) to prevent water damage to the outside loggers without altering outside temperature (Figure 4).
Data from the loggers were adjusted based on each logger's calibration curve, which was attained using regression analysis with simultaneous readings from an independent and well-trusted temperature source (our digital meat thermometer).
Results: These results are so cool... and beautiful, too! I love creating awesome graphs using R coding in R Studio - open source and freely available. : )
Figure 5. Temperature (degrees C) over time measured at different sites (Out and Poly) and heights (0, 0.5, and 1 m) above ground level.
The temperature inside the polytunnel at ground-level was over 2 degrees C higher on average than temperature outside at ground level, with a highly significant Wilcoxon rank sum test p-value < 2.2e-16. Wow!
Unfortunately, my Outside-1m iButton failed to launch (aka I messed it up) so I wasn't able to compare different heights inside and outside of the polytunnel except for at ground-level (0 m). Overall, average temperature inside the polytunnel was higher than temperature outside the polytunnel with the biggest difference at ground-level. Average temperature at ground level inside the polytunnel was above freezing, while average temperature for all other locations/heights was below freezing (Figure 6).
Figure 6. Plot of mean temperature (degrees C) and standard error versus height measured from ground level at 0, 0.5, and 1 m (x axis) for two sites: outside (Out) and inside (Poly) the polytunnel. Asterisks indicate significant differences in mean temperature between Outside and Polytunnel at 0 m, Polytunnel at 0 m and Polytunnel at 0.5 m, and Polytunnel at 0 m and Polytunnel at 1 m.
Conclusions: Increased temperature at ground level (0 m) within the polytunnel reduce plant exposure to freezing conditions, thereby allowing my cold-hardy vegetables, like carrots, kale, and arugula, to survive when outdoor temperature drop below freezing.
Warmer ground-level temperature probably helped keep soil microbes active. The indirect effect of heat produced as a by-product of the microbial breakdown of organic material likely further increased temperature at ground-level.