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Measuring The Heat Trapping Potential of Covered Spaces

Given our relatively cool climate and short growing season, I've accumulated a lot of qualitative experience in growing under cover throughout the last decade, but that experience isn't always the easiest to communicate simply to new growers. There are common misconceptions that our high tunnel allows us to grow vegetables in the winter or that everything is always happier growing under cover. So to help explain the potential and limitations of covered spaces, I employed a collection of temperature sensors to collect some data for us. My hope was that this quantitative information would help clarify the true capabilities of covered spaces. Let's look at where we put these temperature sensors and what we learned.

The general idea with any season extension strategy is to cover a space with some type of material that allows the sunlight to penetrate and warm the space while at the same time preventing the cold air from blowing that heat away. Materials that can accomplish this include glass, twin-walled polycarbonate, UV treated poly, and polyester row cover. In some cases, as with our nursery greenhouse or high tunnel, rigid structures are used to support these transmissive materials. However, as you can also see from the photo below, the use of row cover or plastic can be as simple as pinning it down around your garden beds and letting the material rest on the ground.

To collect our temperature data, I used several of these little Govee temperature sensors. They log their own data constantly and link to an app on my phone via bluetooth and wifi.

The accompanying app allows me to check the temperature or display a graph of the temperature history from anywhere with an internet connection.

As you can tell from the readings on this screenshot, it's unusually warm outside today, but just last week was a very different story. Here's how our nursery greenhouse looked on April 20th when I was getting these temperature sensors set up.

Things look pretty chilly in that photo, but you might be amazed by how warm these season extension structures can become when the sun is shining.

Increasing the Air Temperature

To start off, let's look at some temperature data from earlier in April, to show you the heat trapping power of clear materials. Below is a graph of temperature readings for April 7, 8, and 9 that shows the difference in air temperature between our nursery greenhouse and high tunnel and outdoors. Our nursery greenhouse is glazed with twin walled polycarbonate, has insulated north and west walls and is the most air tight. Our high tunnel is covered with just one layer of 6 mil poly and there are air cracks along the roll bars and corners that allow some air exchange. These differences help explain why the nursery greenhouse is so much better at trapping and holding the heat from the incoming sunlight. During these same three days, the average temperature was 0.4ºC outdoors, 6.4ºC in the high tunnel, and 13.9ºC in the nursery greenhouse.

There are two points I'd like you to note about this data. The first is that despite the significant differences in high temperatures each day, the lows eventually trickle down to similar ranges. These lows are the main reason we can't start using our high tunnel another month earlier. It doesn't matter how warm the temperatures are in the daytime, if the lows are still going to drop well below freezing at night.

The next point I'd like you to notice is the remarkable heat trapping ability of a clear layer of plastic. During the collection of this data, I left the high tunnel and nursery greenhouse closed and unvented to illustrate their full potential. (That is with the exception of April 7 in the afternoon, when you can see a sudden drop in temperature because I opened up the high tunnel door to do some work.) In full sunlight, the temperature of our closed high tunnel can easily jump 20ºC higher than the outdoor temperature. While this is great it some ways, too much heat is also bad for plant growth so season extension is not as simple has throwing a clear dome over your plants and walking away. If you just leave these structures closed, temperatures can surpass 40ºC even on relatively cool spring days, so regular monitoring is essential for success.

Once we start ventilating our high tunnel and nursery greenhouse, the temperature data looks like this...

Our nursery greenhouse is small enough to ventilate automatically with a thermostat and fan, so the maximum temperatures can be kept really consistent. On the 16th and 17th, that fan was unplugged, so you can see what a difference it is making on the other days. The high tunnel ventilation requires someone to roll open the doors or sidewalls, and since I'm not perfect, the temperature control in the high tunnel is less perfect too. You can see a couple of red peaks early in the day, when I was a little late getting out to the high tunnel to open the door.

Increasing the Soil Temperature

Our air temperature data shows that we can significantly influence the air temperature of our growing spaces with clear covers, but the soil temperature matters too. When we seed crops directly, the soil temperature is all that those seeds will experience so it is more important than the air temperature at first. In fact, if we can warm up the soil, we can often seed and achieve good germination rates long before we would could ever transplant our seedlings outdoors.

Now one might assume due to their cost and elaborate structures that the high tunnel or nursery greenhouse are far more effective at trapping heat than a sheet of plastic laid on the ground, but are they really? What if all we wanted was to warm up the soil for our first planting of carrots? To answer these questions I grabbed a few more temperature sensors, wrapped them in tinfoil so I could bury them just below the surface of the soil as shown in the images below.

The three spots I chose to place our soil temperature sensors were a bare outdoor garden bed, a second outdoor garden bed covered with floating row cover and clear poly for nights and cooler days, and a high tunnel bed covered with row cover.

covered field bed
The sheet of poly to the right was pulled over this bed for extra warmth at night and for cool days.

Here is a look at some of the temperature data from these different beds so you can see how much they vary. During these 5 days, the average temperature of the outdoor air was 6.5ºC, the average temperature of uncovered soil was 9.0ºC, the average temperature of of the covered field carrot bed was 14.3ºC and the average temperature of the covered high tunnel carrot bed was 13.8ºC.

It's no surprise that the covered beds reached and maintained the highest temperatures here, but it's remarkable to see how comparable covered field soil is to the covered high tunnel soil. There are even two days where the field soil temperature is higher than the high tunnel soil temperature. The field bed was covered with floating row cover first, then an additional layer of poly for nights and cooler days. On these two days though, I neglected to roll back the layer of poly so the soil temperature continued to climb. The high tunnel soil temperature would likely rise just as high, but at this point, I was opening the tunnel for ventilation to avoid uncomfortably hot temperatures for the germinating carrots and several other crops already planted there.


I hope this data helped to communicate the potential and limitations of using covers to extend your growing season. Here are the primary takeaways to remember when you're considering how to extend the season in your own garden:

  • Covers are very effective in increasing the heat of enclosed air and soil.

  • Covers can create dangerously hot conditions if not vented properly when the sun is shining.

  • Covers can't protect against extreme freezing temperatures but seem to offer around 5ºC of frost protection.

  • Supporting structures for these covers are not necessary when enhancing the growing conditions for small transplants or direct seeded crops.

To learn more about the key components of our nursery greenhouse see this Field Journal post. For more information about our high tunnel construction and full list of parts see this Classroom post (members only). To learn more about the variables we control to optimize. the growth of our vegetable crops, head to my free interactive workshop.


P.S. The carrots that I seeded in these high tunnel and field test beds on April 15 were both starting to emerge on April 28 thanks to the warm soil temperatures. That's a germination period of 13 days, which falls right in line with what we know about how temperature impacts the carrot germination rate. If I had ignored the potential of covers for these early carrots, I'd be looking at a much longer germination period and maybe even a much lower germination rate.

Impact of Temperature on Carrot Germination
The number of days on each bar of the graph indicates the number of days to emergence at that given temperature.

Since our spring temperatures are warming significantly over the next few weeks, I could simply wait longer to plant our first carrots, but if my early carrots mature two weeks later, that actually eliminates a lot of crops that I can potential use to follow carrots in a relay planting combination. We'll be following this early bed of field carrots with corn this season, and there's no way our growing season would have enough time for that combination without an early start to our carrots. Check out this Field Journal post to learn more about relay planting.


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