Greenhouses were initially made of glass. Today, only Bill Gates buys a glass greenhouse.
Greenhouses are sometimes made of double-walled polycarbonate, a clear, rigid plastic that has excellent light-transmission properties and lasts a long time. (The service life is said to be twenty years.) Smaller polycarbonate greenhouses are available, offering the advantages of structural rigidity to resist snow load, and not having to be replaced every few years, like sheet plastic. However, it is quite expensive, the polycarbonate for just one end of my 20×48-ft greenhouse cost $500, and making a large greenhouse with it might require mortgaging your house.
Most homesteaders who want a greenhouse of any real size opt for sheet plastic over a metal frame. Since reading an article in Orion Magazine titled “Polymers Are Forever”, I shudder each time I am responsible for bringing another piece of plastic into the world, since it is not biodegradable, and thus is likely to remain somewhere on the planet forever. These days, I look for every opportunity to substitute a more sustainable alternative to plastic. However, at present, there really is no practical alternative for covering a large greenhouse.
I strongly recommend a proper greenhouse plastic: 6-mil polyethylene sheeting that has been treated to resist ultraviolet breakdown is best, and is readily available from any greenhouse supply. The version I use is guaranteed for four years, and in most cases can easily go five full seasons. However, it’s a good idea not to push your luck and go for that sixth year, even if the cover still seems sound: Like the Wonderful One-Hoss Shay, once it starts to give way it all falls apart at once. If that happens in the middle of winter, you kiss your green beauties goodbye.
I prefer to put two layers of plastic on the greenhouse, with a small, energy-efficient blower inflating the space between. The dead-air space between the two “skins” increases the insulating value of the cover, it can make a difference of several degrees. An inflated “bubble” also sheds snow more readily, and better resists “chatter” in the wind, resulting in better wear of the cover.
Most non-professionals would do best starting with a greenhouse kit, containing all the components for assembling the completed greenhouse. I chose a Paul Boers kit (manufactured in Canada, and distributed to me via Penn State Seed Company in Pennsylvania). The style is “gothic”, that is, the arches come to a rounded peak up top, better for shedding snow, and I paid more upfront to get 1-½-inch galvanized steel pipe (rather than 1-inch) for added strength. If you are in an area with any snow in winter at all (or with frequent winds), make that additional investment.
In his excellent Four-Season Harvest (highly recommended), Eliot Coleman has a design for a modest (12×20) greenhouse that has no foundation, the edges of the plastic sheeting are simply covered with earth. However, most larger greenhouses should have a foundation. When I initially installed my greenhouse, I installed a wooden foundation to attach the “channel lock” (into which the plastic cover is secured). Since I will not use pressure-treated wood on my place, especially in close proximity to food crops, I used 2×8 pine boards, with several coats of linseed oil. Bad idea – don’t repeat my mistake. Any wood in contact with the ground (even pressure treated) will eventually rot out – my less than adequately protected boards did so in five years, at which point the screws on the channel lock began popping out and the plastic cover started looking for the wind.
I have not explored options for composite boards made of recycled plastic, they might be an option. My solution was to install (in the Fall of 2005) a single course of 4-inch hollow concrete block on a small poured footer, then lock a better grade fir 2×4, sealed against moisture, onto the top of the block foundation, using J-bolts pushed into wet concrete filling the holes in the block where needed. The channel lock is still screwed into wood, but it is not wood that is ever in contact with earth or rain. With repeat applications of sealant as needed, it should last as long as I do. Of course, if it ever does require replacing, doing so would be much simpler this time around, I would simply remove the nuts from the J-bolts, take off the boards, and replace them.
My advice to anyone thinking about installing a greenhouse: The bigger, the better. The larger the greenhouse, the more thermal mass inside in the form of soil that is warmed by the sun during the day, and the greater the moderation of overnight temperatures inside the greenhouse. Another reason to go with the largest greenhouse you can afford and have space for is that you will find more and more things to do with your greenhouse. For example, if your greenhouse has excess space beyond what you need for your own winter-greens needs, you can grow green forage crops to cut for your poultry, goats, or rabbits. You will never regret having made that additional stretch for a larger greenhouse, you will likely regret the reverse.
Be aware that most greenhouse kits are made up in stock sizes. Thus buying a larger stock size may make more sense than ordering a smaller custom size. For example, my initial idea was to erect a 20×32-ft greenhouse. I found, however, that by buying the 20×48 stock size, I picked up an additional 320 square feet of growing space (a 50 percent gain) for a mere $100.
If you don’t remember anything else in this article, remember this: If you live in an area where you get snow in winter, brace the hell out of your greenhouse! When we play in snow, it is so light and fluffy, it’s hard to imagine the load it puts on a big structure as it accumulates. A wet, clinging snow has been the death of many a fine greenhouse.
I know whereof I speak: In February 2003, a snowstorm predicted to grace us with two or three inches dumped fourteen instead. I went out the next morning to see my beloved greenhouse crushed, inch-and-a-half steel pipes on the ground like spaghetti. Have you ever seen a grown man cry?
Since that sad experience, I put up 2×4 vertical braces from the earliest possible date for snow in late fall until the latest possible snow date in early spring. The top ends of the braces are drilled to engage the bolts at the gothic peak of the arches, supporting the structure as it takes on snow load. We typically have modest amounts of snow, so I place braces under every other arch. If you get a lot of snow, it might be good to brace under every arch.
Key Planning Questions
Should you erect the structure yourself or hire out installation?
If your skill set includes squaring a foundation and drilling metal to receive screws, putting together a modern greenhouse kit is not difficult, and you will save a lot of money. I spent about $4000 on my greenhouse, and close to half that amount was what I paid to the crew who put it up. On the other hand, you may prefer to pay to have the job done better than you anticipate you can do yourself. For example, the last time I put on plastic sheeting I did the job myself. The result is certainly adequate, but I did not get as tight a “bubble” as when contracting that chore out to someone more experienced.
Do you plan to grow in summer?
Will yours be a cold-season greenhouse only, or do you plan to grow year-round? Since I have plenty of garden space for warm-weather crops, I use my greenhouse for vegetable production in the colder parts of the year only. I found that two solar-powered exhaust fans I installed initially were superfluous, and I bartered them to a friend. If you plan to grow crops in summer, you will need heavy-duty exhaust fans. A greenhouse design incorporating roll-up sides would also be advisable.
As said, I do not grow vegetable crops in my greenhouse in the summer. There are certainly advantages to leaving the greenhouse fallow over the summer: The soil “solarizes” in the intense heat of the sun, burning off potential pathogens and desiccating even the most die-hard slug. However, last year it occurred to me that I wasn’t doing anything in the summer to improve the soil in the greenhouse, equivalent to my practice of cover cropping whenever and wherever I can in the garden. So I grew a cover crop in the greenhouse over the summer. I found that such a strategy requires a lot of water, but makes a big difference in soil quality. My cover crop of choice was cowpeas, since they do well in the concentrated heat and the drier soil of the summer greenhouse.
Bench or earth grow?
In most commercial greenhouses raising bedding plants, flats of plants are grown on tables or grow benches. For homesteaders, however, growing right in the earth floor of the greenhouse is the only reasonable choice. The crop plants have more room for their roots to roam; the earth as observed earlier is a huge heat sink, and stays much warmer than the air temperature at bench level, and growing in earth beds allows improvement of the soil over time as we work with successive crops.