10 Reasons Why Low Tunnels Beat Cold Frames for Winter Gardening

By  |  42 Comments

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