Greenhouse/Cold Frame for a Square Foot Garden
I started a Square Foot Garden this year, and wanted to start the season early for a some frost-sensitive plants. I built a simple bed cover / greenhouse over one of my three 4' x 4' beds to see if it would allow me to plant about 4 weeks prior to our average last frost.
No promises on it working (I don't know myself yet) but it's an easy build if you want to try it yourself.
The framework of the cover is made of 2" and 3/4" PVC pipe plus assorted connectors. The cover is 6 mil clear plastic. The plastic on the sides is held down by 1/2" steel conduit. The four vertical legs sit on top of 18" steel anchor spikes (usually used to secure rebar in a concrete form.) The plastic is held to the frame with cable ties.
The dimensions will necessarily vary depending on the outside size of your beds. Mine are made of 2" x 6" rough cedar, so are 4', 1" - 3/4" wide.
I cut all the PVC on a 12" chop saw, but anything down to a hack saw will work. Conduit was cut with a pipe cutter.
The right-angle-with-takeoff PVC couplers can be tricky to find with all three ports at the right size. The prototype actually uses a 2" right angle, but with a threaded 3/4" takeoff. A thread to slip fit adaptor was used to attach the ribs. If you take this route, remember to subtract the size of the adaptors from two of the rib lengths.
All PVC joints are assembled with standard PVC primer and cement. Apply the primer to the inside of the fitting and the outside of the pipe where they will overlap. Repeat with the cement. Slide the two together while twisting approximately 1/4 turn. Hold steady for 30 seconds for the cement to partially set.
Start with a leg, a right angle connector (with takeoff) and a horizontal support. Cement the two pipes into the 2" sides of the fitting, leaving the takeoff empty. Repeat for the other end of this frame, making sure the two horizontal supports face each other, and that both takeoffs face up.
Cement the two horizontal supports into the 2" openings of a "T" fitting, leaving the 3/4" opening empty and facing up. Make sure the 3/4" pipe is perfectly vertical.
Repeat this process for the other side frame.
Pound a steel spike into the ground just outside your SFG bed on each corner. Use the completed side frames to make sure you get the spacing right. Set the two frames on the spikes with the empty 3/4" fittings facing each other.
Slide the thee 3/4" ribs into the empty connectors to tie both sides together. Do not use cement - you'll need to be able to take the frame apart once the cold weather is over.
Unroll the plastic and drape it evenly over the frame so that it hangs down to the ground plus a few inches. Take a sharp razor blade and run it down the four vertical legs to cut out a triangle at each corner. The plastic should now drape smoothly down all four sides. Trim off any excess that puddles up on the ground.
Take one of the half inch weights (conduit) and roll an inch or two of the hanging plastic around it. (Use tape to hold it temporarily.) Poke a hole just above the conduit so you can wrap a cable tie around it and the encircling plastic. Slide tight to hold the weight permanently to the plastic. Repeat on the three remaining sides.
Poke a small hole on either side of the horizontal supports near their centers and slide another cable tie around the pipe and the plastic to hold it to the frame. Repeat with at least three cable ties on each side of the frame.
Some of all of the sides can be opened by lifting the weighted edges up and onto the top of the frame. (I use a bungee cord to hold them down in case there's wind.) The frame should be left fully or partially open during the day. Otherwise you'll cook the plants. Close the sides in the evening to hold in the heat from the soil.
The frame is high enough to get in to water and tend to plants on a day to day basis, but can also be lifted off entirely and set aside if you need to do more extensive work.
Leave the frame in place till the danger of frost has passed. You may want to try leaving it on a little longer if you have heat-loving plants like tomatoes that will benefit from the higher soil temperature.
Place the frame upside down in an unused part of your property with some weights on top (bags of compost work well) if you're going to use the greenhouse in the fall to extend the season.
The frame can be folded flat(ish) for permanent storage. Remove the plastic by clipping all the cable ties. (The film will photo-degrade over the course of the summer, becoming brittle. Re-cover next year.) Take the frame off the spikes, and slip the 3/4" pipes out of their sockets. Bundle the ribs and weights together and store. Set the two end frames against a shed or garage wall. Pull up the stakes, or leave them in place.
The frame is high enough to allow most plants to grow for several months before they hit the top. Consider removing the plastic and replacing it with bird mesh to keep birds and rodents out. The same conduit weights can be used to hold it down, or you can cable tie the mesh directly to the legs to make a more rodent-proof seal. Make sure you're capable of lifting the entire frame off before you go that route. Don't allow the plants to grow through the mesh or you'll be stuck.
Comments? Let me know.