How Low Can You Go?

February is such a tempting time of the year.  It's still cold, but spring feels like it’s just around the corner, and we are anxious to get things rolling.  One of the big things that we want to do to increase how much food we grow in our backyard is to lengthen the growing season.  If you simply rely on Mother Nature, the frost-free date in central Iowa isn’t until May 10^th.  We aren’t willing to wait that long, so we’re taking matters into our own hands.

Last year we built a cold frame and we plan to use it again this year.  This season we’ve decided to also try a low tunnel.  A low tunnel is essentially the same thing as a cold frame, just a different shape, and the end goal is the same.  It’s a small structure that uses the greenhouse effect to warm the soil and protect plants from the cooler outdoor temperatures.

Lots of places sell low tunnel kits, but we thought it looked simple enough to just build one ourselves.  We even found this great example.  We picked up a few supplies, and built this handy low tunnel in just a couple of hours.

To anchor the low tunnel, we used 6-inch long pieces of ¾-inch diameter PVC fastened to our raised beds.  We drilled a couple of holes through one sidewall of each piece so that we could fit the head of a screw through it.  Then we simply screwed these anchors to both sides of our raised beds spaced about 2 feet apart.

The basic structure of the low tunnel is arched pieces of ½-inch diameter CPVC (chlorinated polyvinyl chloride) pipe.  We looked at regular ½-inch PVC first, but the CPVC was more ductile and only a little more expensive.  It would be easier to bend the CPVC into a hoop shape without cracking it, so we decided it was worth a little extra cost.  The CPVC was sold in ten-foot long sections, but that made the low tunnel a little too tall.  After cutting each piece down to nine feet, the height looked much better.

The ends of the CPVC hoops slide right into the PVC anchors.  Next we added a “spine” of ¾-inch PVC across the tops of the hoops to help make the low tunnel a little more rigid.  The spine is fastened to the hoops with just a couple of plastic cable ties.

The last step was to drape a sheet of 6 mil plastic sheeting over the PVC “skeleton.”  The sheet of plastic should be large enough to reach all the way to the ground on all four sides of the low tunnel.  We’re holding the sheet of plastic to the ground with some landscaping blocks and railroad ties that we had lying around the yard.  This traps the heat inside the low tunnel and stops the wind from blowing our plastic sheeting away.

One really nice thing about this design is that we can always add anchors to our other raised beds so that we can move the low tunnel to any garden box we choose.  This low tunnel should be an easy way to start our veggies earlier in the spring, and keep growing things later in the fall.  It’s a simple and inexpensive way to stretch the Iowa growing season and produce even more fresh food!