A couple of years ago I spent several weeks researching ways to extend our gardening season into the winter months. I planted a greenhouse like this a couple of winters ago, and harvested enough for a couple of family sized salads a week – starting in January! I didn’t plant mine until November 15 – a week after our first frost. (I figured this would be an ‘acid test’!) I plan to plant three this year, starting them 2 to 3 weeks apart and starting the first one uncovered in October.
6 cheap PVC pipes, 1 inch outside diameter by 10 feet; clear plastic, 10 by 20 feet; bricks to hold the plastic down along the edges; enough plastic bottles with caps to solidly circle the perimeter of the bed on three sides; flat black spray paint; scrap wire.
Clear a planting bed approximately 4-5 feet wide by 10 feet long. Situate the bed so that the long sides run east and west. Dig in compost and cultivate deeply. Rake level. Do not mulch. Jamming the ends of the PVC pipes at l east 8 to 10 inches into the ground, erect a ‘Hoophouse’ structure over the garden bed, using all but one hoop. Take the remaining hoop and use it to form a ‘ridgepole’ by wiring it to the underside of the top of the other hoops to make the frame more rigid. Clip and curl the ends of the wires used in constructing the hoop house, so the sharp ends don’t puncture the clear plastic.
Prepare your plastic bottles by rinsing them out, and filling them 3/4 full of water. If you are using clear 2 or 3 liter soda bottles, tint the water inside the bottles dark with ink, or non-toxic black water based paint watercol or, poster paint, etc.). If you are using opaque bottles (laundry detergent bottles, etc.) spray paint the outside of the bottles flat black. Set the bottles along the west, east and north sides of your planting bed. Bury them to 1/4 of their height to stablize them and keep them from falling on your plants.Cut the clear plastic long so that you can pile bricks, rocks, heavy boards (whatever!) and soil on the west, east and north sides of the structure to seal the plastic. Leave the south side long and select something to hold it down when you aren’t venti lating the greenhouse or working with the planting bed.
Plant cold-tolerant crops – hardy lettuces, radishes, turn ips, bunching onions, chinese cabbages, kale, collards, beets, chard. If the weather is warm, water the bed more and lift the plastic on the south side. If the weather takes a cold turn, water the bed less (to prevent fungus and rot) and ventilate only a little or not at all. On warmer days, lift the plastic on the south side for a few minutes to freshen the air inside, then replace. A wind block of hay bales on the north side will moderate cold winter temperatures, and snow should be allowed to accum ulate on the north side for the same reason – insulation. Only remove snow on the north side if the weight becomes too much for the hoop structure. Keep the south side cleared of snow to admit light and warmth.
Practice your normal good gardening techniques in the greenhouse, removing dead leaves and planting succession plants to fill in blank spots. Watch for insect infestations, as this area could become a tropical vacation spot for y our garden pests! (I had a small explosion of cutworms under my mini-greenhouse.) Handpicking will usually keep them under control.
Everything but the clear plastic is re-useable for the next year. If the winter is really severe, two hoop houses can be built, one inside the other, and each covered with plastic to make a double layer of plastic with an insulating space of air between.