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Extend Your Growing Season With A Low Tunnel

Grow your garden by weeks or even months by using a low tunnel for your crops. They're inexpensive, easy-to-build season extenders.

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by Kristi CookMay 19, 2020
PHOTO: Trevor Clark/Shutterstock

Do you get excited about getting a head start on your spring and summer garden? Or are you lamenting that last year’s extra-bountiful fall harvest went by so quickly?

Rid yourself of that anxiety and regret by creating low tunnels to start spring sooner and finish fall later. They require few materials and are easy to construct.

As an added bonus, low tunnels require only minimal management.

Why Low Tunnels?

Protecting your plants from the cold is the most basic use of a low tunnel.

Cool-season crops such as lettuces, spinach, radishes and kale that are started indoors in January or February can be transplanted under the protection of a low tunnel several weeks sooner than when gardening without freeze protection. In my zone 7 garden in Arkansas, this means I can have homegrown salads an entire month earlier than most.

On the flip side of the gardening season, these same crops can be started indoors during the hot and humid days of July and August and transplanted outside in September. Low tunnels keep these delicate crops from freezing well into December, with some years going as far as into February.

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And there’s nothing better than a fresh-from-the-garden salad during the family’s Valentine’s dinner! It’s a win-win no matter which end of the gardening season you want to extend.

However, low tunnels aren’t just for cool-season crops. Perhaps the most exciting use of tunnels is the extension of the summer garden.

Every year big-box stores offer winter-weary gardeners tomato, pepper and other heat-loving transplants weeks before optimal planting weather. And year after year, eager beavers buy up an entire season’s worth of transplants only to lose them within a week or two. (Unexpected cold snaps or cold, drenching rains are inevitable parts of spring.)

Yet again, low tunnels come to the rescue. They protect summer crops from short bursts of cold, as well as the soil from chilling rains, particularly when used in raised beds.

The combined effect of warmer internal temperatures and elevated soil levels spare summer crops from the terrors of spring weather, putting summer crop extension within the average gardener’s grasp without the use of expensive greenhouses.

low tunnel
photowind/Shutterstock

Low-Tunnel Mechanics

While low tunnels do offer summer extension by protecting heat-loving plants through the last days of unpredictable spring weather, the goal of low tunnels is not to create summer-like conditions. Instead, low tunnels limit freeze/thaw cycles by collecting solar heat, blocking chilling winds and avoiding soaking rains.

Temperatures inside a low tunnel often stay 10 to 30 degrees Fahrenheit above outside temperatures
(a little less on cloudy days), depending on the materials used.

So if your transplants need a minimum of 60 degrees, don’t expect those transplants to thrive when outside temperatures are in the low 20s for an extended period of time. That’s when you need a different method of cold protection.

However, for crops that can handle cool weather for longer periods of time, keep them in tiptop shape when temperatures drop into the low teens or single digits by adding an extra layer of insulation to avoid frozen plants.


Looking for a new cool-weather crop to grow? Check out kohlrabi!


Materials Selection

Essentially, a low tunnel is nothing more than sheets of translucent plastic sheeting draped over the tops of semicircular frames.

Most frames have their ends inserted into the ground anywhere from 3 inches to a foot depending on how stable the overall low tunnel needs to be. This is highly dependent on wind intensity and frequency of heavy rains.

However, it’s possible to build frames with a base that allows entire sections of the low tunnel to be lifted off of plantings during harvesting much like a lid.

As for height requirements, the lower the better. The lower height allows for heat to be maintained near the soil, thus keeping plantings healthy and thriving during extra cold weather.

It’s best to determine each crop’s maximum height and allow for an extra few inches when purchasing frame materials so leaves don’t touch the coverings, which can lead to frost burn during freezing temperatures.


Where to Buy

All materials should be readily available at your local hardware store. You’ll need translucent plastic sheeting for the cover and PVC pipe, rebar, conduit or 9-gauge wire to build the support frame.

Hardware stores usually carry rolls of plastic sheeting ranging in length from 10 to 100 feet with widths from 8 to 20 feet. Thicknesses run anywhere from 2 to 3 mils up to around 10 to 15 mils. However, thicknesses up to at least 100 mil are available online and at specialty supply stores.

One key to note when deciding on a thickness is how much light transfers through the plastic to allow for photosynthesis. The thicker the plastic, the less light that enters the low tunnels. Also, be sure to add an average of 5 extra feet of plastic for each end of the tunnel to allow for tightening and tucking of plastic to keep out wind and rain and keep in heat.


Find the right tunnel for your farm’s needs.


Get Creative!

As for the selection of frame materials, creativity goes a long way. There is no right or wrong answer. Select a material you enjoy working with and that is within your budget.

Rebar and conduit are excellent choices and offer significant stability during high winds and heavy snow loads. However, both must be curved into the half-circle shape to allow for draping of the sheeting. Keep that in mind when considering construction and budget.

PVC pipe is often the material of choice. It’s inexpensive, relatively easy to bend (provided it’s warm outside) and virtually indestructible. However, the downside to PVC is concern over chemical leakage into surrounding soil unless using food-grade piping. Plus, it can be impossible to bend in cold weather and doesn’t hold up as well during high winds or heavy snow/rain.

Similarly, 9-gauge wire is inexpensive, very easy to work with and easy to store. (It takes up little room when not in use.)

However, of all the materials available, wire doesn’t do well during long periods of high winds or heavy rains. It tends to bend down towards the soil, often crushing plants. So again, experiment with what’s available and see which material works best in your environment.

Construction

Site selection is relatively easy. Most gardeners use their summer gardening spots due to ease of access and workable soil.

However, keep in mind that low tunnels may need a windbreak if long periods of wind are standard for your garden area.

Also, low tunnels need as much sunshine on them as possible during the day to build up sufficient heat to get through the colder nights. Avoid shaded areas as much as possible.

Another consideration is rainfall and puddling. Avoid areas that become saturated during rainy seasons so crops don’t become waterlogged.

Once you’ve selected a site, work the soil and add any amendments that will be needed during the time the tunnel is in use. Working the soil once the tunnel is in place can be difficult.

Next, set frames in place with an average of 3 feet between each support. In areas with little to no wind or heavy rains, longer distances may be possible. Areas with heavy winds and rains will need to be spaced closer to 2 feet apart.

low tunnel
ESOlex/Shutterstock

Just Add Plastic

Once frames are in place, you can set transplants out, keeping plants a minimum of 3 to 5 inches from the edges of the frames to keep leaves out of reach of the plastic sheeting.

Next, add the plastic sheeting on a windless day. If the tunnel is very long, have a second person help to prevent dropping the plastic on the plantings.

I’ve found it easiest to stretch the plastic out the full length of the tunnel with each person holding the sheeting in a snug bundle at the ends. Then, in unison, we gently drop one side while carrying the entire length over the tops of the frames and dropping that side down to the ground.

I temporarily secure edges to the ground with smooth rocks or sandbags while making adjustments.

Finally, gather the ends tightly and either make a bundle on the ground at the end or secure with a rubber band or twine like a ponytail. Follow the entire length and width of the low tunnel, and place sandbags or even grocery bags filled with dirt along the edges to keep wind from blowing the cover over and allowing heat to escape.

Set up is now complete.

Management

Heat management will consume most of your time when dealing with low tunnels. You’ll need to closely monitor outside temperatures on a daily basis. When temperatures rise close to 30 degrees, raise covers to prevent overheating crops such as lettuce and spinach.

Experience is the best teacher in this area. Plastic thickness, intensity of the sun, specific crop requirements and even day length all play crucial roles in when and how much covers need to be lifted.

Some days, you may need to open the entire cover. Other days may only require ventilation on the sides or ends.

A simple trick to determine your needs is to quickly stick your hand into the tunnel without lifting the cover to feel the inside temperature. Thermometers are also helpful and require only a quick glance.

Toward the end of the day, close the tunnel before temperatures drop to allow time for the tunnel to collect and store heat for the night.

Low tunnels are inexpensive and simple solutions to adding several weeks to either end of the gardening season. With just a bit of time and minimal materials, you can have fresh, homegrown salads for Christmas dinner or extra early tomatoes and squash before your neighbor’s garden is in full swing.

Just remember to be creative, and enjoy the journey!


Sidebar: Extra Insulation

When faced with weather extreme, extra insulation for your low tunnels may be needed temporarily. In my garden, for example,
I tend to use sheets with a 3-mil thickness for the very end of spring while I prefer 6 mil for extra-early gardening or gardening into winter.

To temporarily increase insulation, I add more layers of sheeting as needed and remove them once the harsh weather passes by. Some years, I’ve even added bed sheets when I ran out of plastic. Again, creativity goes a long way with low tunnels, so use what you have in an emergency.

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