My wife and I (with help from a lot of friends) just built a high tunnel greenhouse at Bear Butte Gardens. And, I’ve got to say, it’s a lot bigger job than I expected. But we learned a lot, and it should be pretty amazing once it goes into production! In this article, I thought I would share some of what we learned about this project.
So, what is a “high tunnel”? According to the internet:
A high tunnel is a tall commercial or industrial-sized greenhouse. These are used in commercial agriculture applications to maintain a more reliable and uniform temperature and humidity and to protect plants from variables such as wind.
Depending upon who you ask, the definition of a high tunnel usually includes the characteristics of a hoop-style structure, covered with clear plastic, in which the long sides can be opened (rolled-up) to allow additional air ventilation.
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| Our High Tunnel Greenhouse, nearly finished |
We purchased our high-tunnel as a kit from a company called FarmTek (www.FarmTek.com). The model we have is the “Premium Round Style High Tunnel”. Ours is 30 feet wide, 72 feet long, and 12 feet tall. The price listed on their website for this kit is about $6,000. The good folks at our local Natural Resources Conservation Service (NRCS) office in Sturgis helped us with partial funding for the kit via the Environmental Quality Incentives Program (EQIP) grant. If you are interested in this program, then you should visit the NRCS office and/or visit the NRCS website (www.nrcs.usda.gov).
In addition to the basic kit, we purchased optional rafter support kits and a shade cover from FarmTek.
If you are familiar with where Bear Butte Gardens is located, then you know that we get harsh weather -- mainly the wind. We are located on the top of a rolling hill on the prairie near Bear Butte. In the past, we’ve had wind micro-bursts that have blown our garage door in (collapsed it), blown the front door off our house, blown siding off our house, broken window frames, blown shingles off our roof, as well as several other mishaps. So, when I say windy, I mean it. Due to these extreme conditions, we have chosen to enhance (strengthen) our kit even a bit more. I’ll try to identify these enhancements as we go along.
The basic kit consists of galvanized steel hoops (rafters) that make the frame of the structure. The rafters are located four feet apart. Our kit has 18 rafters (4 x 18 = 72 feet). Each rafter is composed of five sections of curved conduit pipe, secured with “Tek” screws. The rafters are mounted to ground posts. The ground posts are straight pieces of galvanized steel conduit pipe that are three feet long. The ground posts are driven vertically into the ground or secured in concrete. The rafters are connected together by straight pieces of galvanized steel conduit pipe (purlins). The straight conduit pipe comes in lengths of about six feet, with one swagged (male) end and one non-swagged (female) end. Twelve sections of straight conduit pipe are connected together (with Tek screws) to form one entire purlin that is 72 feet long. The purlins are connected to the underside of rafters on the top of the structure. Our kit has five purlins that run the entire length of the structure. The intersections of rafters and purlins are secured with special clamps, bolts, and screws. One large piece of clear plastic is stretched over the structure and secured with a series of “u” channels and “wiggle wires”. The two end walls each get their own piece of plastic covering. The plastic on the long sides of the structure can roll up to a height of about four feet to allow additional ventilation.
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| The kit, unpacked |
This was the first high tunnel kit that we have ever constructed, so the learning curve was interesting to say the least. The instructions that come with the kit (or maybe my ability to comprehend the instructions) leave a lot of room for improvement. Anyhow, I’ll try to give you an idea of how we did it, and also share some of our discoveries, tips, and tricks -- just in case you want to build one of your own!
Step 1 - Planning
What are your objectives? Do you want to extend the growing season? Are you growing for family & friends or for commercial purposes? Do your homework. Search the internet. Watch YouTube videos about high tunnels. Visit someone who has a high tunnel. Ask questions. Contact the NRCS. If you are seeking financial assistance from the NRCS, then get familiar with their requirements. Select a building site. Think through how you will actually grow and harvest the plants -- things like sunlight exposure, pollination, watering and drainage, temperature control, soil nutrition, planting schedule, harvesting schedule, pest control, disease control, etc. What size/shape structure is right for you? How about access to/from the structure? Compare various manufacturers’ products. Plan for transportation (our 2,000 pound kit was shipped by truck to Box Elder and we hauled it from there to Sturgis). Know the warranty info. Plan for additional/optional building materials. Plan for the actual labor and time of constructing the kit.
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| Cedar posts and composite lumber |
Step 2 - Prepare The Building Site
We have a friend with a skid-steer loader who leveled our building site. Before he did his work, I removed the topsoil with my tractor. We used a transit to get the site flat and level. I think we did a fairly good job with this step, but next time, I will work at getting the site even more precisely flat and level. We then placed the stakes, put up string lines, did our measuring, ensured the squareness, etc. It is very important to get things straight, level, and square. We marked where each and every ground post would be installed. Our kit has 18 ground posts on each side for a total of 36. The tasks of pacing the stakes and string lines took several hours. Someone with more experience would probably be able to do it faster.
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| Dirt work |
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| Michelle, using the transit to help get the ground flat and level |
Step 3 - Install the Ground Posts
I happen to have a tractor with a post-hole digger, so we used that to drill holes in the ground, one for each ground post. We mixed one bag of redi-mix concrete per hole and then placed the ground posts vertically in the concrete. In the previous step we installed string lines so that we could get the ground posts set in a straight line, and all to the same exact height. If you have access to a transit, then use it for even more exact height measurement. This was two summer evenings (after my day job) till dark plus drying time for the concrete.
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| The tractor and post-hole digger made the job easier |
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| A corner ground post, with string line |
Step 4 - Install Rafters and At Least One Purlin
This is a job that requires several people. First assemble the rafters on the ground. Then raise the first rafter and attach it to the first pair of ground posts with bolts. Use ropes to pull the rafter into vertical position, and then temporarily stake down the ropes to hold the rafter in place. Then raise the second rafter and attach a purlin between the two rafters. Simply repeat this process for all remaining rafters, working your way from one end to the other end of the structure. We are lucky enough to have another friend who let us use his scaffolding, which made the job much easier, safer, and quicker. I also made a jig to measure the distance between the rafters (exactly four feet). This jig was made from a scrap piece of 2x4 with two notches cut in, four feet apart. This was one fairly long day with several people helping.
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| Assembling the rafters on the ground |
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| Putting up the 1st three rafters and one purlin |
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| Using the 2x4 Jig to consistantly measure distance between rafters |
Step 5 - Install Additional Purlins
After we had all 18 of the rafters in place, we installed the additional four purlins. This takes a considerable amount of time because each intersection of a rafter and purlin has a clamp with two bolts. After the clamp is installed then a couple of Tek screws need to be installed into the conduit piping. This is a considerable job when working on a ladder or scaffolding. 18 rafters x five purlins = 90 intersections. This consumed several summer evenings.
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| Installing additional purlins |
Step 6 - Install Footer Boards
We chose to install the optional footer boards. We used composite (plastic) boards (we are USDA organic certified so we can not use treated wood) about 1¼” thick, about 5” wide, and 12’ long. The footer boards are bolted to the ground posts along each of the long sides of the structure. This was done to add strength and also to get a better seal along the bottom edge of the roll-open sides when rolled down. This took a few hours.
Step 7 - Install Ribbon Boards
We chose to install the optional ribbon boards. Since these boards will not be in the dirt, we used rough-cut, non-treated 1”x4”s 12’ long. The ribbon boards are bolted to the rafters about four feet above the footer boards on each of the long sides of the structure. This was done to add strength to the top edge of the roll-open sides when rolled up. This took about three summer evenings.
Step 8 - Install Rafter Supports
We purchased and installed optional rafter supports. A “rafter support” is basically a rafter “truss”. Our high tunnel has 18 rafters, but we only purchased 8 rafter supports and installed them on every-other rafter. At some point in the future we may choose to purchase and install additional rafter supports on all of the rafters. The rafter supports get attached to the rafters with clamps, bolts, and Tek screws. This took about two or three evenings.
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| Installing rafter supports |
Step 9 - Install “U-Channels”
The kit includes aluminum “u-channels” that are 8’ long. The u-channels are used in conjunction with “wiggle wires” to attach the large clear plastic covering to the framing. U-channels get installed to the ribbon boards and along the two end rafters. The u-channels form a continuous perimeter for attaching the clear plastic covering. This took a couple summer evenings.
Step 10 - Make End Walls Vertically Plumb
Attach a string line between the two ground posts that support the very first rafter. Then hang another string line down (with a weight) from the tip-top of that same rafter. What we want is for the two string lines to intersect and touch. If they do, then that rafter is vertically plumb. If not, then the top of the entire structure will need to be pulled into the correct position. Remember the ropes that we used to temporarily stake down the first rafter? These ropes can be used to pull on the top of the structure. We found that the structure is still flexible at this point. Repeat this process on the very last rafter. If it is also vertically plumb then great! If not, then you may consider some adjustments. Hmmm...
We secured each piece of conduit piping together with Tek screws as we progressed with the construction. With hindsight being 20-20, we should have waited until now to install the majority of the Tek screws. This would have made the “squaring-up” process a little easier.
Once the end walls are vertically plumb, then stake the ropes down again to hold it there.
Step 11 - Construct End Walls
The kit included end walls. Well kinda. The kit included two big pieces of vinyl that are hung from the rafter hoops on each end of the structure. Each piece of vinyl has three zippers. Once mounted, the zippers can be used to open the “door”. That’s it. No additional structural framing. That design might work for a lot of folks, but not for the winds we get. We deviated from the design of the kit and built end walls that are considerably stronger.
For the south end wall, we used six 6”x6” cedar posts (we are USDA organic certified so we can not use treated posts). Two posts were 16’ long, two were 14’, and two were 8’. This accommodated the “arched” shape. The posts were set in concrete, about four feet deep. We then installed 2”x4” purlins between the posts in the usual “post frame” wall structure. The vinyl covering was then installed and attached to the purlins.
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| Preparing to set posts for south end wall |
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| Mixing concrete |
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| South end wall framing |
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| Notice the ribbon board |
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| Outside of south end wall |
For the north end wall, we built a similar structure with cedar posts and 2x4 purlins, but this wall has a 36” steel walk-thru door, and a 7’x8’ roll-up style steel garage door. For the wall covering, we did not use the vinyl included with the kit. We used corrugated, single-wall polycarbonate.
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| Framing-in the garage door |
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| North end wall |
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| Installing polycarbonate siding onto north end wall |
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| We used vinyl to cover the gap between the polycarbonate and the rafter |
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| North end wall with garage door and walk-thru door installed |
All throughout this project we tried to strike a balance between getting things perfectly square, straight, and level; with getting things good enough. When it came time to mount the two doors we discovered that having things perfectly in place is better.
I would say we have at least another 50 hours into constructing the end walls.
Step 12 - Prepare to Install Clear Plastic Covering
This step includes smoothing-down any sharp edges that might come into contact with the plastic covering -- either during installation of the covering, or after the covering is in place. Considering the certainty that wind would blow against the structure and relentlessly rub the plastic against anything making contact, we decided to be very thorough with this step. We either filed-down, sanded-down, or covered-up with duct tape, each and every sharp edge and bolt that stuck out. We did some of this as we installed the purlins to the rafters, but then had to redo some of the duct tape that became loose. Altogether, we have several hours into this step.
Step 13 - Install Clear Plastic Covering
This was the big step everyone was looking forward to! We picked a Saturday with a good weather forecast and recruited four other people (total of six) to help. The plastic was unrolled onto the ground along one of the long sides of the structure. Several ropes were attached to the plastic and then used to gently drag the plastic up and over the frame structure. The plastic is first attached to the structure with “wiggle wires” in the u-channel that is mounted to each end rafter. Then, along each long side of the structure, the clear plastic is attached to the u-channel which is mounted to the ribbon board (about four feet above the ground). A section of clear plastic is left dangling on each side in the area between the ribbon board and the footer board. This will become the “roll-up” side. Looking back at it, I think two to four additional people would have made this step much easier and safer. There was one moment when a bit of a wind came up. I think we all had visions of the wind taking the huge clear plastic sheet and blowing it into the barbed wire fence not to far away. But thankfully, somehow we managed. With six people, this took a few hours.
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The high tunnel greenhouse with the clear plastic cover on
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| The interior with the clear plastic cover on |
Step 14 - Install the Roll-Up Sides
This step consisted of connecting sections of conduit pipe together to form a single piece of pipe 72’ long (the length of the structure). This long piece of pipe is then attached to the dangling plastic. A gear box is then attached to the end of the long pipe. When a handle is used to turn the gear box, the long pipe begins to turn. This will, very slowly, roll the side up or down. “Anti-billow” ropes are attached to keep the roll-up side from blowing in the wind. This is repeated for the other side of the structure. This took four evenings.
Step 15 - Bring in the Dirt!
Since we removed the topsoil before construction, we then had the additional task of hauling the topsoil back into the structure. I’m sure there are a lot of weed seeds in the soil that will be germinating at some point. We have been watering the dirt to encourage some of these seeds to germinate so we can remove the young weed plants before planting a cash crop. We will soon be planting a fall, nitrogen-fixing, cover crop that will enrich the soil through the winter months. We will continue working on soil health, nutrition, and fertility by applying compost, worms, etc.
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| Some of the topsoil is in |
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| All of the topsoil is in |
Expenses
Here is a summary of the expenses:
- High Tunnel Greenhouse Kit $6,000
- Rafter Supports (8) 875
- Shade Cover 550
- Cedar Posts (12) 1,025
- Garage Door (1) 300
- Walkthru Door (1) 55
- Polycarbonate Siding 410
- Concrete (48 bags) 108
- Composite lumber 45
- 1x4s for Ribbon Board 45
- 2x4s for Purlins 100
- 2x6s & 2x8s for door framing 100
- Misc hardware for doors 100
- Misc screws & bolts 100
- Tarps 100
- Vinyl Coated Cable 80
Total: $9,993
The total cost (not including labor) is about $10,000 (including our enhancements). We have been told to expect three to five years of use from the plastic covering before it needs to be replaced -- so that will be a recurring expense every few years. We hope most other components (other than the plastic cover) will last for many years. Conservatively, we hope to generate at least $10,000 of revenue per year from selling garden produce grown in this high tunnel greenhouse. So, the return-on-investment ratio is fairly good.
Benefits
Based on our research, we hope to see many benefits of having a high tunnel greenhouse.
- Longer growing season
- Protection from bad weather
- Improved ability to manage moisture and temperature
- Improved ability to manage weeds
- Improved ability to manage pests
- Improved ability to manage disease
- Ability to grow some plant varieties that otherwise would not grow in this climate
- A more temperate environment to work in -- especially when the weather is poor outside
Altogether, these benefits will result in a higher quality product, greater quantity of product, easier and more convenient gardening work, and reduced risk.
Summary
This has been a wonderful project to work on in the great outdoors! We have grown from the experience, and we have invested in our future. I am oh so looking forward to those cool late-fall and early-spring days being greeted by t-shirt temperatures in the greenhouse!
Please stop in sometime! We'd be happy to show you our high tunnel greenhouse!
Please feel free to contact me with any questions, comments, or suggestions you may have!
Rick Grosek
Bear Butte Gardenswww.BearButteGardens.com
Rick@BearButteGardens.com
