Low tunnels are an inexpensive, DIY way to extend your growing season. Learn how to build a low tunnel that suits your climate and needs.
How to Build and Use a Low Tunnel for Inexpensive Season Extension
Want to eat fresh, homegrown greens all winter long? This video shows a cheap and easy way to extend your growing season into the winter months. (And if your garden soil is frozen right now, remember that you can use this method to extend your growing season on the other side of winter, too, so you can get a jump-start on your spring garden.)
When you build a low tunnel, you’re building a really simple structure. (Note that low tunnels are called by different names in different regions. I’ve heard them referred to as hoop houses, cloches, and cold frames. Those terms get the point across, but each of them technically refers to something else. So for the sake of clarity, we’ll call this a low tunnel.)
Best Materials to Build a Low Tunnel Frame
To build a low tunnel, you start with a structure that’s a simple series of hoops. I’ve seen people use PVC pipe, PVC electrical conduit, steel rebar, cattle panel, and flexible fiberglass rods (like tent poles) to build a low tunnel frame. In my opinion, the best option is PVC—unless you have one of the other materials on hand already. A 10-foot length of 1/2-inch schedule 40 PVC pipe typically sells for under $2, so it’s affordable. PVC electrical conduit is about the same cost, and it should last longer out in the elements.
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My favorite method for securing the posts is driving a piece of rebar into the ground and fitting the PVC over the rebar, as is demonstrated in this video. (It’s comical to think that you could drive rebar 2 feet into the ground in my area, though. It’s solid rock down there, so we use pieces that are about 1 foot long, and we can usually get them about 8 inches deep.) I have also seen many people use pipe straps, screwed into the sides of their raised beds. I think the rebar method is better—especially if your beds are a few years old and the wood has started to break down. Also, the rebar method can be done on any bed or row, even if there is no frame.
The final element is the cover, and this is where I’ve heard a lot of debate about which material is best . . . .
Plastic vs. Cloth as a Low Tunnel Cover
There are two common options: plastic or cloth.
Plastic sheeting allows light to reach the plants, but it doesn’t allow for any air circulation or water penetration. Water may not be an issue if you’re protecting a bed that has drip irrigation installed. But because there is no air circulation, plastic is prone to overheating the tunnel on sunny winter days. If you use plastic, you need to remove or ventilate the tunnel appropriately to avoid smothering your plants with hot, humid air.
Cloth is a better option for air circulation and water penetration. Floating row cover is a cloth material made of woven synthetic fibers that allows hot air out and allows water in, while providing insulation and light penetration similar to that of plastic film. In my relatively warm and dry climate, cloth row covers work very well for low tunnels. Be careful about using old sheets and blankets in wet weather—those can absorb water, and they can actually cool the air as that water evaporates.
5 Considerations When Using a Low Tunnel to Extend Your Growing Season
A handful of tips and pointers:
• Climate: Take your climate into consideration when choosing the material you use to cover the tunnel. Where I live, I need to take advantage of every drop of rain that I get, so I use cloth instead of plastic. If you have abundant winter rains and you need to regulate the soil moisture, plastic might be a better option for you.
• Integrity: If you build a low tunnel that is very long, or if your garden gets a lot of wind in the winter, consider using an additional length of pipe across the top, lengthwise, for structural integrity. Fix it to the hoops using twine or zip ties, not pipe fittings.
• Staples: In the video above, they staple the plastic to the raised bed frame. I would skip that step, and use rocks or bricks to weigh the plastic down instead. You’ll extend the life of the cover and make it easier to ventilate on warm days.
• Lights: You can use a string of Christmas lights inside the tunnel for added warmth. If you do this, you will want to use the old-school incandescent lights. The newer LEDs are more efficient, but they don’t offer much warmth. In this case, you want less-efficient bulbs that use more energy and generate more heat.
• Survival Blankets: You can add a survival blanket on top of your cover for extra insulation on very cold nights. Face the shiny, aluminized side down, and remove the blanket to let the sun warm the soil again on the following day.
Check out “Frost Protection and Extending the Growing Season” from the Colorado Master Gardener Program and the Colorado State University Extension. They tested a low tunnel with 4-mil plastic sheeting, a survival blanket, and a 25 light string of C-7 Christmas lights. With all three of these measures in place, they consistently raised the temperature inside the tunnel between 18 to 30 degrees.
To see some additional creative ways to add heat inside a structure during the winter, read these two great articles from our writing contest. This one is technical: “Mad Scientist Works For Greenhouse Heating Independence Down To -25F.” And this one is practical: “Saving Heat in a Small Winter Hoop House.”
If you want to eat fresh, homegrown greens this winter, but you don’t want to build a structure, here’s a much smaller scale solution that you can put into place right on your kitchen counter: “Grow Sprouts and Microgreens Indoors All Winter Long.”
What Do You Think?
Have you ever built a low tunnel? What’s your favorite way to add or conserve heat in a low tunnel or other cool-weather gardening structure? Let us know in the comments below!
This is an updated version of an article that was originally published on November 23, 2015. The author may not currently be available to respond to comments, however we encourage our Community members to chime in to share their experiences and answer questions!
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Thanks to Natalie Donnelly, John Garlisch, and Nissa Patterson of New Mexico State University—Bernalillo County Extension Service for the nice video.
Thanks to David Whiting, Carol O’Meara, and Carl Wilson of the Colorado State University Extension for the PDF “Frost Protection and Extending the Growing Season.”