How to Build Portable Greenhouse Tops for Raised Garden Beds

Building a portable greenhouse lets the home gardener extend the outdoor growing season by starting early and growing later into the fall. Using a portable greenhouse with raised beds further extends the season. The soil in raised beds warms up sooner than soil at ground level and the portable greenhouse raises the air temperature above the bed. The size of your portable greenhouse will depend on the size of your raised bed. A variety of materials can be used to make the structure. PVC tubing is a good lightweight choice that makes putting up, taking down and moving the greenhouse easy. Heavy-duty clear plastic film or heavy-weight polypropylene fabric row cover draped over the PVC frame provides the thermal protection. The covering can simply be unclipped and peeled back to allow air movement during warmer days.

Things You'll Need

• Hacksaw
• Tape measure
• Eight ¾ inch 90 degree side outlet PVC fittings
• ¾ PVC pipe
• 1 inch PVC pipe
• 6 mil plastic or polypropylene row cover
• Wide flat workplace

Instructions

Preparation and Purchasing Materials

1. Check the mature height of the plants that will be in your raised bed. Corn and other tall plants will require taller greenhouse uprights than lettuce, carrots or sprawling plants such as potatoes.

2. Measure the inside dimensions of the raised bed. The finished portable greenhouse frame will be inside the edges of your bed.

3. Purchase enough ¾ inch PVC pipe to provide four lengths of PVC the same length as the inside dimension of the bed's end, four lengths of PVC the same length as the inside dimension of the bed's side, and four uprights of the height needed to clear your mature plants.

4. Buy eight ¾ inch 3-way L snap fittings. Regular PVC fittings can be used but snap fittings are easier to tap together and take apart.

5. Use your side and end measurements to determine how many clips you need. Clips will be pressed on every 12 inches along the bottom of your portable structure, except at the corners. For example a 6 foot run would require five clips.

6. Buy 1-inch PVC pipe to cut in half to make 3-inch clips. A 6 foot x 4 foot bed would need 16 3-inch clips. Twelve inches of 1-inch PVC pipe will provide eight clips.

7. Calculate how wide a piece of 6 mil plastic or heavy-weight polypropylene row cover you need. The cover must reach from the bottom of one side of the portable structure up over the top and down to the bottom of the other side, plus 1 foot.

8. Calculate how long a piece you need. The ends of the plastic will be folded in. You will need a piece long enough to cover the frame and leave a piece at each end the same length as your end is wide. Standard sizes for poly film or row cover are 6 foot wide by 25, 50 or 100 feet long.

Assemble and Cover the Frame

1. Insert the bottom frame pieces into four of the side outlet fittings. Put the uprights into the outlet fittings at each corner.

2. Put the remaining four outlet fittings on the uprights. Insert the top frame pieces into the side outlet fittings.

3. Tighten the joints by pushing and twisting. Your portable greenhouse will be easier to disassemble.

4. Drape the plastic film or row cover over your frame, letting it drop to either side. Lift one side, roll the covering material up around the bottom of the frame to the inside and push on your clips. Repeat on the other side.

5. Secure the ends to the PVC pipe frame by folding them in like the edges of wrapping paper on a package.

6. Lift one end and roll the covering up and around the bottom of the PVC frame to the inside. Secure the plastic by pushing on your clips. Repeat at the other end.

Tips & Warnings

• Drape plastic film or row cover over the frame and weigh it down with bricks for a quick cover without the fuss of making clips.

• Online sources for PVC furniture building carry a wide selection of snap fittings, clamps, and clamp grips that make putting PVC frames together easy.

• Pipe stakes or poly tube stakes can be driven into wood raised bed edges to secure the portable cover against wind.

• Very long runs of PVC pipe will require the use of tee fittings in the middle and extra uprights for support.

References

• University of Missouri Extension: Building and Using Hotbeds and Coldframes
• New Mexico State University: Hoop House Construction
• Virginia Cooperative Extension: Season Extenders

Resources

• Simplified Building: PVC Arched Greenhouse
• University of Tennessee Extension: Detailed Portable Greenhouse Plans
• Colorado State Extension: Frost Protection