I've mentioned Eliot Coleman and Barbara Damrosch many times before. They are the organic farming gurus on a farm in Maine where they grow and harvest an unbelievable assortment of veggies year round in Zone 5. He talks alot about the use of cold frames, low tunnels, and hoop houses in his book Four-Season Harvest. I've owned this book for a few years and have slowly started to implement many of his proven techniques.
A cold frame is a single sheet of glass or plastic that covers plants. It provides warmer temps day and night thanks to the sun, reduces or eliminates the harmful freeze/thaw cycle of winter, and negates the effects of cold winter winds that remove ambient heat and evaporate moisture. I don't use one currently, but it is a very simple and effective tool to extend your growing season. But one drawback to the cold frame is the cost, size, and weight of materials. Another easy and more affordable option is a low tunnel. This consists of wire or plastic arches draped with plastic to create a mini-greenhouse. Most low tunnels are usually 1-2 feet tall. The edges of the plastic are usually buried to keep it from blowing off in the wind. This works well but makes checking on crops and harvesting a real chore. A high tunnel is an unheated greenhouse large enough to walk into and perform gardening activities. It has the same benefits as cold frames and low tunnels, but is much larger and can be somewhat costly and is impractical for the backyard grower.
I ended up building two structures slightly larger than a low tunnel but smaller than a high tunnel. I guess you can call them mid tunnels. Here's a picture.
I took four 10' pieces of 1/2" PVC pipe and inserted each end into the ground against the inside edge of my raised beds. I then bent the PVC over the bed and buried the other end directly across from itself. I took a fifth 10' length of pipe, shortened it to 8', and then laid it over top of the arched pipes like a backbone. I used plastic zip-ties to attach it firmly to the other pipes. The total height of each tunnel is appx. 36".
I draped 4 mil plastic sheeting over the top of each frame and attached it to the backbone PVC pipe with 1/2" plastic clips from Johnny's Selected Seeds. They are a very tight fit, almost too tight. I first tried clothes pins but couldn't find any large enough to grab the plastic. But I'm sure any type of clip would work fine. I didn't use any more clips along the curved PVC pipes because the clips are very difficult to remove and I didn't want to rip the plastic.
I also hung a small thermometer from a piece of string inside of one tunnel, about 18" from the ground. Last night temps were slightly below freezing and the wheat straw on the ground was covered in frost. I walked outside around noon today and it was sunny and 47F. I could see lots of condensation on the inside of the plastic even though it was cool to the touch. As a result, I was skeptical how warm it could really be in there. I was pleasantly surprised when I pulled up the plastic and was hit with warm, moist air. The thermometer also surprised me. Check it out. Right around 80F.
This is good and bad news. Good for a few reasons: The temp is warm enough to keep existing mature vegetables growing strong. It is also warm enough to germinate a wide variety of hardy crops even in January. Bad for one main reason: If I fail to vent the plastic on an unseasonably warm and sunny day, I risk "cooking" what's under tunnel.
I opted to merely weigh down the edges of the plastic with bricks rather than burying it beneath the soil so I can easily vent the plastic and plant/cultivate/harvest with minimal trouble. I also decided to place the two structures over the beds that contained the least hardy crops. My lettuce and chard were eventually beaten down by the consistently cold and frozen temps so they got the tunnels. I've read that if chard still has fairly protected crowns it can bounce back in warmer weather. So I'm essentially experimenting to see if I can bring them back to life in January and February as opposed to waiting for April or May. My lettuce is definitely completely dead. So I'm using the tunnel to raise temps for germination of new lettuce, arugula, and radish seeds. I've got my fingers crossed.
Here's one last look at my two tunnels and three uncovered raised beds. The front right bed contains my carrots. I've been harvesting carrots all winter even though the greens look rough and have been through many freeze/thaw cycles. The middle bed on the right is loaded with kale that has held up strongly all winter. And the back right bed contains my garlic. I covered it with wood planks because my dogs decided it was fun to run through. They haven't touched it since I put the wood there. I'll remove the wood when growth resumes in the spring.
This setup and these tunnels are very simple to build. It took me appx. 15 minutes to construct each tunnel. Materials costs were fairly low and totaled about $15 per bed. These tunnels are new to me so I'm not sure what to expect as the winter moves along and spring eventually shows up. Hopefully old things will grow back and new seeds will germinate. Check back when you can and I'll make sure to update the blog.