The 2008/2009 Greenhouse

As anyone who knows me would have said was predictable over the spring and summer of the last year my collection of cold sensitive container plants outgrew the space available in the 8x10 greenhouse I built for the previous winter. This meant I was either going to have to reduce my collection to fit or building a larger structure to use for this winter. There was only one correct answer to that particular dilemma so I started accumulating the necessary materials.

Now according to the natural laws that govern the universe this undertaking was not going to go smoothly and it didn't. My first problem was that I couldn't decide how big I wanted the new greenhouse to be! I first figured on going to a 10x12 structure, but quickly realized that it wasn't really going to give me much additional space so I stepped up to 10x16. A fit of greed overtook me though so I decided to go all out for a 10x20 house. As each change in dimension altered the necessary bill of materials Diana just shook her head figuring it was problaby still cheaper than me hanging out in bars.

The second problem proved to be the weather. Hither to now I've tried to have the greenhouse finished by mid-November at the latest in anticipation of our first frost of the season which usually falls within a few days either way of the first of December. Ma Nature had other ideas this year though and treated us to a suprise frost on October 29th! This meant doing the Large Thorny Plant Shuffle in and out of the workshop for the three frosts that occurred before I was able to finish construction. The old gal isn't going to do me that way next year because I'm going to have the thing ready by mid-October from now on!

The third problem made itself apparent when I began to make the bows only to realize that forcing twenty feet of PVC pipe into a ten foot wide bow was placing an unacceptable amount of strain on the joints. So on the fly I expanded the house to a twelve foot width. First though I had to make sure my twenty foot wide plastic would satisfactorily cover the bows while leaving me sufficient material left over on either side to fasten it down. I briefly considered lengthening the house beyond twenty feet but realized I might be exceeding the heating capacity of the available electric power I can run out there so I finally exercised some restraint. After all a man should know his limitations!

The wood frame at the bottom is made of pressure treated 2x6s. The corners are joined by pieces of flat metal bent into right angles with several screws into the wood in each leg. You can find them at your local hardware store in the metal fasteners area for building joists and trusses. They come flat and I bent each one in a vice. As usual you may click on each photo to see the larger version for more detail.


On the insides there are two pipe clamps at the bottom of each leg of the bows. This keeps them from wobbling in the frame once they are tightened down after the pipes have been inserted. A screwdriver bit in a cordless drill makes this an easy chore.

The pipes are one inch Schedule 40 pvc joined at the tops by 4-way fittings for the interior bows and 3-ways for the two end bows. All joints are primed then solidly glued and allowed to cure for at least a day in the sun before bending. The short sections that connect the bows to one another are not glued, but merely held together by friction.

On the long sides I sistered the two boards together using some old pieces of 2x6s that were kicking around. The two long sides used four 10ft boards and the short sides used two 12ft lengths. As the bows are set into place they begin to exert an outward pressure along the twenty foot length of the house causing it to bow outwards. To counter this I drove three stakes cut from 2x2s on each side evenly spaced. I did not otherwise fasten them to the frame in case I later decided I wanted to move the house (which in fact I did several days afterward).

In retrospect I think I'd use 3/4 inch pipe rather than the one inch that I did use if I had to build it again. Slightly more flexibility while still retaining sufficient stiffness with somewhat of a savings in cost.

With the basic frame and the bows in place it was time to frame the ends. My previous greenhouses did not have end framing, but with this expansion there was going to be too much square area for the wind to blow against for that to work. I didn't want to drill holes in the pipe as I figured that would weaken it so I used the method that I use for building chicken tractors and wired it in place.


It's not the prettiest job I've ever done, but it works. The wire is seventeen gauge galvanized electric fence wire which serves as the universal bailing wire around here. Just keep the part running over the top of the pipe as smooth as you can so it won't abrade a hole in the plastic over time.

Once the ends were framed it was time to build and hang the doors. The hinges are on the left hand side for both as you face them so that when I have them propped open they can catch the breeze to funnel it inside. The warm, bright sunny days can run the temperature up pretty high inside so good ventilation is a must.

Here's the house fully covered. The plastic is plain 4-mil twenty foot wide translucent builders polyethylene. It's not UV stabilized so here in Florida it will last through a fall and winter then begin to crack and shred sometime in the late spring to early summer. For my purposes this is OK as by then I no longer need the protection. For the next time I need to cover it I have a roll of six mill poly for somewhat greater durability, but it still won't last more than a year at best. For a more permanent installation I'd buy actual UV stabilized greenhouse plastic which is mail orderable from a number of supply companies.

The plastic is secured to the frame along both sides by using simple firring strips and screws first drilling small pilot holes before putting in the screws to eliminate splitting. You can see the end of one piece in the first photo at the top of this entry. I like to fold the plastic over on itself then secure it with the wood.

The plastic is secured to the hoops by the use of clamps that I purchased from Peaceful Valley Farm Supply.



They comes in various sizes for each width of pipe. You can also get some of the hard to find pipe fittings there as well if you aren't able to find them locally. I'm sure there are many suppliers for these products so feel free to shop around if you want to. The clamps have held up well for me thus far even after several gusty cold fronts have blown through. I think I'm using five clamps per bow leg which makes for a tight seal of plastic to pipe.

The photo above is the house fully covered. When I did the ends I first stretched the plastic upright so that I could fasten it evenly to the bottom frame with the firring strips. This done I then pulled it taught in an upward direction so that I could clip it into place on the end bows. Once it was smooth and even from side to side and top to bottom I could cut the door out. This was done carefully along three sides, but leaving the hinge side uncut for better weather proofing. The cut edges were folded over on themselves then secured in place with more firring strips. So far this has held well through several windy cold fronts.

And here is the house complete with plants, benches, power, fan, and heaters inside. I'm not using proper greenhouse heaters as are recommended because I can't afford them, but I am careful to unplug everything when I water and both heaters are supported off the floor by boards. I covered the bottom in landscape fabric to keep the grass growth down. It's available from most garden center in various widths and lengths. I used three runs of four foot width material in mine and secured it to the ground with the recommended landscape staples. It's not really necessary, but I thought I'd try it this time. It also makes it easy to sweep up inside which mine needs doing with the citrus dropping their old, bug damaged leaves as they grow new ones.

Between the two heaters I have a combined 2500 watts of heating power along with a small eight inch fan to move it around inside. The house passed our twenty one degree night without incident. I don't have automatic vents so I have to use some forethought about opening and closing the doors to either retain heat or let it out. On our coldest days that never climbed above sixty degrees I never opened it at all. Any warmer than that and I'll open at least one door a few inches. Warm sunny days I open both doors wide open.

In a climate that is significantly colder than Florida's I'd either need to reduce the size of the house or use insulation to keep heating costs within reason. Next year I may order one of the clear plastic solar pool covers (think giant sheet of bubble wrap) that some folks use with their houses. This should help significantly in retaining heat, especially in the event of a power failure.

This iteration is as large as I think I care to go in building a greenhouse. If I just had to have more space I believe I'd build a second. What I'm thinking about now though is where I'm going to put our permanent greenhouse and what design I should use. I'm thinking out past by sycamore tree by the corner of the vegetable garden...

.....Alan.