Constructing Low Tunnels to Protect Spring Crops

June 8, 2015

Last week’s post discussed how we use high tunnels on our farm to produce early spring crops.  This proves to be a very productive way to protect spring plantings from the elements and get a jump start on the growing season.  However, most home gardeners do not have the space to construct a high tunnel in their backyard.  Don’t worry, you can use low tunnels to achieve the same results with a fraction of the cost.  When we began farming, we did not have the infrastructure in place that you see today.   I did not know the first thing about using a high tunnel and viewed them as an expensive investment that I did not really need.  Today we have 20,000 square feet of high tunnels, but we still use our old low tunnel techniques.  When starting our farm, low tunnels were the most cost effective way to protect spring crops and we constructed three quarters of an acre of them throughout our field.  If I was still a home gardener, I would use this technique exclusively due to how inexpensive it is.

There are many positive aspects to a low tunnel.  But let’s discuss what they consist of first.  You will need to buy 3/4in PVC pipes, rebar, cinderblocks (or anything heavy to hold down the covers), Covertan row covers (19 weight), and greenhouse plastic.  You purchase the supplies based on the size of the space you are covering.  Here on the farm we use this technique in March when we plant in the field.


Most home gardeners wait until April to plant their broccoli, but we plant Broccoli starting on mid-March out in the field.  Our first step is to prepare the soil at the end of February.  If there is still so covering the ground, we will construct our tunnels over the beds and cover the PVC pipes with the plastic in mid-February.  Our beds are 3 feet wide and 50 feet long.  We can cover 2 beds with each piece of plastic.  We start by hammering in the rebar every 4ft down the edges of the two beds (look at the picture for help).  We take the PVC and slide it over the rebar, the 10 foot PVC makes a semi-circle over two beds. We then cover the PVC with the plastic and weight it down with cinder blocks.  We place the cinder blocks every 4 feet so that the wind cannot blow the plastic off of the bed.  The snow underneath will melt in about 10 days.  When the snow is gone, we take the plastic off of the hoops and unhook the PVC from one side allowing it to swing back and forth on one piece of rebar.  We then continue to prepare the soil for planting.  When the beds are ready we hook the PVC on the rebar and recover the bed with plastic and wait another 10 days.  In 10 to 12 days, the weeds will begin to germinate.  We then use a torch and burn the weeds prior to planting.  We then plant our broccoli in two rows that are positioned on the outside of the beds.  In the picture you can see the 4 rows of broccoli planted in the two beds.  But we do not waste the space in the middle of the Broccoli plants.  We plant two rows of radishes down the middle of the broccoli plants.  The radishes will be harvested before the broccoli completely shades them out.



After planting there are a couple of important steps.  First, we cover the bed with Covertan 19.  The 19 coincides with the strength of the cover.  This is a fabric cover that allows water and sunlight through it, but can protect plants from freezing down to 28 degrees.  This will be the first layer of protection.  Then we place the plastic directly on top of the Covertan.  These materials can cover the plants, but should not be touching the leaves of the plants.  These materials will also warm the growing area just like a high tunnel does.


There are a couple of things to watch out for.  To do this correctly you have to monitor temperatures under the covers.  The double layer of protection has protected our plants down to as low as 18 degrees.  However, you have to be careful not to kill the plants by overheating them.  There are a couple general rules of thumb we have learned through trial and error over the years.  If the day time temperature will not reach 60 degrees, you can leave the plastic completely held down on all ends and sides.  If the daytime temperature is about 55 degrees and it is a sunny day, the temperature under the cover may get as high as 85 degrees.  If the temperature reaches 60 degrees, but it is cloudy, you can leave the sides of the plastic held down.  If the daytime temperature is anywhere over 60 degrees and it is sunny you must detach the ends of your tunnel and roll back the plastic so that the air can flow through keeping the tunnel from reaching temperatures that are too high.  You do not have to roll back the fabric, just the plastic.  If the overnight lows are hovering around 32 or high you will be fully protected with just the fabric, but if they are dipping below 32 seal the tunnel up with the plastic.  Throughout March, you will find that you are adjusting your plastic quite often and this is normal because of the early spring temperature fluctuations.  Once you get to April, you are usually safe to take the plastic off completely and just use the fabric as protection.  I always keep the plastic next to the bed, because inevitably you will get a cold snap in April and need the extra layer of protection. 


The beauty of this system is in its simplicity, and cost effectiveness.  The materials are relatively cheap and can be used for many years.  The fabric also doubles as protection from insects as well.  One of the most interesting parts of this system is how it can aid in plant growth.  Researchers have found that the daytime high is not the temperature that a plant responds to.  Rather it is the average growing temperature of the entire day.  For example, you may have a daytime high of 60 degrees and an overnight low of 30 degrees.  The broccoli plant does not grow better do to the daytime high of 60 degrees.  Instead the plant responds to the average.  In this case, the average temperature is 45 degrees.  However, under the cover the high temperature on a 60 degree day may be 85 and the low may be 33.  Now the average temperature is 59 degrees.  The covers have now increased your average 14 degrees higher causing the broccoli to grow more rapidly. 


We still use these systems outside of our greenhouses do to their many benefits.  High tunnels are easier to manage because we are not fiddling with the plastic every day, but for a home gardener who wants to create one cover to manage, this is definitely the way to go!



This week's share consists of:

Full Share- broccoli, cauliflower, zucchini, cabbage, lettuce mix, escarole, carrots, beets, celery and green kale

Half Share- broccoli, cauliflower, zucchini, cabbage, lettuce mix and escarole

What can I make with my share this week?? Here are a couple of ideas!


Caluiflower Poppers- This recipe was shared with me by friends at Healthy Alternatives (thanks girls!!) and I have to say it's a keeper. You may be unfamiliar and usure about the nutritional yeast but it's definitely worth a stop to your local health foods store to pick it up. This sauce is yummy on any roasted or grilled veggie including cauliflower, broccoli, hakurei turnips and zucchini.


Roasted Cabbage Wedges- Many cabbage recipes call for finely shredding a head of cabbage which can be time consuming and messy. This recipe is so fast and easy and has a few spice suggestions included to give your wedges some zip if you're so inclined!

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