How to Build a Low Tunnel

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In my last post, I described how I am growing cold-tolerant plants under the protection of low tunnels in the late fall and winter months. Today’s post will show you how to construct your own “Living Refrigerators,” potentially allowing you to grow and/or delay the harvest of vegetables all year round depending on your plant hardiness zone.

You might be wondering, “If I’m growing cold-tolerant vegetables anyway, why do I need to bother protecting them in a tunnel?”

Cold-tolerant vegetables are just that – tolerant. They are not cold-resistant. Although they may not be bothered by (and may even improve after) a light frost, many of these plants can still suffer damage from freezing temperatures. This ranges from scorched or yellowed leaves, to a complete dying off of the above-ground portion of the plant with a hard freeze (even though below-ground it may still be viable).

Broccoli raab (rapini) with freeze damage. Note that the damage is primarily between the leaf veins, and some of the dead tissue has fallen away, leaving holes.

 

It is isn’t just freezing of plant tissues that injures, either. Cold temperatures desiccate (dry-out) plants. If you’ll recall your basic biology class you had in high school, all plant cells are encased by a wall… think of a sugar-water balloon (the cell membrane and its cytoplasm) placed inside a small tub (the cell wall) that contains water. Because of its concentrated sugars and other solutes, the cytoplasm (the balloon’s contents) is less likely to freeze than the water between the cell membrane and cell wall (the water in the tub). As the water in this outer space forms ice crystals, however, the surrounding unfrozen water in the space becomes more concentrated with solutes, eventually becoming more concentrated than the cytoplasm. As a result, water will be drawn from the cytoplasm, desiccating the plant.

A hard frost desiccates an unprotected plant on the left, whereas plants in the low tunnel are covered with water droplets and perky. Photos taken the same morning, seconds apart.

 

Although many vegetables will perk right back up after temperatures warm, repeated cycles of freezing and thawing are damaging and weaken the plant. When limp plants make contact with the soil, pathogens may also be introduced to the weakened plants. I don’t know about you, but I like my vegetables to look pretty on my plate, rather than scorched and full of holes from freeze damage or disease.

By protecting plants from freeze damage and disease, your harvest period will be greatly extended. But you can also use low tunnels to give seed germination and early plant growth a boost. Right now it is the week before Thanksgiving, and I need to get a cover crop going in some of the beds. I’m planting Sweet Lorane fava beans, a cold-tolerant nitrogen fixer and “green manure” that I can work into the soil next spring (plus harvest beans if I’m lucky).

How to Construct a Low Tunnel

As adapted from Eliot Coleman’s “Quick Hoop” method of low tunnel construction

This method uses 10 foot lengths of 1/2-inch PVC conduit as “hoops” over the beds. Depending on your choice of row cover and length of your bed, you can build your own low tunnel for less than $50!

1. Prepare the bed, by rototilling or, more preferably, aerating and loosening the soil with a spading fork. Although the length of the bed is determined by the space you have available, beds are most commonly 5 to 6 feet in width; you can make them more narrow if growing plants with a taller height than most greens.

Using a spading fork to aerate and loosen the soil in the former pepper and tomatillo bed.

 

I prefer to plant the bed before I construct the hoops, so that I don’t have to work around them. Plants that will grow taller, like swiss chard or fava beans, are planted toward the center of the tunnel, whereas shorter plants like lettuces are seeded near the border.

 

2. Using a stud bar, drive a 12-inch deep hole into the ground at the corner of the bed.

A stud bar.

I use duct tape to mark 12″ from the bottom of the spud bar, that way I don’t have to keep dragging out the tape measure.

 

3. Create another 12-inch deep hole at the opposite side of the bed. Alternatively, if you have a terraced garden like I do, drive an 18-inch length of rebar (or other small-diameter stake) into the ground at a 12-inch depth on the downhill side of the bed.

Rebar driven to a depth of 12″ on the downhill side of a terraced bed.

 

4. Insert one end of a 10 foot length of 1/2-inch PVC conduit into a hole (or over the rebar if used). As of today, each section of PVC conduit costs less than $2 ($1.89) at my local home and garden store.

1/2 inch PVC conduit slipped over rebar at downhill edges of the bed

 

5. Insert the other end of the PVC conduit into the opposite hole, creating a hoop over the bed. Avoid stepping on the prepared beds (even my girls know better than to “jump on the beds”).

Hoop folded over bed and end about to be inserted into opposite hole.

 

6. Repeat the process of constructing hoops every 5 feet down the length of the bed.

Hoops spaced 5 feet along the length of the raised bed.

 

7. Unroll the low tunnel cover of your choice along the length of the bed. Most row covers can be bought in 10-foot widths, which is what you want in this application. Agribon-19 row covers will provide frost protection down to 28 degrees F, and are great for late fall/early winter protection as they are water and air-permeable. When temperatures start dropping significantly (usually in late December here in hardiness zone 6b), add an additional layer of clear plastic (either greenhouse grade or 6-mil construction grade, the latter being less expensive but less durable).

In this application, I’m using 6-mil construction grade clear plastic, purchased as a 10 X 100 ft section for $64 at my local store. Since I have enough from this to cover 2 beds, each bed will only cost $32 for the row cover.

Unfolded row cover along bed length. In this application, I am using clear plastic to boost the temperature in the beds for seed germination.

 

8. Pull the cover over the bed. At the longest ends of the bed, gather the cover and cinch it tightly with a durable rope (such as woven clothesline). Tie the other end of the rope to a secure stake. Repeat on the other end of the bed, pulling the cover taut.

Row cover end secured to a ground stake.

The row cover is bunched and tied to a ground stake.

 

9. If you are using a lightweight Agribon cover, you need only secure the edges of the cover with whatever weights you have available. I use extra bricks and aluminum fencing that had been left on our property, but you can use 2x4s, plastic sandbags, etc.

Agribon row cover edges temporarily secured for easy access to the tunnel for harvests.

 

You may also consider purchasing a few spring clamps. These are great for clipping row cover to the hoops when you have lifted it to access the tunnels for harvesting.

An assortment of metal and plastic spring clamps secure the rolled up edge of the low tunnel cover.

 

My low tunnel is finished, and it only cost me $14 for the PVC conduit and $32 for the clear plastic cover – under $50 for a 5 x 35 ft bed!

 

How to Secure Row Covers on Low Tunnels

If you are expecting high winds, or when it is time to get additional plastic covers in place for colder temperatures, you will want to secure the cover.

1. Begin by driving a stake into the ground at the base of every other hoop on one side of the bed. Do the same on the other side of the bed, but at the base of hoops that were “skipped” on the opposite side.

Drive stakes into base of hoops at a slight angle.

 

2. Tie durable rope or masonry line to a corner stake. Take the line ACROSS the cover to the stake at the base of the next hoop. Loop the line around the hoop a few times, and then repeat to the other side. You’ll end up with line zig-zagging across the cover, securing it to the hoops and the ground.

Line looped around ground stakes to secure the cover.

 

Line in a zig-zag pattern across the row cover.

 

3. Secure the edges of the cover with weights. If you don’t plan on accessing the tunnel frequently, consider burying the edges with your garden soil.

Row cover edges temporarily secured with weights.

 

4. You’ll want to be cautious and mindful of the outdoor air temperature when using a plastic row cover. If you get a sudden warm spell in late fall or early spring, you could accidentally cook your plants in the tunnels. You may want to invest in a remote thermometer so that you can keep an eye on the temperature in the tunnel.

 

Covering your plants in an unheated low tunnel will not completely protect them when freezing temperatures become their most extreme (January and February here in hardiness zone 6b), but many plants that are considered “cold-hardy” and can survive with minimal damage. These are primarily root crops like beets, parsnips, rutabaga, and turnips, but cabbage, kale, and kohlrabi also do well. Other plants that are less hardy, but will quickly rebound in early spring include parsley and spinach. If over-wintering carrots in tunnels, you might consider adding an additional layer of mulch/compost to protect the roots until you are ready to harvest.

Agribon (top) and plastic row covers in place to extend the season.

36 Comments

  1. darlene

    November 16, 2012 at 6:27 am

    Easy to follow directions! Love the pictures also.

    • Shirley

      October 30, 2013 at 10:19 am

      We live in the Chicago area zone 5 and planning on making a low tunnel. You didn’t mention about cutting slits on the top of the plastic of the tunnel for venting. Is it necessary to have these vents? I’ve heard others say yes. This is our first time doing this. Really enjoy your site! :}

      • Ma Hubbard

        October 30, 2013 at 12:58 pm

        I vent the plastic-covered tunnels manually. I can do it pretty easily since I’m within walking distance from my work, and my schedule is flexible enough that I can pop home if I notice the day is getting too sunny. Whether you use slits or vent manually depends on how easily you can get to the tunnels and vent them. If you go the manual route, you should know that there is the potential to cook the plants if you forget/are unable to vent, as temperatures rapidly rise into the 80s and 90s inside the tunnels on a sunny 45 degree F day. I have primarily seen the slitted covers you’re describing being used for starting plants earlier than normal in the spring, or to extend the season a little longer in the fall — I wish I could offer an opinion on their use for mid-winter gardening, but I just haven’t tried the slits before. I would be very interested in hearing more about your experience, though, so please feel free to come by again and give us all an update. Cheers!

      • Ma Hubbard

        October 30, 2013 at 1:16 pm

        And I should also add what winter-gardening-pioneer Eliot Coleman has to say on the subject, after consulting his “Four-Season Harvest” text (which I highly recommend): “When using these low tunnels to protect spring crops, gardeners solve the venting problem by cutting ever larger holes into the plastic cover (to vent off more and more excess heat) as the spring advances. That works adequately although the frost-protection potential is progressively lessened if a late spring frost comes along. It would obviously not be a suitable technique for over-wintering where some venting is needed in the fall but none in the winter.” Mr. Coleman is also in zone 5, by the way. I hope this is helpful.

        • Shirley

          October 30, 2013 at 2:24 pm

          Thank you! This has been very helpful to me. :}

  2. Joyce Pinson

    November 16, 2012 at 8:06 am

    Gosh…you want to come over and help me put one up? Giggles

    • Ma Hubbard

      November 16, 2012 at 8:51 am

      Sure! I’m also happy to help you with any deer problems while I’m there. ;)

  3. Little Mountain Haven

    February 14, 2013 at 4:53 pm

    Thank you so much for this informative post! we are building some this year, I will be sure to link it to this site!! love your blog!! I will be following from now on!

    • Ma Hubbard

      February 15, 2013 at 10:34 am

      Thank you!!!

  4. little mountain haven

    March 14, 2013 at 12:07 pm

    • Ma Hubbard

      March 14, 2013 at 7:35 pm

      Wow! Thanks so much! Your tunnels look great, by the way… It looks like they are working out well so far!

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  6. Cary Bradley

    November 18, 2013 at 9:34 am

    Great article on low tunnel construction, but I have a question. Did you leave the rebar in place and place PVC pipe over it, or did you use rebar only for making the hole, and then remove. It sounded like some of your holes used the rebar for reinforcement, but would really appreciate clarification. Thanks! Oh, question 2. Do you not use any connectors between the hoops? They stay standing up with no connection? Thanks, again! :)

    • Ma Hubbard

      November 18, 2013 at 10:09 am

      Thanks, Cary! The rebar stays in place and the pipe slips over it — I use a heavy digging bar for making holes. I only use rebar in a few spots on the downhill side of my terraced beds; you could use rebar for the whole thing, but you would need to get a cover larger than 10 ft wide (or cut your pipe) to have enough wiggle-room to place weights on the edges. I’ve never used connectors between hoops, as each end is sunk 1 ft into the ground. If you get heavy snows in your area, you might add a horizontal support, mainly to prevent snow from accumulating into heavy pockets between the hoops. I’ve seen people use fancy connectors, but also something as simple as rope tied across and between each hoop. I’ve only had one hoop ever slip, and that was one where I should have used rebar, had run out, and used a flimsy garden stake instead (which wasn’t long enough to go a full foot in the ground either). And I wouldn’t fudge on the distance between hoops, or that might set things up for problems as well.

      In my next blog post reviewing our winter garden, I’ll show you how the hoops stood up to a good snow. Cheers!

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  8. Maria

    January 10, 2014 at 12:39 pm

    This looks great!!! I can’t wait to try them at our gardens in Williamson, WV. thank you for sharing. Have you ever tried building low-tunnels on any raised beds? The soil at our garden is not very good so we have had to construct many raised beds (about 3 feet high). I assume it would work the same as yours which are directly above the ground?

    • Ma Hubbard

      January 10, 2014 at 3:54 pm

      Thanks, Maria. Great to hear from a neighbor! I’ve never used low tunnels on raised beds, but our extension office is at the moment. Their extension agent, Joel, communicated that their collards and turnips survived (but not lettuces). The main issue with the raised beds, as I’m sure that you’re aware, is that the soil can cool rapidly since it is exposed on all sides. If it were to be a problem, you could always line the side with straw bales through the winter, and then use the straw in the spring for mulching new plantings.

      I’ve been meaning to visit the gardens in Williamson — I hear so many great things about what you are doing there!

  9. Brittany

    March 9, 2014 at 1:22 pm

    I’m so thankful for these step-by -step instructions! I had to tweak a few things as I grow in raised beds, and everything went quite smoothly until it came down to securing the ends of the tunnel… There is just so much plastic to gather together! I just got finished with the construction about 20 minutes ago and the temperature within has already about 0 degrees warmer than outside! Can’t wait to get a jump on the season!

  10. Denise

    April 5, 2014 at 10:43 pm

    I just found your blog and I love it. I have a question about the Agribon row covers, they have Agribon AG 15, AG 19, AG30 and so on, which one would you recommend for the Hickory North Carolina region?

  11. Sally Oh

    June 22, 2014 at 1:08 pm

    I was going thru my bookmarks and found your blog again! So happy — we are in Eastern KY as well. I’m building a low tunnel today from your instructions with a few modifications. We have low beds because the soil here is clay. I’m really not much of a gardener but going to bite the bullet anyway. We’ll feed us and our chickens. (Organic non-GMO feed is expensive, sheesh!) Thanks — looking forward to reading more :)

    • Ma Hubbard

      June 22, 2014 at 9:06 pm

      Awesome! Can’t wait to hear how it turns out!

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  13. Kevin

    September 7, 2014 at 3:29 am

    Just discovered your blog and love the info on winter gardening, I’m at 8000′ in Arizona and this might be the only way I get to grow much of anything since two years in a row my normal “summer” crops have failed to really produce thanks to our late and early frosts as well as the cool late sumer temps thanks to our annual “monsoon” (only rated by the USDA as zone 6a, but first frost is averaged as Sept 10 and last as June 1).

    I do wonder though have you had better results with east/west tunnels or north south tunnels?

    • Cathy

      September 7, 2014 at 5:13 pm

      Very cool, Kevin! Since my garden is on such a steep hillside, I have no choice how I orient the tunnels and they run north/south. I just make sure that taller veggies aren’t planted in the bed where they will shade others. Good luck!

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  16. Tammie Allen

    October 20, 2014 at 3:22 pm

    We were given 8′ poles and 7.5′ – 250′ material…do you have a suggestion as how to make that work? Thanks :)

    • Cathy

      October 21, 2014 at 3:01 pm

      Tammie, the poles should be buried into the ground by at least a foot on each end. Since your poles are 8 ft, once they are sunk in the ground, the exposed surface of the hoop will be 6 feet. So, you’ll have a little less than a foot of material to weigh down on each side of the tunnel. Did Grow Appalachia give you materials?

  17. AM

    October 21, 2014 at 11:15 am

    How tall are your tunnels at the highest peak? I want 5 foot wide beds at a peak height of 4 feet? Is there a formula I can use to determine the length of the PVC to achieve this height?

    Thanks for all the info you share! I am always blessed by it!

    • Cathy

      October 21, 2014 at 3:11 pm

      Based on my math (and this online calculator that computes the circumference of an ellipse), you’ll need PVC that is at least 12.5 feet in length (assuming that you bury the PVC pipe a foot at each end). If you’re using rebar, then your PVC can be 10.5 feet. Check my math, though; I’m assuming a semi-major axis of 2.5 (since your bed is 5 across), and a semi-minor axis of 4 (the height you desire). Best of luck!

      • Ann

        October 21, 2014 at 4:24 pm

        You sound like Einstein!!!! Thanks!

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  19. Josh

    November 9, 2014 at 2:22 pm

    Hi, I am wondering if you have to water the greens in your tunnels during the winter, or if the humidity maintained by the plastic tunnels is sufficient to keep the soil moist.

    Thanks!

    • Cathy

      November 9, 2014 at 3:31 pm

      There is no need to water in winter, due to much decreased plant transpiration and reduced water evaporation due to cooler temperatures. Enjoy! :)

  20. farmer fengel

    November 13, 2014 at 10:18 am

    I have solve the manual method of venting low tunnels by constructing a 1/2 inch treated plywood ($7) on the end of the tunnel with an automatic foundation vent( $16) that opens at 70 deg. and close at 40 deg. I also stapled 1/4 in. pink insulation board to the inside of my raised bed to reduce rapid temperature changes in he soil.

    • Cathy

      November 13, 2014 at 11:57 pm

      Brilliant! Unfortunately my tunnels are too long (~35 ft) for me to get adequate ventilation with the setup you are describing, but it is certainly great for smaller beds. I’d like to invite you to share a photo of your tunnel on my Facebook page; I’m sure everyone would love to see it. I appreciate you sharing your description of it with all of us here — thank you!