The Hoop

A series of hoops or bows make up the primary skeletal structure of high tunnels. These provide support for the outer covering and maintain structural integrity against environmental forces. The spacing of the hoops can vary, depending on the desired structural strength. Hoops are most commonly spaced at 4, 5, or 6 feet. Narrower hoop spacing provides more support for the polyethylene covering, requires the use of smaller diameter metals, and allows easier shedding of rain and snow. However, structure cost increases with more hoops, and there is a potential for increased structural shading. If wider hoop spacing is used, larger outside diameter (OD) metals are needed for structural strength against wind, rain, and snow loads.

End Walls

The size and type of end walls are designed to meet accessibility requirements for people and equipment. Most suppliers carry a variety of end wall framing packages, from simple pass-through doors to framing with larger openings that allow for the use of rototillers and tractors. It is important to remember to include a small door for access if the high tunnel will be used during the coldest part of the year since large openings will greatly reduce the ability to keep the inside temperature warm. End walls can be custom framed using lumber or metal piping if the polyethylene can be securely fastened to prevent air leakage or being blown in by high winds. Polyethylene zip entry end walls are an available option from some high tunnel suppliers, but these are not recommended for High Plains growers due to the risk of premature failure of the closure system from strong winds.

Purlins and Girts

Purlins run horizontally from end to end of the high tunnel, maintaining hoop spacing and increasing structural strength. Typical structures have three to five purlins, spaced at 4 to 8 feet apart. Girts are stronger than purlins and are generally manufactured of square tube or roll form metals. Girts provide greater structural support for polyethylene retaining systems (poly-locks), roll-up wall components, or other equipment. They are installed in positions similar to purlins to maintain hoop spacing. If the supplier provides only one ridge purlin for a particular type of high tunnel, additional purlins or wall girts must be added to prevent structural failure.

Wind Bracing

Overall structural integrity depends on the number of angular wind braces positioned within the structure. At a minimum, these braces should be positioned between the end hoops and the baseboard at each corner. Additional angular bracing can be installed from the ridge and roof purlins and connected to the end wall framing to increase structural strength. This is especially important in high wind locations.

Hoop or Half-Round Style

This is the most common high tunnel profile since it requires the smallest number of structural members and can be easily manufactured and built by individuals with little construction experience. The support hoops of the half-round profile have a smooth, gradual bend that limits the number of stress points on the plastic covering. The hoops bend from the ground on one side of the structure to the ground on the other side to form a continuous arch. This design may limit access or standing room along the sidewalls.

Raised Hoop

The raised hoop design is a modified hoop structure with sidewalls created by extending the vertical ground posts to the desired height (typically 36 to 60 inches). The straight sidewalls allow for greater accessibility along the wall and provide more usable growing space; however, raised hoop structures are taller than typical hoop profiles. The raised hoop design is preferred when using roll-up sidewalls because it prevents rainwater from draining directly from the roof into the growing beds, which can negatively affect plant or flower quality.

Peaked Hoop

These structures are constructed by adding a bend at the apex, or top point, of the hoop or bow to create a distinct ridge. This ridge provides additional strength against crosswinds and creates a steeper roof pitch that sheds rain and snow more quickly. Peaked hoop structures typically have distinct roof and sidewall surfaces that are divided by a wall girt or eave board. This allows for the easy integration of roll-up or drop-down sidewalls for natural ventilation.

Overall Dimensions

Structures appropriate for growing home garden crops may be as small as 8 feet wide by 10 feet long. Commercial production tunnels can be 20 to 30 feet wide and any length. Growers often use high tunnels that are 48 feet, 96 feet, or 144 feet long so that standard 100-foot or 150-foot lengths of polyethylene can be used efficiently. If a longer structure is needed, polyethylene splices are installed to connect multiple lengths of polyethylene. It is important to remember that the larger the polyethylene sheet, the greater the difficulty of installation, and the greater the weight of the covering on the structure.

Determining High Tunnel Width

High tunnel width is a grower’s personal decision based on the desired interior layout and placement of growing beds, walkways, and support trellises, and the type of equipment that will be used inside the structure. The only physical limitation on the area being covered is the size of the available site. The most affordable option is to purchase “off the shelf” high tunnel packages as these often use standard-sized steel or aluminum and polyethylene sheets that result in the most floor space at the lowest per foot cost.

A typical half-round high tunnel is usually half as tall as its width. For example, a tunnel 20 feet wide will be approximately 10 feet high at the center ridge. If the structure is narrower than 12 feet, ground post extensions may be required to allow for more accessible headroom and more usable space next to the sidewalls. Peaked hoop designs have the widest amount of usable floor space, but overall height decreases as the structure is widened, which may result in low headroom space near the sidewalls. Wider tunnels have a flatter roof pitch that may negatively affect the shedding of rain and snow, potentially leading to structural collapse.

Polyethylene Retention

Historically, poly plastics were held in place by fastening wood lath or batten tape over the perimeter edges of the plastic sheet to the baseboard. Although inexpensive and functional, these methods did not hold as securely as a polyethylene lock system (poly-lock) and were time-consuming to install. Polyethylene locking systems consist of two pieces, a receiver base and a locking member. The most affordable system uses an aluminum “C” profile receiving base that has a bent wire (known as spring lock or wiggle wire) locking member inserted into it over the poly sheet (Figure 4). This provides a strong, reusable, long-lasting retainer for plastic coverings and can also be used to hold shading fabrics or insect netting. There are other types of poly-locking systems available from suppliers that function in a similar way.

Topography

Optimal high tunnel sites are slightly elevated, with a 1 percent to 2 percent grade. Avoid moderate to steep locations; these may have shallow soils with poor fertility due to erosion. Cold air may collect or pool in low-lying sites, which may increase the incidence of late spring or early fall frosts in these areas. Evaluate each potential high tunnel site for both soil water infiltration and excessive water runoff. Prime locations have deep, fertile, and well-drained soils, and little potential for cold air pooling. If a grower decides to use a site that is not optimal, careful structural planning and additional site preparation will be required.

Sun, Wind, and Temperature

It is important to choose a site with unobstructed sunlight, gradually modulating temperatures, and good air movement. If vegetation exists in the area around the high tunnel, it can help create an optimal microenvironment for plant growth. Distant trees effectively reduce wind speeds near high tunnels but allow for good air movement within the structure. Shrub rows or ornamental grasses can be planted near high tunnels to create a microclimate and help direct snow deposits away from the structure. Ponds and lakes will “bank” mean seasonal temperatures and modulate daily temperatures. This will keep summer daytime temperatures cooler and prevent early fall frosts; however, spring warm-up can be slower. Production areas near grasslands or forested sites will have cooler environments with fewer temperature extremes than locations adjacent to neighboring fields or suburban developments.

Adjacent Land Uses

Adjacent land uses may affect the grower’s ability to manage the high tunnel environment, crop productivity, plant quality, or organic certification. Traditional agricultural production fields present a challenge to successful high tunnel production. Temperatures and wind speeds vary as crops mature and are harvested. Deleterious insect populations can quickly increase as a result of neighboring crop maturation or production activities. Windborne dust and debris can affect specialty crop quality, as can the increased potential for agricultural chemical drift that can result in plant injury or death. Nearby animal holding areas, such as pastures and feedlots, increase the risk for contamination of edible fruits and vegetables due to animal waste pollution.

High tunnels near expanding urban or suburban areas or major transportation corridors can be affected by air and storm water pollutants, increased heat and runoff from pavement, and changes in topography associated with development.

Neighbors

A high tunnel can affect neighboring property in either a positive or negative way. What may be of interest and excitement to a grower may not be visually appealing or may be a safety concern to a neighbor. Show respect to neighbors by carefully locating and constructing an aesthetically pleasing, high-quality structure. Neighbors have the ability to positively or negatively affect the business.

Direction of the Sun

Warming the growing beds is critical for early season production. This is accomplished by solar radiation being absorbed by the soil. Any obstruction of light or shading will reduce solar radiation absorption. Orienting the structure north-south will allow the sun’s energy to reach the maximum floor area. This orientation also allows light to penetrate the foliage of crops of different heights with a minimal amount of plant shading (Figure 10).

Many publications suggest that regions north of the 40th parallel should position a high tunnel in an east-west direction. This could allow more sunlight to penetrate the tunnel during the winter when the sun angle is low, but consideration must be given to shading that might result from growing taller crops.

Wind

It is important to position a high tunnel to take advantage of prevailing winds. The goal is to provide good ventilation inside the structure without creating a large wind block. The large expanse of polyethylene covering can easily turn into a large sail that can lift the structure out of the ground. By positioning the structure with the narrowest side (the end wall) into the prevailing wind, less resistance will occur. This orientation also limits the amount of snow that can accumulate on the leeward or opposing side of a high tunnel, reducing the potential for structure collapse.

Because high tunnels are temporary structures that typically use lighter gauge metals and fewer structural components than permanent structures, sites with high winds may require the use of heavier grade metal, installation of more structural hoops (or bows), and/or increasing the amount of wind bracing (Figure 11).

Locations Previously in Production

The productivity of an existing field will be known, but improvements should be considered prior to installing a high tunnel over any existing site. A deep ripping of the soil will enhance aeration and break up any hardpans that may have developed over time. Incorporation of composted plant material or animal manure through surface tillage improves organic matter content. Improved soil conditions will encourage microbial activity and beneficial organisms, contributing to better breakdown of compost and increased fertility.

New Locations

If a new site has been selected, development of the growing beds should begin the season prior to production. Prepare untilled soils by clearing the existing vegetation, using mechanical removal if possible, to avoid contamination by herbicides. The site should then be deeply tilled and rocks and debris removed before doing the final surface tilling. Use a soil ripper for deep tillage and avoid plowing to prevent a hardpan from developing in the root zone. Cultivate compost or organic matter into the upper 6 to 8 inches for improved soil structure and fertility. Compost is often available at local recycling sites. Rotted alfalfa bales and composted manure from barnyards can be good soil amendments if the source is known.

Plan

Prior to the purchase and construction of a high tunnel, visit the local building code governing authority to understand the legal requirements for a high tunnel and its placement on the property. The agency or department responsible will be able to provide guidance in selection of a high tunnel that meets building codes in the area.

Order Materials

Growers should search for various suppliers and identify the most suitable structure to meet their needs. Carefully evaluate each structure for the amount of pre-engineering of the structural components and the level of customer support provided by the supplier. Determine whether construction will be possible by following a clear set of instructions or whether the installer will need to custom fit all the components at the site. Contact the supplier regarding the lead time required between ordering and delivery. Some companies will have structures ready for shipping while other companies may only manufacture to order. Once the order has been placed, complete the initial site preparation and structure layout.

Receipt of Materials

Plan ahead for receipt of the materials being shipped. Due to the size and number of components, the high tunnel should be shipped directly to the construction site. While a forklift is useful for unloading the high tunnel parts, it is not absolutely necessary if enough helpers are available (Figure 12). Verify that the shipment is complete and parts have not been lost or damaged.

Sort Materials

Identify and sort all parts prior to construction. Match the location where each part will be installed with the manufacturer’s instructions and construction plans. Install each component as directed to ensure the safety and durability of the structure. Many suppliers pre-size and predrill various components and also note in the instructions which parts need to be sized to fit properly. If cutting is required, verify the location of each cut before making it.

Labor Assistance

The initial layout of a high tunnel can be completed by an individual, but additional help will be needed for construction. Helpers must be physically able to lift and carry components and have previous experience using tools. A team of two or three people will be needed to lift the high tunnel hoops into position and will speed up the process for fastening supporting components and hardware. An assistant can hand tools, supplies, and parts to the person on the ladder installing purlins and end walls. Several people will be needed to help install the polyethylene covering.

Structure Layout

Layout accuracy is critical for ease of construction, application of coverings, and operation of high tunnel equipment. A string line and corner stakes will be needed to exactly place and square the corners of the structure. To maintain trueness of the sidewalls, measure along the string line and identify ground post locations with flags, marking paint, or stakes (Figure 13).

Set the Ground Posts

High tunnels are generally classified as temporary structures, which, according to many building codes, will allow ground posts to be directly inserted into the soil. Verify this with the local governing agency. Ground posts are tubular steel sections that are driven into the ground using a post driver or sledge hammer, or pressed directly into the soil with the bucket of a heavy tractor. The use of a ground post driver cap will prevent post distortion during installation. If the tops of the ground posts are damaged during installation, all posts will need to be cut off uniformly, horizontal to the soil surface.

If pier installation is desired or required by building code, all ground post locations need holes that are drilled using a screw auger at the recommended depth (Figure 14). Remove loose soil at the bottom of the holes. Install a bolt through the ground post that will be embedded in the concrete to prevent the post from lifting after the concrete has set. Re-establish string lines after drilling the holes so ground posts may be inserted into the poured concrete in straight lines and at a level height.

Frame Erection

Bows will be shipped as multiple-piece units requiring assembly on-site. Assemble the bow sections on a level surface near the construction site so they will be square and true upon completion. Carefully position each completed bow near its ground stake position without bending the unit (Figure 15). Any distortion in the bows will cause difficulty in the installation of precut and predrilled components.

Erect the first two or three bows by tilting them upright into position. Fasten the bows to the ground stakes using bolts or self-tapping screws, as recommended by the supplier. Next, install the ridge piece (or ridge purlin) and any additional purlins according to the manufacturer’s instructions. Square the structure so it is perpendicular to the ground and maintain this position by installing the diagonal wind braces at the first two end wall corners.

Following the initially erected bow unit, the remaining bows will be systematically tilted upright into position and fastened to the ridge and purlins, maintaining a true structure. Once the final bow is positioned, install the remaining angled wind bracing.

End Walls and Baseboards

Pre-engineered complete high tunnel structures include metal end wall framing components, baseboards and/or roll-up walls with skirts, and swinging or sliding entry doors. By following the manufacturer’s instructions, installation of these components should be easily completed as long as the structure was squarely positioned and constructed.

High tunnel structures that were not purchased as a complete unit may require the owner to buy local materials to finish the structure. Lumber will need to be purchased locally for end walls and baseboards. Carefully consider the use of treated versus non-treated lumber. If the wood will come in contact with the soil, treated lumber is commonly used; however, the use of treated lumber might affect organic crop certification. The grower should contact the organic farmer certification program to learn which lumber and wood treatment products are acceptable.

Wood baseboards should be installed to provide a barrier between the high tunnel growing beds and the outdoors (Figure 16). If roll-up walls are used, the baseboard creates a sill for the roll tube to close on. Without a baseboard, cold air can easily pass under the roll-up wall, and rodents and pests can more easily enter the structure. Baseboards set on top of the ground provide little insulation from cold temperature conduction through the soil from outside to inside the structure during winter and early spring. An additional option is to install a construction foam barrier (¾ inch to 1 ½ inches thick) behind the baseboard and bury it a minimum of 16 inches, which improves insulation.

Entries and Doors

Entries are typically designed based on personal preference and accessibility requirements needed for equipment to be used inside the high tunnel. Doors can be constructed using lumber framework covered with polyethylene film sheeting or rigid panels. Doors may be hung using rolling door glides, similar to a barn door, or heavy-duty gate hinges. The entry method selected should close tight enough to prevent wind leakage into the high tunnel. For the Great Plains region, end wall curtains with zipper closure systems are not recommended as high winds will cause them to fail. Sliding door entry systems work best since they can be opened incrementally, require minimal opening space, and remain unobstructed if snow accumulates against them.

Installation of a Polyethylene Roof Covering

Polyethylene locking systems are the most effective method of holding plastic sheeting snugly on the structure. A tight covering prevents excessive movement and flapping that can quickly weaken the plastic. Polyethylene plastic expands when it is hot and contracts when it is cooled. For this reason, it is best to install the polyethylene on a very warm, calm day. In the Great Plains, winds often calm down in late afternoon and early evening.

Before installing polyethylene, distribute the locking pieces around the structure within reach of the lock base channel. Have tools available to install locks and place step ladder(s) inside the high tunnel to assist positioning of the sheet plastic. There are two methods for positioning polyethylene sheeting over the high tunnel structure.

Securing Polyethylene Covering

For best fit of the covering, the polyethylene should be positioned squarely over the structure with few obvious wrinkles. To ensure correct positioning, visually line up a gusset crease with a purlin or ridge piece (Figure 17). By temporarily locking the polyethylene on one end bow peak, the poly can be pulled from the opposite end bow to pull out any creases or overlapping poly on the ridge.

Final locking of the polyethylene should be done in the following order:

• Install locks on end bow “A” (Figure 18).
• Pull poly away from end bow “A” toward end bow “B” and lock in position.
• Install locks on side wall “1” (leave any small creases in position and do not pull to one end).
• Pull poly away from side wall “1” toward side wall “2”, then install remaining locks (leaving small creases in position).

If the polyethylene was installed on a warm day, the plastic will tighten as the days cool. By doing this in the summer, an extremely taut covering will result in the fall when the weather cools, and small creases will disappear.

Sidewall Venting

Roll-up or drop-down walls are options on high tunnels that increase air movement within the structure. If installing walls that open, the hip or wall girt these are mounted on must be parallel to the ground with the height of the opening equal on both ends of the structure. Wall blank panels at least 18 inches wide should be used on end bows to overlap the roll-up wall ends (Figure 5) and prevent cold air infiltration. Ropes should be installed outside the roll-up or drop-down walls from the baseboard to the eave or hip purlin to prevent “blowout” of the wall covering (Figure 19). Inexpensive hand driven roll-up machines are available to allow for variable positioning of the roll-up wall. Drop-down walls are much more complex to install and require several cables, pulleys, and clamps to operate.