High Tunnels

High tunnels are greenhouses without permanent foundations that are used to extend the growing season and enhance the environment for crop production. 

Site Selection. Avoid sites with inconvenient access, excessive water, poor quality soil, high winds, or low light levels. Ideally tunnels have year-round access, even when crops are not being grown, to allow for snow removal and other maintenance. Existing or potential access to irrigation water is essential, and access to electricity is desirable for inflation fans and mechanical air movement, although some growers have made use of micro-solar power systems to support these loads. It’s desirable to have good access roads and be close to wash/pack facilities. When siting your first tunnel(s) keep in mind future tunnel locations, so that your “build out” over the years allows for efficient access, materials handling and potential for multi-tunnel heating systems, etc.

The site’s topography should allow for drainage of “worst case” storm water and snow melt away from tunnels. A relatively level site is important to minimize structural stress on the tunnel due to uneven snow load. Moderately breezy sites can be helpful for passive ventilation, but high-wind sites create risk of damage to structure and/or plastic covering. Trees can provide a windbreak but consider their future height when locating tunnels to avoid shading and also note that dense hedge rows or locations too close to wooded areas can reduce passive ventilation.   

Tunnels should be slightly elevated compared to the surrounding soil in order to allow water running off the cover to drain away from the interior, and to allow snow melt to move away from the tunnel when the ground is frozen. On some sites it is advisable to create a raised pad for tunnels. Some growers install tile drainage, French drains, or curtain drains along the inside or outside of tunnels to carry excess water away from growing areas. Water running through/under a tunnel takes away soil heat, prevents good root growth, and can create muddy working conditions. Orienting tunnels along east-west axis provides optimal light for winter production, and a north-south axis is best to avoid shading inside the tunnel in other seasons, though most crops will have more light than they can use in the summer.

Construction. Do not skimp on the structural integrity of tunnels, as this can lead to collapse in bad weather. Plan for extreme snow and wind. Well-set ground posts, cross-ties, and other features that anchor the tunnel and keep it rigid are essential. Doors and vents should close securely to prevent winds from opening them in storms and seal well to help retain heat. When building a tunnel, avoid driving equipment over future growing areas, as this can create compaction. Installing large doors in end walls or having removable / roll-up end covers to allow for tractor access can make tillage and addition of bulk soil amendments easier than with small equipment. Head houses or other structures make sense for tools and equipment, seed, potting soil rather than taking up valuable growing space in the tunnel.

Zoning and codes. Before you build, contact your state and local agencies to find out about regulations and tax policies for high tunnels. Some states and towns may require building permits; setback requirements and building codes vary among municipalities. Some consider tunnels to be real property (subject to tax) and others do not. It may be helpful to be very clear with local officials that the structure is not permanent and is used for producing agricultural crops.

Soil quality. As in the field, tunnel crop production will benefit from deep, well-drained, fertile soil that is not compacted. On most sites, soil amendments such as compost, peat moss or coir will be desirable to increase the organic matter level to optimize tunnel production. Lime and nutrients should be added based on soil tests prior to production.  Fertilizer recommendation should be calibrated to the higher yields and longer growing season expected in tunnels compared to the field.  On sites with poor native soil, compaction and/or drainage problems, soil can be imported either into the entire tunnel, raised beds, containers or by using ‘grow-bags’ of pre-fabricated media.

Since tunnel soils are not exposed to regular leaching from rainfall, soluble salt levels can build up over time negatively affecting plant growth. Salts dissolve into ions in soil solution and come from the application of fertilizers and composts or manures. Crops remove some of these salts in their tissues, but the excess remains in tunnel soils, unlike in the field. Strawberry, green beans, and certain herbs are very sensitive to salts, but even tolerant crop such as tomato and spinach can show reduced vigor with very high levels. Salts tend to accumulate especially in the top few inches of soil, as they move upwards with evaporation and can effect germination of winter crops while transplanted crops such as tomato may be more tolerant to high salt levels.  Deep tilling will remix those salts into the soil profile. Salt injury can be exacerbated if soils are allowed to dry out. Excessive salts can be reduced by diluting with the addition of peat moss, coir or topsoil. Irrigating with a large amount of water can move salts down in the soil profile, but is often impractical. Removing the plastic cover over winter is perhaps the easiest way to leach salts out of the root zone.

Ventilation. Air exchange in tunnels is essential to avoid high temperature, high humidity, and low CO2 in tunnels, leading to plant stress or disease. Passive ventilation using roll-up sides is common in tunnels, though some tunnels use mechanical ventilation with fans to pull air through the tunnel. Generally, you must pick one or the other or use them at different times. Fans pull from the point of least resistance, so running an end-wall fan with the sides rolled up simply pulls air from around the corner, not from the other end of the tunnel. When sizing fans for ventilation the basic rules of thumb are 8 CFM/ft2 (of growing space) for summer cooling and 2 CFM/ft2 to remove humidity during cooler months. Note that this guidance is for peak ventilation needs.  Staged fans (e.g., one small, one large) or variable speed fan controls can help moderate the ventilation for various times of the year.
Passive ventilation is less than ideal in locations where tunnels are crowded together, there are lots with trees or other significant wind breaks, or in calm sites with little wind. A dense crop canopy later in the growing season also reduces passive ventilation.  

Installing a ridge-vent (along the top peak of the tunnel roof) will greatly increase the effectiveness of passive ventilation, though these can be costly and they make installation of plastic cover more complicated. Some growers who have ridge vents have installed “cat walks” to ease maintenance.  These can make plastic replacement and repairs easier. Gable vents high up on end walls can also improve ventilation by acting as outlets for warm humid air in warmer seasons and by allowing for low volume ventilation in colder weather. A 24″x24″ gable vent on each end wall is recommended for a 30’x96′ tunnel. These can be made of plywood and manually operated with hinges, ropes or cables and tie-downs. If using a louvered vent, be sure it has a flanged seal to close against. Thermostatic wax cylinder actuators may also be used which require no electricity, are relatively inexpensive and are passively controlled by the wax cylinder based on temperature.

HAF Fans. Horizontal air flow (HAF) fans are hung from the inside horizontal structural tubing to mix the air inside a tunnel to create consistent growing conditions, they don’t improve ventilation.  They are for circulating and mixing the air inside the tunnel. When installed and used properly, they ensure that plants and any control sensors are seeing the “average” conditions of the space. The first fan should be placed about 10′ to 15′ from one end wall to pick up the air that is coming around the corner from the other side. Subsequent fans should be located 40′ to 50′ apart to keep the air mass moving. In a 30′x100′ greenhouse, four fans are required, and the total fan capacity should be 6,000 CFM (2 CFM/ft2). The “empty” corner, where there is no fan can sometimes become a spot without air flow. Check to make sure you can feel air flow in all locations in the house.  You may have to add fans or reorient the ones on the end to promote adequate mixing flow. If a tall crop such as tomatoes is grown or if there are hanging baskets, a slightly greater capacity is needed to overcome the additional air flow resistance. Small, 1/10 to 1/15 horsepower circulating fans work well in providing the air movement needed. A permanent split capacitor motor can save as much as one-third the electricity of the more common shaded pole motor. Some growers have used inexpensive, simple box fans and just plan for frequent replacement. High efficiency vane-axial fans can increase the “throw” of each HAF fan meaning you need fewer fans to provide the same mixing flow.

Tunnel covering. Typical high tunnel covering is greenhouse grade 6 mil polyethylene rated for 5 years. Using two layers, separated by air blown between the layers, reduces heat loss during cold season production and provides stability under windy conditions, reducing damage to the plastic. Solid plastic “spacers” are available to separate two layers of plastic in locations without electricity.  Some greenhouse plastics have additives to enhance durability and performance. UV stabilizers are essential to slow degradation of plastic. Anti-fog and anti-drip surfactants make water condense and run down to the sides of the structure, rather than bead and drop on the plants below. IR radiation-blocking additives reduce heat loss at night. UV-absorbing films have the potential to suppress certain insects and diseases. In summer, plastic may be covered with shade cloth to reduce light intensity and temperature. Shade cloth is rated by the percent of light blocked. Whitewash is also used by growers to help keep tunnels cool in summer. 

Heating. When it’s cold outside, growers may want to heat the air and/or soil inside a tunnel, and this can be done using permanent heating systems, or emergency systems for coping with unusual conditions. Whatever system is used, a temperature warning system is important to provide notification when heating (or cooling) is urgently needed. To determine what size heater is needed, one must calculate the heat loss when a certain minimum temperature is desired inside the tunnel when there is a certain outside temperature. More information on heating, ventilation, and other engineering issues can be found at the University of Massachusetts greenhouse and floriculture web site and on the University of Vermont Extension agricultural engineering blog.

Irrigation. Water must be provided to replace that lost by evapotranspiration (ET), the combination of soil surface evaporation and water loss from plant leaves. Drip irrigation is an efficient way to deliver water and nutrients in tunnel, keeping the foliage dry, which reduces disease pressure. Drip tape is usually 8 to 10 mil thickness and is laid on the surface or buried an inch or two. Flow rates of drip tapes vary, a medium–flow tape provide 0.5 gpm per 100 feet. High-flow tape with 1.0 gpm flow is useful to prevent clogging and reduce irrigation time. Drip lines should be spaced on the soil surface to assure that the entire bed or row is wetted –light textured soils have less capillary movement of water than heavier soils, so more drip lines may be needed to prevent dry areas in the bed in which roots will not grow. Many systems are available to add soluble fertilizer to irrigation water. Fertigation is a good way to provide plants with the nutrients they need over the season, or to supplement fertilizers applied at planting. Irrigation water quality should be tested. Water with high pH and high alkalinity may lead to increased soil pH over time.

A mature crop of tomatoes may require 2.5 quarts of water per plant per day, whereas winter greens may grow well only on existing soil moisture, and if irrigated, may develop disease. Determining the optimal amount and timing of irrigation is complicated since it depends on the crop, its stage of growth, soil texture, sunlight, temperature and humidity. Use of soil moisture sensors placed at several locations and depths in each tunnel, combined with irrigation and crop performance records can help determine best practices on your farm. 

Pest management. The tunnel environment differs from the field, so the type and timing of insects and diseases also differs. For example, spider mites and aphids are more frequent pest in tunnels than outdoors, and foliar diseases are typically more problematic in tunnels due to humidity levels. The use of biological insect controls is more practical in tunnels than outdoors due to the enclosed space, high crop value, and extensive information developed for many crops. See the University of Vermont’s high tunnel pest management web site for more information. Maintaining the area around the perimeter of the tunnel with mowing and trimming is a passive way of minimizing mammalian pests by reducing cover.

Pesticides. Outdoors, pesticide residues break down after application by exposure to ultraviolet radiation and rainfall. Inside tunnels, plastic coverings reduce U/V and rain, and as a result, pesticides break down differently. Each state’s pesticide regulatory agencies may have different interpretations of whether high tunnels are considered open fields or greenhouses, however it is safest to consider a high tunnel a greenhouse from the perspective of pesticide labels. The label may 1) specifically state that the product may be used in greenhouses and may provide different guidelines for greenhouse and outdoor use, 2) specifically state that the product may not be used in greenhouses, or 3) may not mention greenhouse use at all. The Environmental Protection Agency’s current position is that, a label does not have to specify greenhouse as a site, provided the crop is on the label, in order to use the product in a greenhouse. If the label has multiple sections, and one of those sections is for greenhouse application, then the label must be followed explicitly for greenhouses with no exceptions. The rate for outdoor applications on those crops is for outdoor use ONLY and CANNOT be used for those crops in the greenhouse, since those crops were not included in the greenhouse section of the label. Using a pesticide inside a greenhouse where the label does not mention greenhouse use can increase risks to workers or plants. Also, when using a fumigant or smoke generator for an entire greenhouse, every crop in the greenhouse must be listed on the product label. We advise against applying a product in high tunnels unless the label specifically allows its use in greenhouses.