10 Reasons Why Low Tunnels Beat Cold Frames for Winter Gardening

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Low Tunnels Winter Gardening

In my last post, I shared with you how it’s desiccation, not really cold temperatures per se, that is most damaging to cold-tolerant plants. While it’s true that cold frames and low tunnels provide a growing environment that is slightly warmer than ambient temperature, the increased relative humidity in these structures and the protection they provide from winter’s harsh winds allow plants to survive at MUCH colder temperatures than they otherwise would. So while lettuce should theoretically die at 28° F (-2.2° C), it has survived and thrived in my low tunnels that registered 10° F (-12.2° C) inside when temperatures outside dropped to -8° F (-22.2° C). That’s pretty powerful protection!

Because cold frames have been around a lot longer than low tunnels, many more gardeners are familiar with them for winter growing; I usually don’t have to explain what a cold frame is to participants at my winter gardening workshops. Cold frames differ in design and materials, but are essentially a box with a hinged transparent lid (most commonly made of glass). Cold frames can be expensive and permanent architectural additions to the garden, or cheap structures assembled from scrap lumber and recycled glass windows.

When I bring up low tunnels, on the other hand, I’m usually met with blank stares. Perhaps it’s because we can’t agree on a name for them (they’re also referred to as Quick Hoops, Caterpillar Tunnels, and Mini-Hoops), or simply because they are so relatively new to the backyard gardening scene (even among commercial growers, they’ve only been around for a few decades). Low tunnels are constructed from metal or PVC pipes that are bent in an arc over garden beds, then covered with fabric row cover or clear plastic sheeting so that the plants within are well-protected.

A low tunnel opened after the Polar Vortex of January 2014, contrasted with unprotected plants (below) and an unopened tunnel (above).

A low tunnel opened after the Polar Vortex of January 2014, contrasted with unprotected plants (below) and an unopened tunnel (above).

Which of these structures is better for growing in winter? You can’t go wrong with either, but here are 10 reasons why I chose low tunnels over cold frames in my backyard garden. You just might decide that they would be a better fit for you, too.

 

1) Let the Sunshine In!

The main limiting factor in winter vegetable production is light; once day length drops to under 10 hours of sunlight, plant growth stalls (which is why NOW is the time to be planting your fall and winter garden). Winter shadows are also lengthened, thanks to the winter sun traveling nearer to the horizon rather than directly overhead as in summer. The wood or brick walls of a cold frame can significantly shade parts of the growing bed during the day, resulting in poor or uneven growth. Low tunnels have no shady spots — since they are transparent on all sides, they make the most of any available sunlight, maximizing yields.

2) A Few Meals versus Many

Speaking of yields, you’d need to plant much more kale to last you a month in winter than a month during summer; plant growth in winter simply can’t compete with summer’s warm temperatures and abundant sunshine. Harvesting a few fresh greens out of a cold frame in winter is awesome, but if you want to eat REGULARLY from the garden in winter, it will take much more than just one 3 foot by 4 foot (3′ x 4′) cold frame. In contrast, my low tunnel garden beds average 4′ x 40′ — you’d have to round up a LOT of windows to cover the same amount of area in cold frames.

3) How Low Must You Grow?

Any vegetables grown in a structure are necessarily limited by the dimensions of the structure, especially in winter; any leaves that touch the glass window of a cold frame or plastic film of a low tunnel will freeze. Since taller cold frames will shade the plants within (back to Reason 1), most cold frames are low structures, most commonly 12 inches high at the front and 18 inches high at the back to allow a sufficient angle for catching the sun’s rays.  Low-growing vegetables like spinach, lettuce, and beets are fine for cold frames, but where are the large sprouting broccoli and cabbages going to go? In a low tunnel. They can easily reach 3 feet in height depending on the length of conduit that is used to construct them.

Want to grow rapini (broccoli raab)? It probably isn't going to fit in a cold frame.

Want to grow rapini (broccoli raab)? It probably isn’t going to fit in a cold frame.

4) Let Me Level with You

We don’t all have gardens in the most ideal locations. My challenge? My garden is all on hillside, but I’ve learned to love it by transforming it a system of terraced beds. Because low tunnels are modular and lightweight, I can easily carry the materials to their destination in the garden. Lugging heavy lumber and glass windows up a hillside? I don’t think so.

The hillside garden in winter; I'd hate to slip and crash into a glass-covered cold frame.

The hillside garden in winter; I’d hate to slip and crash into a glass-covered cold frame.

5) For the Mechanically-Disinclined

Low tunnel construction requires no saws, no hammers, no screws, no levels, no measuring tape. Can you dig a hole with a digging bar? Congratulations, you can construct a low tunnel.

6) Winter’s Over. Now What?

My city homestead is situated on less than 5/10th of an acre, so space is at a premium. To store my low tunnels, I simply roll up my fabric or clear plastic covers and store them in the attic for next year, and pull up the PVC conduit hoops, laying them flat and out of the way beside my storage building. I wouldn’t have anywhere to go with cold frames on my small lot, and even if I left them in place and worked around them, I would still need to store the glass lids out of the way (did I mention how accident-prone I am?).

7) A Reason for Every Season

That PVC conduit need not go to storage, however. In spring and summer, I connect it with elbows and build large tunnels that will support bird netting over the berries in my landscape. The large size of these tunnels allows me to pick berries with ease, and I don’t have to worry about new canes or branches growing up through the netting. Can a cold frame do this?

Easily picking black raspberries beneath bird netting suspended by PVC conduit

Easily picking black raspberries beneath bird netting suspended by PVC conduit.

Feasting on blueberries. Bird netting suspended by PVC conduit tunnels keeps our berries safe from the mockingbirds and catbirds.

Feasting on blueberries. Bird netting suspended by PVC conduit tunnels keeps our berries safe from the mockingbirds and catbirds.

8) Excuse Me While I Vent

If you use a cold frame or a clear-plastic covered low tunnel, you absolutely WILL have to vent them anytime it is above 40° F (4.4° C) on a sunny day or risk cooking your plants. Unlike cold frames, low tunnels can easily be covered with breathable fabric row cover instead, getting around the need to vent, yet still affording freeze protection. Fabric row covers (also known as frost blankets) come in many thicknesses, which affect light transmission and the degree to which they protect plants in cold temperatures. Of my 12 winter garden low tunnel beds, I usually keep only two under clear plastic sheeting — the rest are covered with medium-weight Agribon-30 fabric row cover (anytime temperatures are forecast for the low teens and below, I throw on an extra layer). Exchanging clear plastic or glass for fabric results in a whole lot less work for you.

9) Thar She Blows!

Vented cold frames and tunnels should circulate air, but not at the expense of them acting like sails and flying away. If you’ve used cold frames, you’ve probably had this experience: a propped-open lid that caught the wind and ripped away (shattered glass in the garden is not fun). I once invested in a very expensive double cold frame with polycarbonate panels along the sides (to get around the shadow problem mentioned in #1). A few weeks into use, a straight-line wind came through and ripped the lids away (along with the tops of some of my plants it was supposed to protect).

My expensive lesson in cold frame vulnerability.

My expensive lesson in cold frame vulnerability.

Yes, there is also a good chance that low tunnel clear plastic covers will blow away, but the risk is minimized if they are properly secured with ropes criss-crossed across the tunnels and tied to ground stakes (see my low tunnel construction guide for an example) — no need to untie them to reach your veggies, as the sides will slide up easily for you to reach in and harvest. And by the way: don’t think you can get by without securing your plastic-covered low tunnels with rope — everyone else has already tried it, and they had weights, rocks, and logs bigger than yours.

Note the rope criss-crossing this low tunnel, but  veggies are still accessible.

Note the rope criss-crossing this low tunnel, but veggies are still accessible.

10) Saving Green While Growing Green

Remember that double cold frame that blew away on me? It cost me $179. Even if it had remained intact, I would have been paying roughly $12 per square foot of growing space ($179 divided by a cold frame footprint of 14 square feet). In comparison, I can build a 4′ x 40′ low tunnel (with cover) for less than $50 (that’s only $0.31 per square foot of growing space!). Even if you use recycled windows and lumber for your cold frames, time is money, and the time you spend searching for materials (if they aren’t readily available) and constructing frames still costs you, especially when you consider that a 4′ x 40′ low tunnel can be installed in about 20 minutes after just one trip to the hardware store for PVC conduit, clear plastic sheeting, and rope.

 

Special Considerations

I’m certainly not one to bad-talk any method of winter growing that works for someone, so please understand — low tunnels simply work better for me, and I suspect they are an inexpensive and effective alternative for most of North America. Low tunnels are not as structurally sound under heavy snow load as cold frames, so if you regularly get heavy, wet snows, you’d need to stay on top of the storms and keep your low tunnels brushed off from time to time. Here in the mountains of eastern Kentucky, most of our snows register a few inches, with a few topping out each winter at 6-9 inches (although well over a foot of snow isn’t out of the question and happens periodically) — in the rare event we get significant snow, I go out with a broom and brush off the tunnels.

Tunnel failure is much more likely to occur if hoops are spaced greater than 5 feet apart, especially at tunnel ends (if you look closely at the picture for reason #4, you can see that the right end of the tunnel is beginning to collapse — a result of me trying to stretch the hoop spacing to 7 feet apart) . Also keep in mind that metal conduit will hold up better than white PVC conduit (usually found precut to 10′ lengths in home supply stores), which in turn is MUCH stronger than coiled black electrical conduit. DON’T try to save some pennies on the cheaper black electrical conduit and cut it to length — if it is flexible enough to withstand coiling, it will fail under even a light snow. So don’t be a cheapskate and risk tunnel failure — low tunnels are plenty inexpensive enough to construct without skimping on hoop materials or spacing.

 

Have you used both low tunnels and cold frames for winter growing? Share your experience in the comments.

 

Additional Winter Gardening Resources:

These books are in my personal library, and I highly recommend them (these affiliate links cost nothing extra to you, but generate advertising fees that help me support this blog):

 

What vegetables could you be growing this winter? Check out this extensive list: my Winter Vegetable Planting Guide. And for more winter gardening resources from me, including how to build an inexpensive low tunnel, click the winter gardening tab above.

 

Web-based Resources

Not sure about your plant hardiness zone? Find out what it is here: United States and Canada

42 Comments

  1. Jaime

    September 5, 2014 at 12:47 am

    Do you have to monitor water levels or water weekly with low tunnels through the winter?

    • Cathy

      September 5, 2014 at 5:23 am

      Once winter arrives and there is a need to add the clear plastic, I never have needed to water. Winter gardening authors Eliot Coleman and Nikki Jabbour also do not have to water in their climates (Maine and Nova Scotia, respectively). Great question!

  2. Rosemarie Thompson

    September 5, 2014 at 11:56 am

    Thanks for sharing your recommendations. I live in the South, so we have a long growing season. However, last winter we had two ice storms. I won’t tell you what that did to my garden! I don’t need heavy protection because we don’t get snow and rarely get ice storms. So, after reading your post, I believe low tunnels would be best for my garden. I have 21 raised beds, so choosing two or three for low tunnel growing would work well. I’ll take a look at your plans and hand them to my handsome “yard boy” (I call him my Garden Wilson). It’s way too hot to plant fall crops yet, but it’s not too early to get supplies for low tunnels. Thanks again!

    • Cathy

      September 5, 2014 at 1:54 pm

      Glad you found it helpful. Thank you for stopping by!

  3. Sue

    September 5, 2014 at 1:03 pm

    Thank you so much for this. I was going to have my husband build a cold frame. It would be very expensive. He will be thrilled not to have to build one.

    • Cathy

      September 5, 2014 at 1:52 pm

      You are most welcome!

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  6. Janet

    September 23, 2014 at 7:09 am

    Since I found your blogsite a few months ago, my husband has finally been convinced to try a low tunnel in our garden. We just finished building a 4×8 raised bed 11 inches tall and plan to buy some kind of row cover for it. Will anything be different for a raised bed instead of an in-ground row? I’m anxious and excited to watch the progress this fall and winter. I have a thermometer sensor to put in the bed that sends it’s info to the base unit in the house. Your successes are very inspiring!

    • Cathy

      September 24, 2014 at 8:14 pm

      Thank you, Janet. Plants in raised beds are more susceptible to freeze damage, simply because the soil is elevated and thus isn’t as “insulated” as in-ground plantings. With that said, you still have a marvelous chance at success. Even in last winter’s Polar Vortex, an elementary school that I work with here had few losses in their raised bed/low tunnel garden, as well as my local extension office. You are definitely on the right track by having a thermometer sensor in your tunnel — keeping things on the cool side will definitely increase your chances of success (so that your veggies maintain their adaptations for freezing weather). If you’re able, give me an update in the future — I’d love to hear how your garden does this winter. Best wishes!

      • Janet

        September 24, 2014 at 9:57 pm

        If the raised bed project goes as planned, I will be glad to let you know the results. I am in zone 6a, so we’ll see. Also for the record, you are a great inspiration to us. I have been gardening the traditional way (Dad’s way) for years and I am hopeful and excited about our ‘upgrade’. I guess you can teach an old dog a new trick!

  7. Sue Mosier

    October 3, 2014 at 7:07 pm

    Would you please tell me what you are using to hold up the row cover? It is red. My husband gave me a clamp used in wood working, but I could not squeeze it to get it open. I know, sad. Thank you.

    • Cathy

      October 3, 2014 at 7:54 pm

      I use plastic spring clamps with an easy-release trigger (you can find them at home supply stores equivalent to Home Depot or Lowe’s). Some of them can be harder to squeeze open than others, so I suggest going to your local hardware store or home supply store and trying different ones until you find one to your liking. Good luck!

      • Sue Mosier

        October 3, 2014 at 8:11 pm

        Thank you so much, Cathy.

    • Nancy

      November 11, 2014 at 8:45 pm

      You can get plastic clamps that snap on 1/2 ” PVC hoops. You can find them at lots of places online but these are the best price I’ve found – http://www.amazon.com/White-Snap-Clamp-Inch-Inches/dp/B00AIE89CY/ref=sr_1_3?ie=UTF8&qid=1415756297&sr=8-3&keywords=pvc+clamps

      I actually saw them in half in a small hand miter box so I have twice as many and I sand the corners so they don’t tear the plastic. They work great both to hold down the plastic and to cinch it up, although in high wind you still need to use crisscrossed rope.

      • Christine

        January 17, 2015 at 12:52 pm

        Thank you for the link , bought me 2 pkgs, just what i needed to complete my preparation for a floating row cover.

        Thank you Cathy for the title of your post , itgot my attention because i did not like theYooutube vids I saw on winter gardens with smooshed winter green frostbitten y very low glass covers, i ean what green will grow to 2 -3 inches , and not be frostbitten by the freezing glass? I dont want to harvest my greens and find slimmy nubs and slushy green for dinner like a deer! So your article told me i was on the right track, i hope to find your agribon fabric at our local Lowes or HD, you use that under plastic sheeting right?

        • Cathy

          January 18, 2015 at 2:45 pm

          In the past I have used Agribon under plastic sheeting, but this year all of my beds are just covered with Gro-Guard fabric row covers only (I double up covers when single-digit temps are forecasted). I’ve never seen Lowes or HD carry fabric row cover — I order mine from Deerfield Supplies. Good luck!

  8. Sue Mosier

    October 14, 2014 at 9:03 pm

    I need to buy plastic for the row cover. Does the greenhouse grade plastic allow more light through compared to the 6 mil? Do you have any recommendations where to buy it? Thank you.

  9. Nancy

    November 11, 2014 at 8:59 pm

    This is a good post Cathy, thanks. I started with three small cold frames using windows set over boxes and was so pleased to have a little bit of fresh lettuce during the winter that I tried two low tunnels last year. Mine also went through the polar vortex here in VA with no problem. This fall I have three 4′ X 10′ tunnels. I’ve abandoned the cold frames and instead have lashed those windows to the western end of the tunnels. Now when the wind really gets whipping it never blows the plastic in on the ends.

    • Cathy

      November 13, 2014 at 12:16 am

      Glad to hear that you prefer the tunnels over cold frames, too. And what a great way to still get some use out of those cold frame windows — way to grow!

  10. Famer Fengel

    November 17, 2014 at 12:19 pm

    Cathy,
    I am trying winter gardening for the first time. I have constructed a 4 by 10 ft. wood raised bed and attached 7ft. PVC hoops to the side every 30in. On the west end I cut a 1/2 in.2 by 4 ft. treated plywood and attached it with 2 electrical conduit clamps on the end hoop. I cut an opening in the plywood and attached an automatic foundation vent ($16) that opens at 70 and closes at 40 deg. I covered the hoops with 4 mil. plastic secured by chis- crossed parachute cord . Planted spinach and kale and am hoping for the best! 3 inches of snow fell last night.

    Fred Engelhardt in Ohio

    • Cathy

      November 22, 2014 at 6:14 am

      This system sounds great… if my tunnels were shorter in length, I would be trying it for sure. In fact, I just might have to make some shorter beds next year. 🙂 Thanks for sharing!

    • Nancy Sanders Farmer

      November 22, 2014 at 7:18 am

      Hi Fred,

      You have constructed the same set up I have.

      Last winter I tunneled 2 raised beds (same size as yours) and it worked great. I was home enough to be vigilant venting the plastic but when I made an early spring tunnel I also put an auto door in a plywood end. It worked beautifully. For that time of year I taped some mosquito netting around the inside of the opening and had cabbage worm free broccoli.

      This winter I have 3 tunnels and am just making the 3rd door this weekend. I am using greenhouse vent hinges which are more expensive ($39.) I hadn’t heard of foundation vents. I’ll look for them.

      You might also try some bottles of water in your tunnel to store some of that daytime heat and release it at night. I’ve lined the edges the beds with 1/2 gal milk bottles (HDPE) this year. You can also lay them on their sides in between rows.

      I’m still experimenting with night time covers, trying to find something that is easy to put on in wind and which sheds water and snow.

    • Christine

      January 17, 2015 at 1:05 pm

      Is the foundation vent easy to install? how about on a 2×4 post i can plant on the inside of the bed? How much is it?

  11. AJ Rawlek

    December 14, 2014 at 12:58 pm

    What do you know of walipi green houses that are used mainly in South America in high altitudes and cold climates? I’m in Calgary, Alberta and we can get to -40°F/C. I have heard that these can be used in far greater colder temps. Any thoughts would be great.

    • Cathy

      December 16, 2014 at 3:47 pm

      I’ve never used a walipi and have no experience with them. It sounds like you are in the same growing zone as a gardening friend of mine… have you visited http://www.northernhomestead.com?

  12. kaybecomingbeehive

    January 19, 2015 at 5:37 pm

    Hi Cathy! I’m thrilled to have Mother of a Hubbard…thank you for your wonderful work. Inspired but your winter gardening, I set up a hoop house and everything was going great until very recently. Either mice or voles have found my greens and, in just a few days, have eaten everything. Have you ever had a problem with critters eating your bounty? And, if so, what’s your solution. Kind regards…Kay

    • Cathy

      January 20, 2015 at 3:35 pm

      Ugh! How aggravating! I’ve never had problems with critters eating my winter crops. Eliot Coleman puts out box traps — essentially a wooden box with a hole for voles to enter, baited with a mousetrap inside. Many winter greens will grow back from the crown… hopefully you can catch the voles and get some regrowth. Good luck!

      • kaybecomingbeehive

        January 20, 2015 at 3:52 pm

        Cathy…thanks so much for getting back! You’re so lucky not to have had this problem. I like the idea of the box traps. I don’t really want to kill the pesky pests but I do want to be able to grow salads, veggies and the like for the family throughout the winter. Love your blog…thanks for the great effort you make to share your knowledge. All the best!

  13. Christine

    January 20, 2015 at 1:25 pm

    Now what? I’ve got my 2 4’x8′ low tunnel built, and warming the soil nicely. I’ve got my heirloom cold resistant seeds waiting on the counter. First i wanted to wintersow these in milk jugs until they sprout, then transplant them in the tunnels, but why not plant them directly in the tunnels, they are just like the milk jug except bigger, right?

    • Cathy

      January 20, 2015 at 3:39 pm

      You can absolutely winter sow in a tunnel! I do this frequently (about to sow some mache and other greens this week, actually), but you can also let the seeds get a head-start on germination. I germinate my spinach, lettuce, peas, etc indoors, and as soon as I see the seed coat breaking, sow them into the tunnel. This provides a major jump-start (since even under a tunnel, soil temperatures still stay cold)… just be careful not to let the seeds get so far along that you risk breaking off the little root that is first to emerge — the seed can’t recover from that.

  14. Jaime

    January 22, 2015 at 12:57 pm

    I tried the low tunnels, worked a lot better than a larger hoop (6 feet in the center covering 2 rows of raised beds) but i would invest in an entry side with am automated vent system and thermostat..i work very long shifts and lost a good portion of my plants due to wildly fluctuating spring temps in winter (50-60s)..wasn’t home to open sides up

    • Nancy

      January 23, 2015 at 3:33 pm

      You can use temperature sensitive greenhouse hinges if you construct some ends for your tunnels. I installed auto-open doors on the end of my 10′ raised beds by using these – see link below. I made plywood arches with narrow doors which open sideways. They self-regulate, opening at about 70 degrees and closing about 60. Work great.

      http://www.amazon.com/gp/product/B0036EJ9HW/ref=oh_aui_detailpage_o06_s00?ie=UTF8&psc=1

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  16. Cindi

    February 1, 2015 at 10:04 pm

    this is what I use on my cold frame. I am going to try low tunnels next winter and I think I will make wooden ends with one at each end.

    http://www.menards.com/main/p-2426224-c-13258.htm

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