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Building a Temporary Greenhouse

By Michael Greenstein

With many of us in the SF Bay area growing tropical or subtropical Bonsai, I was motivated to design and build a temporary green house for over wintering these type of trees. For me, I wanted to grow Bougainvillea as bonsai, but these trees were marginally hardy in the SF Bay Area, and a couple of deep freezes would kill the Bonsai trees. I use the green house when the temperatures drop to 45°F degrees overnight – that’s usually from about November to April.

So here is a description of how to build a low cost, temporary green house to overwinter subtropical bonsai. Let’s look first at what the finished greenhouse will look like.

Finished Greenhouse

Finished Greenhouse

There are three elements to the greenhouse:

  1. A heated bottom plate (plywood)
  2. Frame made from 3/4-inch PVC, available in the plumbing department of Home Depot, etc
  3. Polyethylene sheeting walls

Begin by deciding how big your greenhouse needs to be to hold your cold-sensitive trees. The easiest way is to gather all of your trees together and see how small an area you use. The bottom of the greenhouse will be plywood – which can be bought in a variety of pre-cut sizes at Home Depot, Lowes, etc. 

The frame supports the plastic sheeting that keeps the warm air inside. Your frame should be a few inches taller than your tallest plant. Don’t make it unnecessarily large because you’ll be heating a space that isn’t needed. Make a note of the size because you’ll be buying piping and fittings in a bit.

Buying the Pieces

The heat is provided by “soil warming cables” that you’ll mount to the plywood sheet. I use ones that are rated 4Watt with the thermostat set at 72F. You can buy these cables from Home Depot or other sources. You’ll also need some Metal Insulated Cable Staples to hold the cable to the plywood. Find these in the electrical department.

The “Gro-Quick” soil warming cables are available at Home Depot
The “Gro-Quick” soil warming cables are available at Home Depot
Metal Insulated Cable Staple (Image courtesy of Lowes)
Metal Insulated Cable Staple (Image courtesy of Lowes)

The frame consists of a top and bottom rectangle of pipe and four vertical pipes to connect the two rectangles.

Frame with board in place
Frame with board in place

As you can see, the frame is a slightly larger than the plywood base. That makes it easy to sit the frame on the ground – and use all the wood base to hold plants.

Take a look at the four corners of the bottom frame. Each corner uses a PVC fitting like this –

Three-way elbow fitting - slip, not threaded
Three-way elbow fitting – slip, not threaded

To keep assembly easy we’ll just push the plastic pipe into these fittings. Glue isn’t needed. So make sure you buy “slip” fittings – that is without threads inside. You’ll need eight of these fittings.

PVC pipe comes in two “weights” – light weight for underground sprinklers or heavier duty pipe. For this project buy the heavier duty because the thicker walls of the pipe stays more rigid and and will be less likely to bend when you’re assembling / disassembling your greenhouse. Also PVC pipe is cheap so buying the heavier version will only cost a couple of extra dollars.

Normally PVC pipe comes in 10-foot lengths, but Orchard Hardware sells it in shorter lengths – and will even cut to your specification. If you decided to cut the pip yourself you’ll need a hacksaw or a pipe cutter tool – very cheap. Ask someone in the plumbing department for help.

For the bottom and top frames you’ll need four lengths of pipe that are slightly wider than the piece of plywood and four lengths that are slightly longer than the other edge of the plywood. To connect the top and bottom frame you’ll need two 

If you are using a 4×4 sheet of plywood you’d want these sizes of pipe:

  • 8 pieces 52″ long (width of the rectangle)
  • 4 pieces 18-36″ long , depending on how tall you want your green house to be

Assembling the Pieces

Layout the soil-warming cable on the plywood. Then nail the uninsulated cable staples into the plywood. Leave them loose enough that the wire can be moved around a bit if needed – and avoid hitting the cable with your hammer.

Cable stapled to the plywood
Cable stapled to the plywood

Now cut the PVC pipe if you didn’t buy pre-cut. Assemble the bottom frame rectangle by pushing the cut pipes into the corner fittings. The corner fittings should be aligned so that all of them have an opening pointing up. The pipes that connect the bottom and top rectangle will go into this opening. When you make the top rectangle it will be identical – and then you’ll turn it over.

Bottom frame corner
Bottom frame corner

Now push the remaining 4 pipes into the corners of the bottom rectangle. Then place the top rectangle on top of these pipes and push down.

Installing Your Greenhouse

Place the greenhouse where it can get some sun and near an outlet or run an extension cord. 

Here is a view looking inside with the poly sheet draped over the top. I use one long piece of sheeting to go up one side, over the top and down the other side. I tuck the sheeting under the frame to secure it. I use a second sheet to go from the back to the top to the front. This last sheet I use as a front door and I secure it with a couple of bricks to hold it in place. In the following photo you also see that I put a layer of pea gravel on top of the board to act as a thermal reservoir for the heater cables.

Finished Greenhouse
Finished Greenhouse

Using the Greenhouse

If the weather is warm I open up the plastic. But when the temperatures get below 45°F degrees overnight I close up the plastic and turn on the warming cable(s). They’re thermostatically controlled and won’t over-heat the greenhouse.

I also place a tray of water in the green house to keep up the humidity. It is important to check the green house at least once a week to make sure nothing has dried out after watering and that no mold or fungus is growing and aphids haven’t invaded.

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