How to build a high tunnel to protect your plants This video shows how to build a greenhouse over your plants so that you can extend the growing season into the winter. Take some rebar and put it in the ground around your plants. Then take electrical conduit bent into half circles and put it on the rebar to form supports for the greenhouse plastic. Take some 3mm greenhouse plastic and cover the arches that you created. Use a zippered piece of plastic on the front of the arches so that you can easily get inside the greenhouses that you have built. To secure the greenhouse plastic roll up the bottoms using pieces of wood and then put bags of dirt or sand on them to hold the plastic down tight to the ground. Also put bags on the inside to secure it further. You can plant all year long inside of these low cost greenhouses.