High tunnels: what are, how they are different from greenhouses, and the products more frequently cultivated in them in the US

For many decades, high tunnels have been one of the essential tools used by agricultural producers to improve agriculture processes worldwide and thus obtain better quality products during any time of the year.

While they can sometimes be confused with greenhouses, the truth is that high tunnels have very particular characteristics that separate them from that category, which besides, have allowed them to become one of the most viable and effective options, for example, to save a larger amount of capital by significantly lowering installation and maintenance costs.

What are high tunnels?

When we talk about high tunnels, many people often call them “hoop houses,” although in simple words it can be said that they are structures erected in a land arranged for planting, which can be ventilated naturally by having large doors, as well as sidewalls that can be rolled or extracted, according to the climatic requirements of each time of the year.

These structures have a single or double layer of a polyethene film that is stretched over hoops, either made of metal or polyvinyl chloride (PVC).

High tunnels don’t have electrical services, heating systems or mechanized ventilation, so today some agricultural producers are responsible for providing additional heating to these structures through unventilated propane heaters, to protect their crops from low temperatures, especially during the arrival of spring and the end of autumn.

To maintain crops with the proper hydration levels, agricultural producers generally implement manual or drip irrigation, although some prefer to install sprinkler systems to produce a wide variety of crops directly in the soil.

Differences between high tunnels and greenhouses

• Temperature control systems

One of the main differences is that high tunnels are not cooled or heated through machinery or artificial mechanisms and depend basically on the natural ventilation that enters thanks to the removal or winding down of the walls of the structure that work as side vents, so less temperature control is required. Greenhouses, on the other hand, do need electrical mechanisms or other fuel sources to heat and cool, while they have an automated superior ventilation method that helps them achieve specific temperature and humidity levels within the area, so temperature control is strict.

• Agricultural taxes

Another aspect of contrast between the two is the fact that high tunnels aren’t perceived as permanent structures that remain in the same place for a long time and, therefore, are not subject to some agricultural taxes, for example, land taxes. Instead, structures such as barns, farms and greenhouses are subject to this type of taxes since they are perceived as individual properties.

• Construction material

High tunnels are generally made of plastic (rigid and flexible), which is a little more economical, while greenhouses often use a wide variety of materials such as polyester, rigid plastic, even flexible plastic combinations, as well as the use of glass, which is more expensive, not only in terms of construction but also regarding the maintenance of the structure.

• Land for cultivation

Crops in high tunnels are usually found directly in the land arranged for planting, while in greenhouses crops can be found in containers or several types of hydroponic systems where land use is not required for plants to thrive.

 

 

High tunnels in the United States

According to information published on the website Scientific Research, although high tunnels have been used in this nation for more than five decades, they have not been adopted so quickly by fruit, vegetable and flower growers in the United States, compared to the rest of the world where, by 1945, high tunnels or plastic structures were already used to grow horticultural products. We could say that the use of these structures isn’t that old.

The construction of the first plastic-covered structure of this kind in the region is attributed to Emery Emmert in 1953 and remained in use until 1959. The goal of the structure was to save costs compared to one made of glass. In the early 1990s, Otho Wells MD and Brent Loy MD from the University of New Hampshire built the first eight high tunnels for research in the United States on their experimental farm.

Plastics used for cultivation under high tunnels initially had certain weaknesses, for example, low UV resistance, difficulty to heat or cool the structure, as well as to manage the ventilation that would minimize the condensation levels, which explained why they had to be replaced each year, as they deteriorated easily and their lifespan was shorter.

Thanks to technological advances, this has changed and today the replacement can be done after 3 or 5 years, depending on the climate risks of the area and the material used to manufacture the plastic films. For example, nowadays it’s common to find plastic films with high resistance to ultraviolet rays, which gives them a longer lifespan and durability.

Currently, the technology applied in plasticulture allows to add up to 7 layers in the manufacture of these plastics, adding different properties to each layer to meet the most demanding needs of agricultural producers depending on the geographical area of the crops, climate risks, special characteristics of the crop, etc. In this sense, we invite you to read our article on the importance of the quality of agricultural plastics and the tests that are carried out to guarantee that you buy agricultural films that meet each of your specifications.

The popularity of high tunnels has been growing exponentially since they are an ideal tool to extend food production seasons, while yield levels rise and the quality of vegetables, cut flowers and fruits are optimized.

Thanks to conservation programs led by the USDA, more than 13,000 high tunnels have been built, with an estimated investment of more than $61 million by the end of 2014. Currently, many farmers from different parts of the world are erecting multi-bay complexes that allow hosting more structures of this type to boost large-scale production.

According to Scientific Research, the number of agricultural operations producing food crops under protection in the United States has now tripled since 1979 and sales have multiplied by 30 over the same period. The forecast is that the use of high tunnels in this nation will continue to increase as demand for fresh horticultural products obtained in increasingly local areas and close to consumers thrives.

 

 

Products that are grown in high tunnels in the United States

Among the different products that are mainly sown in high tunnels stand out the cultivation of cucumbers, lettuce, tomatoes, eggplants, melons, paprika and summer squash, along with other smaller tree fruits such as berries.

Using these structures to obtain greater quantity and quality of horticultural products generates significant benefits: they allow to create a special protection environment to prepare for severe weather events, raise the temperature of the soils more quickly after the arrival of spring and use the appropriate amount of water to avoid compromising the integrity of the soils due to the appearance of fungal diseases as a result of waterlogged soils.

Thanks to their properties, different microclimates can be created in high tunnels, which benefit food production almost all year round regardless of the season.

For example, in countries where the climate is generally temperate, high tunnels help extend crop growth seasons by creating a much warmer environment for plants; while in nations with more tropical climates, these structures allow to extend the farming season by protecting crops once the rainy season comes, which brings with it a lot of moisture. They also provide shade and decrease the high temperatures when the hot season arrives.

It is important to know what types of high tunnels exist to choose the one that best suits the requirements of the land and the type of crop to be sown. For example, there are single-bay or multi-bay tunnels that are ideal for fruit trees and small fruit crops, but since they have a large film surface on the roof are susceptible to very strong winds and cannot tolerate snow loads, so it’s recommended to remove these plastics during the winter and put them back on in spring or summer.

Such tunnels may require daily maintenance and therefore, must be placed on well-drained soil with drainage corridors or grooves between the structures, as well as in areas where access is easy during all production seasons, guaranteeing that they are located far enough from the main roads to avoid any damage caused by agricultural machinery and other vehicles.

They are generally erected in a production field, so it’s necessary to project what their estimated growth will be to take preventive measures. Usually, high tunnels are firmly fastened to the ground using metal poles (some with screw-type structures that provide more grip) that are nailed to the ground at about 0.75 m deep, or also using reinforcement bars to leverage railings in moving tunnels.

The optical, thermal and mechanical properties of various plastic films used to cover the high tunnels were analysed by performing several studies, discovering that the multilayer material has a better performance than the monolayer, especially when it comes to resistance to the abrasive effect of wind and sand.

Also, the initial light transmission was greater in the monolayer, but after sand and wind abrasion, the opposite was observed, less light entered through the monolayer than with the multilayer system. In regions with warmer climates, where extending the season is not a problem, polypropylene fabric is often used to protect plants from insects and the incidence of sunlight.

If you have any questions about the most suitable agricultural plastics for your type of crop or the geographical area where you are, as well as tips for their installation in high tunnels, don’t hesitate to contact our experts whose data you will find on this page.