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The C2X4P Greenhouse.
A simple do it yourself “lean-to” greenhouse.

By Doug Sweet
Central Ohio Cactus and Succulent Society

           Six years ago I moved from Michigan to Ohio due to a change in my career path.  This created a dilemma; the house I moved into is much smaller than my Michigan house.  My cactus and succulent collection and hobby had grown considerably over the years.  Forty years ago it consisted of about two dozen species squeezed onto several windowsills.  When I purchased my first Michigan home, in Detroit, I splurged and built a 12 ft by 14 foot sun-porch, with a southern exposure and floor to ceiling single paned glass, specifically for the plants.  This porch accommodated an ever burgeoning collection and enabled grow out of a large number of cacti from seed.  Something I never succeeded at while my plants were subjected to the low light levels behind double paned insulated windows.  I ended up moving again, from Detroit proper, to another larger house in the suburbs of Detroit.  This time I made sure I purchased a house that already had a spacious sun-porch.  Unfortunately this sun-porch, although spacious, was hung with double paned insulated windows so again, my cacti and succulents had to endure winters of relatively low light levels as they cowered in the reduced light intensity behind double insulated windows.  This necessitated I move all my plants outdoors again in the spring and then back indoors again in the fall.  This shock set the plants back a bit as they readjusted to higher light levels living outdoors again.  I did not have to move plants outdoors and back indoors in the brightly lit, single paned sun-porch of my previous residence.  So I began to appreciate having an appropriate built greenhouse/sun-porch.  That is one that allows plenty of light transmission to keep the plants happy and healthy both in the winter and summer….year round.  
           Then my life was upended and I ended up moving to work in Ohio.  Fortunately I was able to move gradually.  My wife and kids stayed behind in Michigan to finish off the school year, try to sell my Michigan house (which I still own), and tie up loose ends.  This process was over a year in the making.  In the meantime, one of my loose ends was what to do with all my plants?  There was no way they would fit on the windowsills of my Ohio house!  I could invest in a bunch of fluorescent light fixtures but my wife and I had already gone that route once before many years ago.  (We were both into growing African violets, Streptocarpus, Episcias and other Gesneriads under indoor lights for many years.)  I really didn’t want the added electricity expense of running 16 or more banks of lights.  What could I do?
           I ended up selling the vast majority of my plants.  I had huge mounds of gorgeous Mammillarias, large clumps of various Aloes, Haworthias, and Gasterias   Many of my own Echinopsis hybrids and assorted clumps of Rebutias and Sulcorebutias.  I was able to sell a fair number to a local, privately owned, nursery and garden center.  As long as I busted up the clumps and potted each plant neatly into the nursery’s specified pots with top dressing and labels, I was able to get a bit of cash for many but there were still too many left over.  Fortunately I found a cactus hobbyist in southern Michigan who also sells semi-commercially out of his large roadside greenhouses on his spacious property.  He offered to take quite a few plants as well.  My heart was breaking to bust up clumps of plants I had grown for 15 or 20 years.  I tried to keep one pup from each of my favorite species to grow on into massive clumps again someday.  It had to be done; I had no room in my new Ohio residence. What was I going to do?
           The answer came unexpectedly.  As many Central Ohio Cactus and Succulent Society members know, I’m also an avid tropical fish enthusiast.  In the basement of my suburban Detroit home I had built a “large” basement fish room consisting of 42 twenty gallon fish tanks on cinder block and 2X4 racks.  I had the same dilemma with the fish room.  I had to sell off virtually all the fish as I prepared to move to Ohio.  The entire fish room would have to be dismantled and removed before I could even begin to think about selling the house.  It is pretty hard to show and sell a house when the basement is wall to wall fish tanks!  So in the process of dismantling the fish room I had a stack of 2” X 4” lumber and cinder blocks that needed to be disposed of……   Bing!  The light goes on in my head!

           Build a greenhouse out of the lumber and cinder blocks from the fish room!  I was pretty cash strapped at the time of the move so everything made perfect sense.  I would limit the size of the greenhouse to the lumber and blocks I had available.  My Ohio house has a long wall with a southern exposure.  In the southeast corner of the house is a basement door that opens outdoors into a stairwell leading up and out of the basement.  The plan came together quickly.  This basement door would be access to the greenhouse and a source for heat.   The stairwell would be the service area to gain access to the plants.  I planned the foundation to be cinder blocks lined up end to end outlining the floor of the greenhouse.  Several bags of quick-crete were purchased to fill in the cinder blocks.  (But the cinder blocks are not cemented to each other, only filled in and lined up end to end as tight as possible.)  Every cinder block has a short, 30 inch tall, two by four riser or stud equally spaced apart (on 18 inch centers) cemented in one or the other hole of the cinder block.  This riser supports a two by four rafter that then runs back to the house’s aluminum siding at a 40 degree angle.  The sides of the greenhouse have higher risers originating from the cinder blocks to appropriate heights that match the slope of the roof.  The rafters are also tied together with 2 X 4 pieces and to the handrail that protects the stairwell opening. 

         Ground was prepared for the cinder blocks, sod was removed and soil leveled and compacted.  Some of the cinder blocks actually rest on the concrete pad that served as a small porch at the top of the stairwell.
           This entire “skeleton” or “frame” was screwed together with deck screws.  So basically the entire greenhouse can be disassembled if need be (in case I move again) or when and if I can upgrade to a better greenhouse.  Each cinder block with its riser can be picked up and moved once screws are removed from the other wood supports.  The 2X4s were also painted white to match the house’s aluminum siding and to reflect light.  
           The complete floor dimensions of this greenhouse are 7 feet. X 18 feet giving about 126 square feed of growing area.  This square footage was increased some by several benches (from our old African violet growing system) that enabled me to stack some plants on a top rack and keep a fair number underneath the benches at least in the winter.  
          The entire skeleton or frame was covered with two layers of greenhouse 6 mil poly-film.  I purchased several rolls of two different types of white fabric tape (almost like seat belt material) that was used to fasten down the poly-film over the frame.  The poly-film was laid over the frame, and this tape was lined up on the rafters to hold the poly-film in place.  Short deck screws and washers were used to cinch down the tape and the poly-film in place.  The poly-film was cut large enough so several feet extra was available all around the greenhouse.  The extra length of poly-film was then buried under several inches of dirt all around the greenhouse.  Thus effectively holding it down in place and preventing it from blowing up in high winds.  
       Initially my plans were to have two layers of poly-film so a blower could be used to inflate the outer layer away from the second layer and form a nice insulating layer.  I abandoned this plan in order to have the poly-film better attached to the rafters.  So the ceiling is still double layered for strength but not separated by an insulating gap.  So, I lose some heat but I feel the extra fasteners and double layer have held the plastic in place better keeping it from blowing, flexing and eventually weakening.  This poly-film top has weathered many rainstorms, ice, snow, hail, etc. over the last six years.  I was able to separate the poly-film on the walls by stretching and fastening in different areas to create an insulating space.   So the vertical walls do have some insulting effect.  
           I installed a small computer fan with miniature louvers, activated by a thermostat control, in the apex portion of the roof.  This small fan does a good job keeping the greenhouse from overheating during the spring and fall.  During the summer an additional box fan is needed to keep the greenhouse within acceptable temperature range.  (This fan runs pretty much all the time anyway.  Ventilation, ventilation, and more ventilation, basically moving air is essential to keeping cacti and succulents healthy.  This keeps fungal spores from germinating in damp and dank recesses on their epidermis).  I have also opened the ends of the greenhouse during some summers to allow cross ventilation and adequate cooling.  This is easily accomplished by removing the poly-film attachment screws from one or two support 2X4s on the ends and peeling open the two layers of film.
           Heat in the wintertime is simply furnished by leaving the basement door open with a  blowing air into the greenhouse.  I do have a small Polonis electric furnace that can be plugged in to add extra heat during the absolute coldest days of winter.  (I’ve rarely had to use this.)  This greenhouse does run a bit cold.  During the coldest nights of winter the temperature can drop down to about 38 degrees F.  During most of winter the temperature is 40 to 45 degrees F at night and into the low 50s during the day.   Since this greenhouse does run cold, some plants do not do well.  I’ve lost all my Sanseverias, they don’t like cold roots.  The leaves wouldn’t die, just the roots.  So I would re-root the leaves each spring after the roots died, but the same thing would happen the next winter.  This eventually led to their demise.  Some asclepiads don’t like it this cold either.  Most Aloes and Haworthias seem to tolerate it, as long as I keep them in the central area of the greenhouse and not close to the walls of poly-film.  I have lost Haworthias and Kalanchoes positioned nearest the furthest corner away from the door opening (presumably the coldest spot).  Most South American cacti do fine including Echinopsis, Lobivia, Rebutia, Trichocereus, Parodia and Notocactus.  Any cacti which may be slightly temperature sensitive, like some Mammillarias and other Mexican species I tend to keep closer to the door just to be sure they don’t experience any frost.  

           Plants in this greenhouse flourish.  I don’t think I will ever go back to a glass or plexi-glass greenhouse ever again!  The thin poly-film seems to let in much more light than glazing.  Right away I noticed plants growing better, more compact, deeper darker coloration, better spines, and increased flowers!
           The grade of poly-film I purchased was rated for four years of use.  The greenhouse is now about six years old and the poly-film is just now starting to show signs of fatigue. I will probably need to replace it this summer.  As the film ages its transparency seems to drop, it gets a bit cloudy either due to U.V. degradation or perhaps microbial growth (algae, fungi or bacteria) growing on it.  It also becomes a bit more brittle.  Various things apparently must fall from the sky too.  Small perforations are showing up in a few places.  Not sure if sticks fall from nearby trees, birds drop stuff, perhaps small icicles falling from the eaves troughs in winter, or small stones flung up from passing cars on the nearby road, but you definitely start to get perforations.  These perforations can be temporarily patched with transparent repair tape you purchase for just this purpose when you buy the poly-film.  
           An added bonus is this greenhouse acts as a solar heater during the spring and fall.  If the air circulating box fan is positioned to blow through the greenhouse it moves a tremendous amount of heat from the greenhouse right into my basement during sunny days.  Unfortunately this added heat is offset during the middle of winter when my basement is obviously losing considerable heat to the greenhouse.
           I can list some other disadvantages.  First, I made the green house low to save wood and construction cost, and so that one sheet of poly-film would cover the ceiling, walls and out onto the ground without seams.  Thus being so low the furthest west portion of the greenhouse, away from the stairwell, can only be accessed by crawling on your hands and knees.  I’ve positioned plants that don’t need to be moved around much (a huge golem jade tree) in this corner.  Still, it is a bit awkward to move plants at times due to the low ceiling.  It is not bad near the stairwell where you have working room to stand.  Only in the area away from the stairwell this is a problem. It would be nice to have a second doorway outside but then this would take away plant growing area.
           Trees near to the greenhouse have grown progressively larger over the last six years.  So during the summer the greenhouse is shaded to an extent.  This has necessitated I put many plants outdoors on a well lit deck.  In a way this is a disadvantage but it also is a benefit too.  Since the trees are leafed out and partially shade the greenhouse, the last two summers I did not need to open the ends to add ventilation.  Plants that need a little less light can remain in the greenhouse, which doesn’t get too hot or too illuminated for the shade tolerant species.  Once leaves drop from the trees in the fall the greenhouse is again fully illuminated just about the time plants come back inside for the winter.  
           The total cost for this greenhouse was about $120.00 to $200.00 (since most of the wood was salvaged from my fish room.) That covered the poly-film, quick-crete, deck screws, and fabric tape.  I did have to buy just a couple more two by fours to complete the project because I didn’t have enough from the fish room salvage.  I did not have to purchase the cinder blocks either because these were salvaged from my fish room.  If you had to buy all the material to build a similar sized greenhouse I would expect the cost should only be about $400.00 to $500.00.  I don’t recall what the computer fan cost and I already had some thermostats that I wired to turn the ceiling fan on and off.  No matter, this was a very inexpensive greenhouse that served me well over the last six years.
           One caveat to a greenhouse constructed this way.  This would not be up to code for a bona-fide addition to a house.  Code typically calls for a foundation with rat-walls etc for any structure.  Depending on your local code you might get by with something like this if it was termed a temporary and not a permanent structure.  This is one reason why I made mine to completely disassemble.  It could be termed a temporary structure.  You might also be able to get by with such a structure in areas zoned agriculture.  Since all kinds of animal pens, sheds, etc. are allowed.  In the city would be a different matter.  Then again, it would not be too difficult to actually pour a concrete foundation with rat walls and add a frame structure so that it might be up to code.  If you didn’t attach something like this to a house, and it was positioned in a garden, it could simply be termed a cold frame, and might be perfectly acceptable as long as your neighbors didn’t mind.  That is always a concern, especially in neighborhoods with homeowners associations or other “standards” of conduct for what is in your yard.  Check your local building codes first (and homeowner associations, etc.) before committing to a project like this.  
           Things I might do different if I built another greenhouse like this?  Build it high enough so one can walk around in it better.  Then it would need larger sheets of plastic or more creative ways to seam two pieces together.  Install a larger ventilation fan on the ceiling too.  The small one does a pretty good job but it could be better on really intense sunny spring days.  I would probably also box in and louver a fan(s) on a side wall(s).
           Finally, why is it called a C2X4P greenhouse?  First it sounds kind of neat!  Second, the C stands for cinder block, 2 X 4 is the lumber used, and P stands for poly-film.  There you have it.  If a greenhouse seems out of your reach because of budget reasons, build a C2X4P greenhouse.  Mine has been fantastic!  Only build yours a bit bigger…..  No sooner did I build mine that I had enough seedlings started to overfill it!

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