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.