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Vol. 7(7), pp. 11-17The McAllen International Orchid Society JournalJuly 2006

Building an Orchid Greenhouse

Cliff and Sandee Pelchat

Our interest in orchids has always centered on the study of them in situ. We have traveled across the US and into Central Mexico finding orchids, recording them on film, and studying their habit and habitat. Because of this we have always put off building a greenhouse--our reasoning being that such an undertaking once constructed would require too much of our time. Of course there were other reasons also, such as the fact that we hadn't really settled down to live in one place for more than a couple of years. We have always had a couple of orchids that we kept on a windowsill or a cart on the front porch, but had never really grown them in a serious way. This year we built the greenhouse and this article is a description of what we did.

Before getting started we came up with the following criteria for our greenhouse:

  1. It has to be sized large enough to provide adequate space for growing orchids and at the same time fit in the space available in our yard.
  2. It has to withstand Oklahoma weather
    1. High sustained winds of 30 - 40 mph
    2. Large hail
    3. Hot dry summers
    4. Winters of temperatures as low as +9° F. for a few days/year and the occasional -9° F.
  3. It has to be economical to operate and be as maintenance free as possible when it comes to controlling the climate inside the structure
  4. Be aesthetically pleasing and blend in with the house and neighborhood
    1. not violate any of the homeowner association covenants
  5. The cost of the finished greenhouse was not exceed $5,000.00 including climate control

Taking into account the above criteria we decided that a "lean-to" structure could be placed on the outside south wall of the house. Besides providing part of the greenhouse structure, the outside south wall of our house is brick and would act as a heat sink in the winter. Of course it is a heat sink in the summer as well and a system to offset this would also have to be considered. The other advantage to building the greenhouse onto the side of the main house is that utilities such as water and electric could be easily run into the structure.

The Internet was an invaluable tool that we used to search for a green house. If you "Google" greenhouses on the WEB you will get a plethora of choices from small cold-frame structures to giant industrial setups. The 3 that we found most helpful are http://www.charleysgreenhouse.com, http://www.littlegreenhouse.com, and http://www.greenhousekit.com/julianalean-to.htm. We found that most of the greenhouses offered followed the same theme with the following selections available:

  1. Cedar frame with polycarbonate or glass glazing
  2. Plastic, snap-together frames with polycarbonate glazing
  3. Aluminum frames with glass or polycarbonate glazing

We decided that an aluminum frame would be best. Aluminum is better than wood because it won't require the maintenance that wood does. Aluminum is also better than plastic for 2 reasons: (1) it provides for more rigidity; and (2) plastics, no matter how they are made, tend to become hardened and brittle over time because they are susceptible to the effects of ultra violet (UV) radiation. There might be those that will argue that modern plastics are made to withstand the effects of UV, but in this case we had to go with Cliff's experience with plastics of various types over the last 30 years. Without going into great detail Cliff has yet to find a plastic that is not affected by UV. Yes, they can be treated for the effects, but over time there is still some degradation and we didn't want any degradation in the structural frame.

The next step was to consider the type of glazing. Tempered glass was eliminated as a choice because a typical Oklahoma hailstorm would have made short work of the flat roof on a lean-to structure. Fiberglass sheathing is often used, but even treated fiberglass is highly susceptible to UV and it doesn't provide very much insulating value. Based on our research we decided to use a twin-walled polycarbonate sheathing.

Polycarbonate Sheathing

Polycarbonate is an amazing material. It is soft while at the same time rigid. It has an insulating airspace that can be double or triple walled (it looks like cardboard when viewed on edge). It is highly UV resistant and you can get it with one side treated to block UV light altogether. It is kind of like placing a set of sunglasses on your greenhouse.

Putting it all together

Fig. 1. Skeletal structure of proposed Juliana lean-to greenhouse.

With the above considerations in mind we decided to purchase a Juliana lean-to greenhouse kit 16 feet long by 6.5 feet wide (Fig. 1). It has an aluminum frame and 6 mm. twin-wall UV treated polycarbonate glazing. The Juliana greenhouses are made in Scandinavia and are very well constructed. The kit also included 2 roof vents and the base. The lowest price we found for the kit was $2,168.00 including shipping. We also purchased two automatic roof vent openers, an exhaust fan, automatic opening exhaust louvers, thermostat, humidistat and a water misting kit. We deferred purchasing cooling and heating equipment until after the house was constructed and will discuss our approach to those needs later in this article.

Greenhouse Construction

As stated above, the Juliana Greenhouse is well constructed. It has all of the characteristics of fine German engineering such as you would find in a well-engineered German automobile. All of the pieces fit together precisely and were held in place by ingenious slotted aluminum bolts. The only drawback to the kit was that the instructions for assembly were written primarily in German and the English translation was terse and used British rather than American terms. For example the glazing instructions consisted of a few short sentences with the note that "The central 15 cm. of the sheets are cleaved without closing the channels below (emphasis added)..." The interpretation for this instruction was: "place a 15 cm. long bead of silicone in the bottom middle of the panel to attach it to the frame." Our assessment is you need to be pretty mechanically inclined to put this thing together. If you have ever found it a challenge to assemble a child's bicycle or a backyard barbecue grill then you are going to be extremely challenged to figure out how to put this kit together!

We figured it out mostly by referring to the parts list and matching the parts with the supplied diagrams. The instructions also recommend 2 people for construction. We laid out each of the sections in our garage and pre-assembled each before carrying them out to mount on the foundation. Two 8-foot long 2 x 4's were used to brace the gables and walls into position before bolting them together.

Before assembly we excavated and leveled the area next to the house where the greenhouse would be erected. We then laid out the base making sure that it was square with the house and each corner. Using a post-hole digger, holes 18 inches deep were dug at each corner from the house and the center of the front wall. Each hole was poured with concrete using 8-inch concrete tubes (available at Home Depot or Lowe's) as molds to create peers for the base to be bolted to. The steel brackets for the base were suspended in the tubes and made level with each other. Once the concrete set, the base was bolted to the brackets and to the wall of the house using ¼-inch Redhead anchors. Each of these anchors has 520 PSI of anchoring force. The top portion of the greenhouse has a Redhead anchor every 2 feet for a total of 8 anchors. Combined, the green house has 5,200 PSI of anchoring force securing to the house as well as the brackets that are anchored into the concrete peers securing it to the ground.

Fig. 2. Paved walkway through lean-to greenhouse.

Fig. 3. Assembled greenhouse structure on south side of home.

Once the foundation and base was ready it was not difficult to bolt the pre-assembled sections to it. It took about 4 evenings working from 7pm to 9pm to get it completed. The glazing of the polycarbonate took a little longer because we had to wait for days where there was no wind. We left the glazing off of the front wall until we had finished preparation of the flooring on the inside. The flooring consists of ½-inch diameter river stone (also called Aztec gravel) placed over landscape cloth to a depth of about 7 inches. It took 3 ½ tons to completely fill the floor to the top of the base and the open wall allowed us to dump the stone in with a wheel barrow rather than carry it in one bucket at a time through the door. Pavers were laid down the center for a walkway (Fig. 2). Finally we finished off the outside by covering it with 50% black UV resistant polyester shade cloth and a façade of brick around the base to match the brick on the house. The 50% shade cloth was used to further offset the intense solar radiation we have in Oklahoma (Fig. 3). We have subsequently determined that even more shading will be needed for some species of orchids. This shading will be added to sections of the greenhouse once we begin populating it with plants according to the requirements determined at that time.

Heating and Cooling

The temperatures in Edmond, Oklahoma range from highs as much as 105°F. in the summer to as low as -9°F. in the winter. Both of these temperatures represent the extreme records that have been set in the last 100 years. Nevertheless these extremes need to be considered when designing a greenhouse that could easily house thousands of dollars worth of orchids.

The heating requirements are easily calculated - taking into consideration the greenhouse construction, the desired inside minimum temperature and the lowest outside temperature in the winter we came up with a requirement of 17,000 BTUs/hour to maintain an inside temperature of 65°F. when the outside temperature dropped to -9°F. As mentioned before a temperature of -9°F. is an extreme, so a smaller BTU/hour heat source could suffice. In fact, at the time of this writing we haven't yet put in a heating unit. We are going to wait until the weather cools down to refine the calculations to take into consideration the brick wall that acts as a heat sink. We will do this by recording the day and night temperature gradients on the inside and outside the greenhouse as the weather cools down. This was the same approach used for determining the cooling requirements.

Fig. 4. Temperature differences within greenhouse over a two week period. (Red line: outside temperature; blue line: inside temperature; green line: temperature alteration.)

Fig. 5. The greenhouse's swamp cooler. (outside view)

Fig. 6. Inside view of the evaporative cooling unit.

We determined by experimentation with misters that evaporative cooling would be able to achieve at least a 10°F. difference between the outside and inside temperatures (Fig. 4). Based on this determination and guided by the recommended air volume exchange for this size greenhouse we determined that a portable window-unit evaporative cooler capable of moving 2,200CFm would do the job. It was purchased, installed and plugged into a thermostat control that controls both the cooling unit and the exhaust louvers at the opposite end of the house (Fig. 5) shows the outside aspect of the evaporative cooler. The inside view of the evaporative cooler is shown here (Fig. 6).

Humidity Control

Fig. 7. Misters, misting lines and humidistat.

Fig. 8. Automatic valve for humidistat system.

The misters installed for cooling experiments have been converted to an automatic watering system and a humidifying system for use during the winter and during those really dry summer days of August when the swamp cooler won't provide enough humidity. For humidity we have 3 misters connected to an automatic valve and a humidistat (Fig. 7)(Fig. 8). The humidifying misters point toward the brick wall. As humidity tends to drop as temperature rises the misters kick on when the brick heats up, and the water vaporizes when it hits the wall raising the humidity and dropping the temperature at the same time. Humidity measurements for the entire greenhouse hover around 70% relative.

Watering System

Fig. 9. Manual valve control of misters.

The watering system consists of 6 misters that point over a built in plant shelf constructed from vinyl coated shelving found at the Home Depot. At this time these misters are controlled with a manual valve, (Fig. 9), but we think we will be adding a timer type control in the future depending on the requirements of the plants. Since the piping is PVC and PEC FLEX tubing we are able to easily make changes to the water circuit to meet any need.

Fig. 10. Temperature differences following installation of the evaporative cooling system.

After installing the swamp cooler we continued to monitor the temperature differences and the results are quite satisfactory. A chart showing the temperature difference since installing the swamp cooler has been provided (Fig. 10). As you can see the difference between inside and outside temperatures increases as the outside heat climbs. Since a thermostat controls the swamp cooler the night temperature inside the greenhouse would climb relative to the outside because the swamp cooler would shut off. We were able to get the nighttime temperature differential down to zero by lowering the thermostat and adjusting one roof vent to automatically open in the evening to equalize inside and outside temperatures. This has the effect of making the swamp cooler run all night long when the outside evening temperatures remain in the high 70's and above.


We have already placed a couple of the plants we had sitting on various windowsills and they are doing great. One plant, Bletia purpurea, which is a Florida/Mexico species has shown more vigor than we have ever observed before while the Epicat species. has bloomed like never before because of the additional light available to it. We think this greenhouse is going to be perfect for our immediate needs and give us some experience in growing orchids. Everyone we talk to says we will soon outgrow it, but that remains to be seen. In the meantime we have the best of both worlds - a small hobby greenhouse while not giving up our ability to continue our field trips to discover orchids in the wild. We will continue to periodically submit updates as to our experience.

Copyright © 2006 Cliff and Sandee Pelchat