Easy Low Tunnels Extend Your Season

Salad, as fresh as it gets!

Here in USDA Zone 6a today, November 22, I am enjoying a fresh salad from my own garden.  A month from now, closer to Christmas, I will still be picking fresh greens from my garden and saving about $6.00 a week by not purchasing bagged salad greens from the grocery store.  All thanks to a very simple and easy method known as low tunnels, quick hoops or floating row covers.

I built a cold frame one year to grow lettuce throughout the winter.  It worked well, but it took a lot of effort to build, including digging the pit to heat the soil bed passively with manure.  It required a lot of attention to monitor the daytime temperatures and prop open the lid on very hot days and also a great deal of mental fortitude to remember to go back out and close it before bedtime.  A particularly strong wind grabbed my plexiglass lid one day and slammed it up and broke it.  I duct-taped it back together and went on.  Really though, the main issue I had with the cold frame was that it did not give me much room to grow enough things to make it worth my while.

Then I read about low tunnels.  This is the second year I have used them and I am very happy with this set up.  It is mid-November and I am enjoying the tasiest salad greens and radishes; vegetables that are typically grown within a very narrow season.  The spinach I've been able to grow with this method is the sweetest, crunchiest spinach I have ever tasted - so good I can't help but eat it straight from the garden, plain.


Spinach thriving under the protection of a low tunnel.

Given how very easy low tunnels are to construct and how cheaply you can obtain the necessary materials, I don't know why more gardeners aren't using them.

Any of your cold hardy plants can be overwintered in them, such as:

• spinach
• lettuce
• kale
• swiss chard
• radishes
• beets
• carrots
• bok choy
• turnips
• some herbs

Sweet, crispy radishes harvested in late November.

They can also be used to protect an everbearing strawberry bed so you can squeeze a little more fall harvest time out out of them.  You can start most transplants sooner for a jump on the season, such as tomatoes, peppers, squash and melons.  You can start sooner or extend later a crop of broccoli, cabbage or cauliflower while simulaneously protecting them organically from cabbage loopers (by blocking out the moths so they can't lay their eggs.)  Seeds for peas, carrots, beets and also onion sets can be sown in late fall and covered with a low tunnel to urge them to germinate and grow faster and earlier than they are traditionally planted in the spring, giving you a 1-2 month headstart.  There are some subtle variations in the growing methods for all of these, but the basic set up is the same.

Last year was more or less an experiment for me and all that I grew was lettuce.  I planted my lettuce in a regular bed in October and allowed it to germinate and begin growing while the daytime temperatures were still rather warm.  Once the time came to anticipate my first frost, I set up the low tunnel.  My simple low tunnel kept my lettuce safe throughout the entire snowy winter, one of the coldest winters my area had seen in years.

Once spring arrived, I took the cover off and was enjoying fresh lettuce long before any of my regularly planted rows of spring lettuce were large enough to harvest.  Growth enhancement is the biggest benefit of using a low tunnel.

Lettuce growing outside of the low tunnel is puny compared to the lettuce growing within.

The lettuce will only grow when the temperature inside the cover is warmed up enough by the sun.  In the deepest part of winter it is held in stasis, but resumes growing just as soon as the days become more mild.

This year I put more thought into it.  In October, I tilled a long row on the perimeter of my garden and sowed spinach, two varieties of loose leaf lettuce, radishes and turnips.  They germinated in the last 70+ degree days of Indian summer, then I implemented my low tunnels.

I use 5' sections of 3/4" PVC pipe to create the hoops.  I take 18" sections of rebar (narrow enough that the PVC can fit over) and drive them a foot into the ground opposite each other on either side of a narrow row about 3' apart.  I set the rebar in this manner at 5' intervals all along the row.

Then you push one end of the PVC pipe over the stub of rebar and arch it across to the corresponding rebar on the other side and push it down over that rebar stub as well.  Repeat the process all the way down the row. 

Be sure not to forget about your rebar when you go to till next spring.  Remove them all for the sake of your tiller's tines.

Another method of setting the arches is to use 10' sections of PVC pipe and use a spud bar to drive holes into the soil about a foot deep, then push the ends securely into the pre-set holes.

Some people will use 10' sections of 1/2" EMT (galvanized metal electrical conduit) which is also very cheaply obtained from your local hardware store.  It requires a jig and some bending to form the arches, but since it is more rigid, it is easily pushed down into the soil without any need for rebar anchors.  Here is a video demonstrating that method:



I used the 3/4" PVC because I already had a lot of it on hand.  Any of these will work well and all can be salvaged and reused year after year.

This series of arches is then simply covered with 4 or 6 mil clear plastic sheeting and weighted down with rocks along the sides of the tunnel and closing the ends.  A 10' x 25' roll of plastic sheeting will run you about $20.  The plastic will also hold up to several seasons of use.



Rocks weigh down the plastic and keep it in place even through strong winter winds.

Depending on what you are growing and how hot you expect it may get inside of the tunnels, you may need to take a razor knife and cut a series of short vertical slits along the length of the plastic for air circulation.  You don't want the temperature inside the tunnel to exceed 90 degrees.  For a tunnel that is used to overwinter, you would want to leave the plastic intact and just open the ends on unseasonally hot days.  If you were only using your tunnels to protect your brassicas from cabbage loopers, then a fine mesh, lightweight fabric (sold by specialty garden suppliers) would be the way to go.  The fabric will protect from frost, but not hard winter freezes.

One side of the cover can be lifted up to harvest your veggies.  On very cold days, it's best to do this no later than early afternoon.  Pick a mild day to harvest if you can.  You don't want to give away any heat gained from the sun before it goes down for the night.


Lettuce ready for harvesting.

Black plastic can be used for mulching and will also help warm the soil.  That would be especially useful when starting warm weather crops early.

Today I thinned out my spinach and lettuce, leaving about 6" between those that remained so they can get full and leafy.  I also found some chickweed and young dandelion greens to add to my salad.  I had to work quickly and get the cover back on - the chickens think this spinach rocks too!