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.