How to Expand a Greenhouse

You can extend a polytunnel by adding more ribs and plastic.

One of the simplest, most inexpensive greenhouses that amateur gardeners can build is a polytunnel. A polytunnel, which is also known as a hoop house or a high tunnel, is created by stretching polyvinyl greenhouse plastic over a series of plastic arches. One advantage that a polytunnel offers over a traditional greenhouse is that it can be disassembled seasonally, easily moved or expanded should you need to add space for greenhouse production. Expanding a polytunnel is as simple as adding more arches and plastic to the tunnel frame.

Things You'll Need

• Screw driver
• Utility knife
• Wire cutters
• Rubber mallet
• Wooden stakes
• String
• Measuring tape
• 3 lengths of PVC conduit pipe measuring 24 feet in length
• 6 rebar pieces measuring 24 inches long
• Galvanized wire
• T post driver
• Plastic zip ties
• Construction staples
• Construction staple gun
• Polyvinyl greenhouse sheeting
• Polyvinyl greenhouse repair tape
• 1x2 plywood battens

Instructions

1. Using a screw driver, carefully pull up the staples that bind your polyvinyl greenhouse sheeting to the sides of your greenhouse. Remove the sheeting and fold it up for reattachment later. Remove the wire from the sides of the greenhouse using wire cutters and discard it.

2. Cut through the plastic zip ties that anchor one door end of your greenhouse to the T-shaped fencing posts that hold it in place. Carefully pull the end piece off of the rebar stakes that anchor it into the ground and set it aside for later use. Extra care must be taken in this step not to puncture the polyvinyl that covers the end of the greenhouse.

3. Work the T-shaped fence posts out of the ground by wiggling them back-and-forth until they are loose. Set them aside for later use.

4. Measure the distance between the arches of your green house. Multiply this distance by the number of new arches you will be adding (for the purpose of this article, we are adding three ribs, so we will multiply by 3) to get the length of your addition. Add this length to the end of each wall, and drive a stake into the ground at the new wall's end on each side.

5. Tie a string between the wooden stake and the rebar stake at the former end of the greenhouse to act as a guide. Space the rebar stakes equally along this new length and drive them two-thirds of the way into the ground. Each stake should be directly across from its counterpart on the wall at the other side of the greenhouse.

6. Place a piece of PVC conduit pipe over each rebar stake on one side of the greenhouse. Flex the pipes over into arch shapes and place the other ends over the rebar stakes on the other side of the greenhouse. Pull the wooden stakes out of the ground and cut the string.

7. Measure the length from the door posts to the edges of the arch in the door end of the greenhouse. Use this measurement as a guideline to drive the T-shaped fence posts into the ground near the door posts at the new end of the greenhouse.

8. Lift the door end onto the rebar at the new endpoint. Affix the door's posts to the T-shaped fence posts with zip ties.

9. Stretch galvanized wire from one end of the greenhouse frame to the other over the top of the arch and the sides of the greenhouse walls, wrapping the wire around each arch when you reach it. This will improve the stability of the the greenhouse. Then Stretch wire from the top corner of the doors down to the lower corner of the arch over the top of the greenhouse frame to make large X shapes. Staple this wire to the wooden reinforcements in the ends with the staple gun and staples.

10. Lay out your polyvinyl greenhouse plastic. Cut a second sheet of greenhouse plastic that is long enough to cover the new section of the greenhouse using a utility knife. Align the two pieces so that their ends touch.

11. Cover the end of one piece of greenhouse plastic with two strips of the double-sided polyvinyl greenhouse tape. Peel off the backing for the other side of the double-sided sticky greenhouse tape. Carefully pull the end of the second piece of plastic over the tape so that the two pieces of greenhouse plastic overlap. Smooth out any bubbles in the tape.

12. Drape the greenhouse plastic over the greenhouse. Pull the ends taut, and staple the plastic to 1x2 plywood battens to weight the plastic down.

References

• Extension.org: Construction of High Tunnels: Resources for Organic Farmers
• Washington State University: Poor Man's Greenhouse A Simple, Plastic-Covered Hoop House
• The Door Garden: Building Greenhouse Doors

Resources

• The Door Garden: How to Build my 50 Dollar Greenhouse