I want to use root-zone heating to grow cold-tolerant crops under floating row covers in my hoop house this winter. Is 16 inches too deep to run the tubing at 7,000-foot elevation?
Answer: Sixteen inches may be a bit deep for your situation as it may not provide effective root zone heating. It would be more effective to move the system closer to the root zone at 12 inches. You could also install a thermal break (insulation) under the radiant tubing under each growing bed, and insulation around the perimeter of the tunnel. It may be possible to not use any energy inputs by doubling the layers of plastic on the tunnel with an air space between the layers. With the double plastic and plastic row covers inside the tunnel over each bed, you should be able to grow the right variety of crops over the winter with no energy inputs.As a rough estimate, based on 750 Btu/hour/liner foot of radiant tubing, you need 270,000 Btu/day. Assuming an average of five sun hours a day in your location and a 24-square-foot collector, you need 11 panels and a 400-gallon solar storage tank. Again, these figures are for optimum production. You could start with fewer panels and a smaller tank and still get some benefit. The key will be to get the tubing close enough to the root zone to transfer heat to that area. If you could install a double layer of plastic with air blown between the layers, and use plastic instead of geotextile row cover cloths, you might be surprised at how well the right varieties of crops would do with no extra energy inputs.For more information on season extension techniques for cold climates, see the ATTRA publication Specialty Crops for Cold Climates, available at https://attra.ncat.org/attra-pub/summaries/summary.php?pub=369.