This is your short and sweet guide to growing a cold season garden with a low tunnel. Learn the essential supplies and steps to take to use a low tunnel successfully for growing in cold and freezing temperatures.
Cold season growing when the temps are dropping below 0C / 32F is a little bit like playing Russian roulette with your plants. It can be very rewarding, and perhaps some of the most special harvests you get in the season, because you are growing during a time that most gardeners do not get any harvests.
In this blog post you will learn the key points to help you improve your success with gardening even after the temperatures drop. In the video below I will share with you how I applied these steps and tools to troubleshoot my own low tunnel and save all my little spinach plants from some very cold nights.
Using a low tunnel to extend your growing season is not a set it and forget it endeavor. To grow successfully with a low tunnel you will require extra supplies such as the low tunnel structure itself. I strongly recommend an outdoor thermometer so that you actually know what is taking place in YOUR growing space, and in particular, inside the low tunnel itself. The weather app is not going to be accurate enough. Your growing space is its own unique microclimate.
Typically a low tunnel will create an environment where the inside temperature is approx. 4-6C / 39-42F higher than the outside ambient temperature overnight. During the day time if the sun is out you can expect it to gain significantly higher temps, but only while the sun is directly hitting the low tunnel to create that greenhouse effect.
The reason you gain a few extra degrees overnight under the low tunnel is mainly due to the thermal mass of the earth. During the day the sun warms the earth, then at night it slowly releases the warmth to be captured by the low tunnel.
Pay Close Attention
You may need to open the low tunnel partially or fully during the day time to avoid cooking out your plants when the sun hits the low tunnel. In the evening you will need to close it again. Potentially you will need to open and close it multiple times in the day if the weather is changing a lot.
The time of day you open and close it in the morning and the evening will also be determined by the weather. If the outside temperature is staying below 10C / 50F I will usually leave the tunnel closed during the daytime. Above that you can start to open it in small amounts to see how it affects the inside temperature. Adjust the amount of opening accordingly.
As you will see in this video I got very lucky that my plants survived a challenging 48 hour period. The primary challenge was the fact that I was away from the house for a 13 hour period and was not able to open and close it at the appropriate times. Leaving your low tunnel unattended for a full day like this is inadvisable. If this is your typical day and you don’t have support to assist in maintenance of the low tunnel, then this is not a good option for you.
Spinach Cold Tolerance
Spinach is considered a semi-hardy crop. Meaning it can tolerate frost. Typically it’s ok down to about -6C / 12F. Do not let the day time temps creep up past 22C / 72F. Cold tolerant plants don’t like to get cooked.
3 Ways To Increase The Temperature
Step 1: Adding layers is the first step to take to help your plants get through an expected big temperature drop. I like to start with throwing old sheets over top of my low tunnel. It’s fairly quick and easy to do which is why it is my first step. You will need to remove the sheets during the daytime to let the sun in.
Step 2: Use a row cover fabric or thermal fabric specifically designed to hold heat around cold tolerant plants. Lay this inside the low tunnel right on top of your plants. This adds an extra layer of protection right close to the plants. This can be more work as you need to get right in your low tunnel to lay the fabric properly. Often this fabric lets light through so you may be ok to leave it on during the day time on especially cold days. Long term though between the plastic of the low tunnel, the row cover fabric on your plants and reduced daylight hours, growth will slow to a crawl.
Step 3: Add thermal mass such as a bucket of hot water. In this video I used a couple of buckets of hot water to help me recover heat within the low tunnel when it dropped too low from being open too long during the day. Once the temp came up, the low tunnel held it long enough to get the plants through the night comfortably.
As you will see in the video I used all 3 of these methods on a couple of very cold nights to ensure everything made it through. The thermometer that I use has a gauge that sits inside the low tunnel and a display I keep in the house. This helps me to know easily if I need to take any steps to increase or decrease the temperature in the low tunnel. I would not be without it.