I have often considered constructing a greenhouse at my urban homestead, and have created numerous low tunnel and high tunnel systems for growing later into the fall and earlier in the spring. The question for me has been, what kind of a greenhouse do I want? What size should it be and what materials should I make it out of. Do I invest in a prefabricated aluminum and glass Elizabethan structure, or is a rough and ready DIY set up the most efficient and economical one?
The key to greenhouse success, for me, has been adopting a system that allows me to take an incremental approach to design and implementation. Instead of going all in on a $5,000 commercial greenhouse right at the beginning, I found it better to start with a simple high tunnel greenhouse for a relatively modest investment. But this will allow me to answer questions about where to locate it for optimal placement, what size it should be, and how I can best use the space.
Then, after I test it out and answer a few questions, I can begin to improve the existing structure. Finally, I can confidently make a more substantial investment, knowing that I'm getting exactly what I need.
YEAR ONE
Step one. Identify the location for the greenhouse. This will give you a sense of what size you can accommodate. Consider 8x10, 10x12, 10x16. Go out into your yard and physically outline the working space you are talking about and see which one actually makes sense.
Step two. Using concrete blocks outline a foundation for your greenhouse at the size you selected. Level your blocks from corner to corner, digging down as needed and checking with a level to make sure that the foundation is perfectly level. Leave an empty space in the front for the doorway. Doors are typically 30" - 36".
Step three. Spend some time to level the floor inside the foundation perimeter. You don't want to create a pit that will trap water, so make sure that it can drain away. At the same time, you want to level the floor so that it is flat. Set the concrete blocks aside, and then cover the entire area with industrial landscape fabric that blocks light and resists tearing. Replace the concrete blocks into their former location, resting directly on the landscape fabric and keeping it stretched tight.
Step four. Use pressure treated 2x6, and lay them flat along the top of the concrete blocks, creating a wooden surface. Drill holes through them and drive rebar through the 2x6s and into the ground. These should be 2 feet on center. The rebar should be 36" or longer so that the rebar sticks out the top of the concrete block by about 12". This will form the spikes for the pvc walls.
Step five. Using 3/4" pvc electrical conduit, join two 10' pieces together to form a large hoop. Place one end of the pipe over the rebar on one side and bend it over to insert the other end over the rebar spike on the other side. The spikes and the pvc ribs should be 24" apart. I use pvc fittings at the very top where the two 10' lengths join and then space the joints apart with connecting lengths of pvc.
Step six. Using 2x4s, frame up the ends of the greenhouse to add structural support. Create an opening for a 32" storm door at one end, but it can be covered by plastic to start. At the hip intersection, extend a 1x3 or 1x4 board to keep even spacing between the ribs.
Step seven. Create a double ridge pole from 2x4s, set 32" apart (based on the width you left for the storm door. Secure the hoop ribs to these ridge poles. Use mounting brackets where each pipe meets the ridge pole, two per hoop. Using furring strips, attach an 8' diagonal brace reaching from the hip down to the 2x6 base, to keep the hoops perpendicular.
Step eight. Use plastic sheeting to cover the end panels. Cover the door end of the greenhouse separately from the sheet covering the hoops.
Then, use a sheet of 10x25' 6 mil plastic sheeting to cover the entire greenhouse, from the back to just meet the front door panel. Using 1" pvc pipe to hold the plastic sheeting in place at the base. Attach a handle at one end so that the sides can be raised by rolling them up on the pipe.
Step nine. Use plastic folding tables and wire framed shelving inside the greenhouse to create a working space. This will get you started for a relatively modest expense and the components will be waterproof and resilient. Then, as you get a better idea of what you want, you can upgrade them to the cedar potting bench of your dreams.
Step ten. Consider adding a shade cloth over at least one end, during the summer when temperatures can be hot inside the greenhouse. Also consider your options for lighting, if you want to use the greenhouse as a workspace after dark.
YEAR TWO
After having used this temporary greenhouse for an entire year, you should have a better answer to several questions.
This will give you a clear idea of the size of greenhouse you need. Were your chosen dimensions adequate or should you really consider going a size larger? Do you need more space to move around inside to work? Was there room for shelves and work surfaces, seed starting and potted trees overwintering? Everything that we have done so far has been with an eye to minimal investment and an impermanent installation that can be taken apart and resized.
Each of the components can be made more secure, more sturdy, more permanent.
For example replacing the sheeting on the ends with polycarbonate panels. You can upgrade this to use clear polycarbonate panels, creating a more solid end.
Upgrading to UV stabilized greenhouse film. You can exchange your draped plastic door for one you construct on hinges, or install a regular storm door in the end with a handle that latches.
Consider the floor of the greenhouse and covering the landscape fabric with washed gravel.
YEAR THREE
When this plastic begins to break down in 2-3 years, consider upgrading to greenhouse film
By year three, about the time the plastic sheeting wears out, you should consider replacing the pvc hoops with a framed roof covered with clear corrugated panels.
Alternatively, you can consider buying a commercially constructed polycarbonate and aluminum greenhouse from Costco or Harbor Freight and installing it on top of your concrete block foundation.
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